Archives of Seismological Society of America recent issues

[Articles] Inhibition of Very Strong Ground Motion in Response Spectral Attenuation Models and Effects of Site Class and Tectonic Category

Zhao, J. X., Rhoades, D. A., McVerry, G. H., Somerville, P. G. — 2009-06-07

In current ground-motion models, the uncertainty in predicted ground motion is usually modeled with a lognormal distribution. One consequence of this is that predicted ground motions do not have an upper limit. In reality, however, there probably exist physical conditions that limit the ground motion. Applying the usual uncertainty distribution in probabilistic seismic hazard analysis may lead to ground-motion estimates that are unrealistically large, especially at the low annual probabilities considered for important structures, such as dams or nuclear reactors. A recently proposed statistical procedure to compare the actual and expected numbers of predicted spectral accelerations exceeding a given value gives clear results when applied to a ground-motion model developed for Japan from a very large strong-motion data set. It shows that, for increasingly large spectral accelerations, the actual number of exceedances becomes progressively less than the expected number of exceedances. The pattern of this discrepancy depends on the site class and the earthquake tectonic category. These results suggest that assuming a normal distribution for the prediction errors of an attenuation model (empirical ground-motion prediction equation) is likely to result in overestimation of the extreme values of spectral accelerations.

[Articles] Making the Most of Available Site Information for Empirical Ground-Motion Prediction

2009-06-07

This article proposes a new framework for the inclusion of site effects in empirical ground-motion prediction equations (GMPEs) by characterizing stations through their one-quarter wavelength velocities and assessed confidence limits. The approach is demonstrated for 14 stations of the French accelerometric network (Réseau Accélérométrique Permanent). This method can make use of all the available information about a given site, for example, the surface geology, the soil profile, standard penetration test measurements, near-surface velocity estimated from the topographic slope, depth to bedrock, and crustal structure. These data help to constrain the velocity profile down to a few kilometers. Based on a statistical study of 858 real profiles from three different regions (Japan, western North America, and France) physically realistic profiles are generated that comply with the information available for each site.

In order to evaluate the confidence limits for the shear-wave velocity profiles and derived site amplifications for each station, a stochastic method is adopted: several thousand profiles are randomly generated based on parameters derived in the statistical study and the constraints available for each station. Then, the one-quarter wavelength assumption is used to estimate the amplification for each station. It is found that a good knowledge of near-surface attenuation (i.e., or Q) is mandatory for obtaining precise amplification estimates at high frequencies. Nevertheless, the proposed scheme highlights the important differences in the uncertainties of the site amplifications, depending on the information available for a given station. We suggest that these results could, therefore, be used when developing GMPEs by weighting records from each station depending on the variability in the computed one-quarter wavelength velocities.

This approach relies on the assumption that local site effects are only one-dimensional, which is far from true, especially in sedimentary basins. However, most GMPEs only model one-dimensional site effects, so this is not an issue specific to this study. Finally, a way to improve this technique is to use earthquakes or noise recorded at the stations to further constrain the shear-wave velocity profiles and to consequently derive more accurate one-quarter wavelength velocities.

[Articles] Numerical Study of Ground-Motion Differences between Buried-Rupturing and Surface-Rupturing Earthquakes

Pitarka, A., Dalguer, L. A., Day, S. M., Somerville, P. G., Dan, K. — 2009-06-07

Recent ground-motion observations suggest that surface-rupturing earthquakes generate weaker near-fault ground motion than buried earthquakes. This difference is significant in the period range of 0.3–3 sec. Contributing factors to this phenomenon may include the effect of fault zone weakness at shallow depth on rupture dynamics and rupture directivity during earthquakes.

We present results from numerical experiments of spontaneous dynamic rupture and near-source ground-motion simulations of surface rupturing and buried earthquakes and discuss mechanisms for the observed ground-motion differences. The surface-rupturing earthquake is modeled with a shallow zone of 5 km thickness containing areas of negative stress drop (within the framework of the slip-weakening friction model) and lower rigidity. Surface-rupturing models with this weak zone generate lower amplitude ground velocity than do models without this modification.

Observed ground-motion differences between surface and buried events are qualitatively reproduced by imposing higher stress drop in the buried earthquakes than in the surface earthquakes, combined with introducing a deeper rupture initiation for buried rupture, enhancing upward rupture-directivity effects for the latter events. In the context of our simplified model parameterization, then, the observed differences in ground motion could arise from combined effects of relative weakness of the shallow layer of faults, the relatively larger stress drops of buried ruptures, and a tendency of near-fault sites to record strong upward directivity from buried ruptures.

[Articles] Near-Fault Strong Ground Motions Recorded during the Morelia Normal-Fault Earthquakes of May 2006 in Mexicali Valley, B. C., Mexico

Munguia, L., Glowacka, E., Suarez-Vidal, F., Lira-Herrera, H., Sarychikhina, O. — 2009-06-07

By the end of May 2006, a series of earthquakes had occurred in the Mexicali Valley, Baja California, México. This activity, which included an M_w 5.4 event, developed in the vicinity of the Cerro Prieto geothermal field. The earthquakes occurred on the Morelia fault, one of the east-dipping normal faults in the Mexicali Valley. Cracks and fissures visible on the ground after the stronger earthquake defined two parallel fault segments of 4 and 2 km length, the shorter segment being 1 km to the east of the other one. Down-dip displacements of up to 25–30 cm were measured at some places along this surface break zone.

The ten larger events of the series were recorded by strong-motion stations that operate in the epicenter area, so that accurate hypocenter locations were obtained on the basis of P-wave arrival times from the strong-motion recordings. The estimated shallow depths, combined with the earthquake magnitude, explain the strong shaking felt by residents and some damage observed on the Cerro Prieto geothermal plant.

The M_w 5.4 event produced peak ground accelerations that go from 0.002g at CUC (on rock) to 0.5g at GEO (on sediments), at 13 and 1.7 km from the epicenter, respectively. The station GEO recorded closer to the epicenters and on the hanging-wall side of the fault. Static ground displacements and a predominance of the strike-normal over the strike-parallel velocity components were determined from the acceleration records of this station. These and other ground-motion characteristics are also seen on pseudovelocity and absolute acceleration response spectra calculated from data of the larger event. Altogether, the observed ground-motion characteristics provide useful insights into the levels of ground shaking that near-fault structures in the Mexicali Valley should be designed to withstand.

[Articles] Predicted Ground Motions for Great Interface Earthquakes in the Cascadia Subduction Zone

Atkinson, G. M., Macias, M. — 2009-06-07

Ground motions for earthquakes of moment magnitude (M) 7.5–9.0 on the Cascadia subduction zone interface are simulated based on a stochastic finite-fault model and used to estimate average response spectra for firm-site conditions in Vancouver, Victoria, and Seattle. We also express the response spectra as ground-motion prediction equations (GMPEs) for Cascadia events. The simulations are calibrated by modeling the wealth of ground-motion data from the M 8.1 Tokachi-Oki earthquake sequence of Japan. Adjustments to the calibrated model are made to consider average source, attenuation, and site parameters for the Cascadia region.

We perform best estimate simulations for a preferred set of input parameters. Typical results suggest mean values of 5%-damped pseudoacceleration in the range from about 100 to 200 cm/sec², at frequencies from 1 to 4 Hz, for firm-ground conditions in Vancouver, Victoria, and Seattle. Uncertainty in stress drop causes uncertainty in simulated response spectra of about ±50%. Uncertainties in the attenuation model produce even larger uncertainties in response spectral amplitudes—a factor of about 2 at 100 km, becoming even larger at greater distances. It is thus important to establish the regional attenuation model for ground-motion simulations. Furthermore, combining data from regions with different attenuation characteristics—in particular Japan and Mexico—into a global subduction zone database for development of global empirical GMPEs may not be a sound practice.

Time histories of acceleration for the stochastically simulated motions are provided for reference sites in Vancouver, Victoria, and Seattle. An alternative set of motions, based on lightly modifying real recordings from the Tokachi-Oki earthquake to match expected conditions for Cascadia cities, are also provided. These alternative records have similar spectral content to the simulated motions but contain additional complexity and more realistic phasing. The provision of alternative record sets allows users to conduct studies to determine the importance of these effects for structural response.

[Articles] Sedimentary Basin Effects in Seattle, Washington: Ground-Motion Observations and 3D Simulations

Frankel, A., Stephenson, W., Carver, D. — 2009-06-07

Seismograms of local earthquakes recorded in Seattle exhibit surface waves in the Seattle basin and basin-edge focusing of S waves. Spectral ratios of S waves and later arrivals at 1 Hz for stiff-soil sites in the Seattle basin show a dependence on the direction to the earthquake, with earthquakes to the south and southwest producing higher average amplification. Earthquakes to the southwest typically produce larger basin surface waves relative to S waves than earthquakes to the north and northwest, probably because of the velocity contrast across the Seattle fault along the southern margin of the Seattle basin. S to P conversions are observed for some events and are likely converted at the bottom of the Seattle basin. We model five earthquakes, including the M 6.8 Nisqually earthquake, using 3D finite-difference simulations accurate up to 1 Hz. The simulations reproduce the observed dependence of amplification on the direction to the earthquake. The simulations generally match the timing and character of basin surface waves observed for many events. The 3D simulation for the Nisqually earthquake produces focusing of S waves along the southern margin of the Seattle basin near the area in west Seattle that experienced increased chimney damage from the earthquake, similar to the results of the higher-frequency 2D simulation reported by Stephenson et al. (2006). Waveforms from the 3D simulations show reasonable agreement with the data at low frequencies (0.2–0.4 Hz) for the Nisqually earthquake and an M 4.8 deep earthquake west of Seattle.

[Articles] Evidence of Two-Dimensional Amplification Effects in an Alluvial Valley (Valnerina, Italy) from Velocimetric Records and Numerical Models

Lenti, L., Martino, S., Paciello, A., Scarascia Mugnozza, G. — 2009-06-07

The results of field surveys, velocimetric records, and numerical models are reported with the goal of analyzing the local seismic response in a section of the Nera River alluvial valley in central Italy. Alluvial deposit geometry and dynamic properties were defined by geological and geophysical investigations. 2D amplification effects were pointed out by horizontal-to-vertical spectral ratios (HVSRs) as well as by horizontal-to-horizontal spectral ratios (HHSRs) to a reference station; HVSRs were derived from both noise and weak-motion records, while HHSRs were only computed from weak motions. Where travertine deposits are interlayered within the alluvia, 2D amplification effects are shown only by HHSRs. The observed amplifications: (1) consist of peaks of the amplification functions A(f) in the range 1–10 Hz, (2) generally correspond to two frequency ranges whose values mainly depend on the valley shaped ratio and on the local heterogenities of the alluvial deposits, and (3) can be regarded as due to a 1D resonance combined with lateral wave propagation. Numerical models via the finite difference method were performed by the Italian National Institute for Geophysics and Volcanology Web Interface for Seismological Application and by the Fast Lagrangian Analysis of Continua codes. The obtained results: (1) show a good agreement with HHSRs, (2) prove that some 2D amplification effects are constrained by both the shape ratio of the valley and the heterogeneity of the alluvial deposits, (3) reveal focalization effects within the alluvial deposits strictly related to the shape of the valley, and (4) allow the comparison of the A(f) functions in both linear and nonlinear dynamic behavior and the computation of the shear strains due to ground motion within the alluvial deposits.

[Articles] Determination of Shallow Shear-Wave Attenuation in the Mississippi Embayment Using Vertical Seismic Profiling Data

Ge, J., Pujol, J., Pezeshk, S., Stovall, S. — 2009-06-07

We used vertical seismic profiling (VSP) data collected in four shallow boreholes (about 40 to 60 m deep) to study the shear-wave attenuation in the Mississippi embayment in southwestern Tennessee. The source was an air-powered hammer that produces repeatable SH waves that were recorded by monitor geophones deployed on the surface very close to the source. The spectral ratio method was used to estimate the shear-wave quality factor (Q_S). The method assumes that the amplitudes of the seismic waves decay exponentially in the frequency domain. The spectral ratio was computed using a VSP trace at a certain depth and the corresponding monitor trace. Using the source monitor trace as reference eliminates possible artifacts that may be introduced by changes in the source-ground coupling. The slope of a least-square line fitted to the logarithm of the spectral ratio versus frequency gives the attenuation coefficient (z) for that depth. Then a straight line is fitted to (z) as a function of z, and the slope of this line is used to estimate an average value of Q_S. For the four sites, Shelby Farms, Covington, Brownsville, and Jackson, the Q_S values are 25.3±2.6, 18.6±3.4, 18.4±7.6, and 16.4±2.6, respectively. To assess the reliability of these results, synthetic VSP data were generated from the first cycles of the monitor traces recorded in the four sites and these Q_S values. Except for Brownsville, good matches with correlation coefficients equal to 96%–99% were obtained between the synthetic VSPs and the actual VSPs, indicating that the Q_S values we obtained are reliable. In addition, these values are also consistent with recent results obtained by others using seismic data generated by small earthquakes recorded by the Japanese strong-motion network Kiban-Kyoshin (KiK-net).

[Articles] In Situ Measurements of Nonlinear and Nonequilibrium Dynamics in Shallow, Unconsolidated Sediments

Lawrence, Z., Bodin, P., Langston, C. A. — 2009-06-07

We present results and in situ measurements from a field experiment in which a large seismic shaker truck is used to induce nonlinear and nonequilibrium dynamics in shallow, unconsolidated sediments. An array of accelerometers was deployed adjacent to the shaker truck to record strong ground motions exceeding 1 g. We determined high-strain Rayleigh-wave dispersion across the array in the band from 5 to 60 Hz. Rayleigh-wave phase velocities at frequencies above ~10 Hz are wave amplitude-dependent and a function of the driving-force amplitude; as driving force was increased phase velocity decreased, consistent with nonlinear dynamics. We demonstrate the existence of a temporary, nonequilibrium state occurring in the near-surface soils during and after the induced nonlinear behavior. Nonlinear conditioning is demonstrated by measuring changes in Rayleigh-wave phase velocity for input signals with the same applied driving-force amplitude. A logarithmic slow dynamic recovery process is observed by analyzing the temporal variation in velocity of the noise field produced by the shaker truck when sitting idle. Measurements from weak-motion seismic surveys taken before and after shaking show compelling evidence that induced nonlinear behavior in the shallow sediments is reversible. We demonstrate that an active source, field-based approach has the potential to expand our knowledge of how sediments respond to strong ground motions, provide additional insight into the poorly understood slow dynamic recovery process, and possibly even lead to a new, site-specific, and noninvasive technique for characterizing the nonlinear properties of sedimentary deposits.

[Articles] On Baseline Corrections and Uncertainty in Response Spectrafor Baseline Variations Commonly Encounteredin Digital Accelerograph Records

Akkar, S., Boore, D. M. — 2009-06-07

Most digital accelerograph recordings are plagued by long-period drifts, best seen in the velocity and displacement time series obtained from integration of the acceleration time series. These drifts often result in velocity values that are nonzero near the end of the record. This is clearly unphysical and can lead to inaccurate estimates of peak ground displacement and long-period spectral response. The source of the long-period noise seems to be variations in the acceleration baseline in many cases. These variations could be due to true ground motion (tilting and rotation, as well as local permanent ground deformation), instrumental effects, or analog-to-digital conversion. Very often the trends in velocity are well approximated by a linear trend after the strong shaking subsides. The linearity of the trend in velocity implies that no variations in the baseline could have occurred after the onset of linearity in the velocity time series. This observation, combined with the lack of any trends in the pre-event motion, allows us to compute the time interval in which any baseline variations could occur. We then use several models of the variations in a Monte Carlo procedure to derive a suite of baseline-corrected accelerations for each noise model using records from the 1999 Chi–Chi earthquake and several earthquakes in Turkey. Comparisons of the mean values of the peak ground displacements, spectral displacements, and residual displacements computed from these corrected accelerations for the different noise models can be used as a guide to the accuracy of the baseline corrections. For many of the records considered here the mean values are similar for each noise model, giving confidence in the estimation of the mean values. The dispersion of the ground-motion measures increases with period and is noise-model dependent. The dispersion of inelastic spectra is greater than the elastic spectra at short periods but approaches that of the elastic spectra at longer periods. The elastic spectra from the most basic processing, in which only the pre-event mean is removed from the acceleration time series, do not diverge from the baseline-corrected spectra until periods of 10–20 sec or more for the records studied here, implying that for many engineering purposes elastic spectra can be used from records with no baseline correction or filtering.

[Articles] Regions of Reduced Static Stress Drop near Fault Tips for Large Strike-Slip Earthquakes

Lovely, P. J., Pollard, D. D., Mutlu, O. — 2009-06-07

We present a case study of slip distributions for the 1992 Landers (M_w 7.3) and the 1999 Hector Mine (M_w 7.1) earthquakes in California’s Mojave Desert. Slip distributions, as determined from geophysical inversion of geodetic, strong ground motion, and teleseismic data, are complex, heterogeneous, and often exhibit linearly tapering or concave-upward patterns toward segment tips, distinctly different from the elliptical slip distributions characteristic of uniform stress drop. Mechanical interaction of discontinuous fault segments fully explains the reduced slip near the southern termination of the Camp Rock/Emerson fault segment of the Landers rupture; however, numerical models demonstrate that such interactions are insufficient to explain slip distributions observed at other segment terminations. Numerical models demonstrate that long (5–25 km) zones of reduced stress drop in the vicinity of some rupture segment terminations can explain the slip distributions for these large earthquakes. Zones of reduced stress drop are implemented as regions of increased Coulomb strength. Slip distributions are improved 30%–70% relative to models with uniform stress drop. Regions of reduced stress drop appear to play a relatively greater role near segment tips at which rupture terminates than near segment tips at which rupture jumps to a nearby fault segment. Similar results are obtained implementing discrete stepwise and spatially linear reductions of stress drop. Plausible mechanical explanations for such zones of reduced stress drop include heterogeneous fault strength or friction, spatial or temporal changes in pore pressure, geometric complexity of the fault surface, heterogeneity of normal tractions resolved on the fault surface, inelastic deformation, and dynamic rupture effects.

[Articles] Source Scaling and Site Effects at Vesuvius Volcano

Galluzzo, D., Del Pezzo, E., La Rocca, M., Castellano, M., Bianco, F. — 2009-06-07

The site-corrected source scaling pattern is estimated for local earthquakes (0.9≤M_D≤3.6) at Mt. Vesuvius. The dataset comprises 35 low-to-moderate local earthquakes recorded by 14 three-component seismic stations during 1993, 1996, and 1999.

Site-transfer functions in the frequency range 1 Hz–25 Hz are estimated from the spectra of S waves and coda waves and from the horizontal-to-vertical (H/V) spectral ratios. We applied the direct spectral ratios method to S waves, considering as a reference the average spectrum and the inversion method to S waves and coda waves. The site amplification on the coda waves was also compared with that evaluated using the wavelet transform. The standard deviation associated with the experimental results is computed for all of the used methods.

Results indicate a general agreement among the methods, and the site-transfer functions show interesting features. The highest amplifications are found for frequencies lower than 12 Hz for sites located at lower altitude. The methods based on coda waves show highest amplification with respect to the methods based on S waves for most of the sites located in the summit part of the volcano. This can be a phenomenon of coda localization, which consists in the trapping inside the upper part of the volcano of scattered waves. The H/V spectral ratios do not show total agreement with the other methods, mostly for the sites located in the summit part of the volcano. The discrepancies among the results obtained in this work are also due to the different normalization applied in the methods of analysis.

Generalized inversion method allowed us to estimate the source scaling of the site-corrected source seismic spectrum for the investigated area. The source scaling obtained in terms of seismic moment and source radii shows that the seismicity of Mt. Vesuvius is characterized by stress drop as low as a few bars (10 bars) except for the event of ( ). The scaling pattern shows an apparent linear relationship between source size and seismic moment (for M_D≤3.3) but the statistical test shows that the linear trend has low reliability.

[Articles] Dynamic Driving of Small Shallow Events during Strong Motion

Fischer, A. D., Sammis, C. G. — 2009-06-07

High-pass filtering (>20 Hz) of acceleration records from the 1999 Chi-Chi, Taiwan, and 2004 Parkfield, California, earthquakes reveals a series of bursts that occur only during strong shaking. Initially interpreted as originating from asperity failure on the Chelungpu fault, bursts observed during the Chi-Chi earthquake were subsequently determined to be a local effect within about 1 km of the seismic stations. Similar bursts were observed at the U.S. Geological Survey Parkfield seismic array during the Parkfield earthquake and were constrained to originate less than 20 m from the instruments. Such small shallow events cannot result from the triggered release of stored elastic energy because rate-and-state friction rules out stick-slip instability on such small, shallow patches. Our hypothesis is that the bursts are not triggered but are driven by simultaneous shear and tensile stresses near the surface during the strong motion. At 2 Hz, SV- to P-wave mode conversion at the free surface produces tensile stresses to depths of 70 m. Where standard triggering releases stored elastic energy and adds to the incident wave field, this new driving mechanism takes energy out of the 2 Hz strong motion and reradiates it at high frequencies. It is thus an attenuation mechanism that we estimate can contribute 3% to the net attenuation in the very shallow crust.

[Articles] Structural Geometry and Slip of the Palos Verdes Fault, Southern California: Implications for Earthquake Hazards

Brankman, C. M., Shaw, J. H. — 2009-06-07

The Palos Verdes fault (PVF) is an active structure in southern California comprised of several segments that together form a complex fault system. The fault has been active since the Miocene and is a major regional seismic source. Using marine petroleum industry seismic data, we define the geometry of the fault and offsets of Tertiary stratigraphic units that constrain the along strike segmentation and slip rates on the fault system. In San Pedro Bay, patterns of deformed stratigraphic units suggest a deep fault geometry that dips moderately to the southwest and was initially formed as a Miocene normal fault. Plio–Pleistocene transpression resulted in the reactivation of this normal fault and its subsequent inversion with oblique, right-lateral reverse displacement. To the south, the Lasuen Knoll segment dips steeply to the northeast and is not a reactivated Miocene-age structure but rather formed in middle Pliocene time. Plio–Pleistocene transpression has linked these and other segments together to form the currently active PVF system. The resulting complex fault geometry and slip patterns suggest that the PVF is a highly segmented fault, which may rupture in moderate-size earthquakes that involve individual segments or large events that are able to propagate across significant geometric discontinuities. Estimates of moment magnitudes based on empirical relationships with rupture (fault) area indicate the possibility of M_w 6.6–6.9 earthquakes for single-segment ruptures and M_w 7.1–7.3 multisegment ruptures, with recurrence intervals ranging from 181 to 534 yr. Given the high inferred slip rates on the fault and its proximity to the urban population, these events pose significant earthquake hazards to metropolitan Los Angeles.

[Articles] The 1911 M~6.6 Calaveras Earthquake: Source Parameters and the Role of Static, Viscoelastic, and Dynamic Coulomb Stress Changes Imparted by the 1906 San Francisco Earthquake

Doser, D. I., Olsen, K. B., Pollitz, F. F., Stein, R. S., Toda, S. — 2009-06-07

The occurrence of a right-lateral strike-slip earthquake in 1911 is inconsistent with the calculated static stress decrease imparted by the 1906 rupture at that location on the Calaveras fault, and 5 yr of calculated post-1906 viscoelastic rebound does little to reload the fault. We have used all available first-motion, body-wave, and surface-wave data to explore possible focal mechanisms for the 1911 earthquake. We find that the event was most likely a right-lateral strike-slip event on the Calaveras fault, larger than, but otherwise resembling, the 1984 M_w 6.1 Morgan Hill earthquake in roughly the same location. Unfortunately, we could recover no unambiguous surface fault offset or geodetic strain data to corroborate the seismic analysis despite an exhaustive archival search. We calculated the static and dynamic Coulomb stress changes for three 1906 source models to understand stress transfer to the 1911 site. In contrast to the static stress shadow, the peak dynamic Coulomb stress imparted by the 1906 rupture promoted failure at the site of the 1911 earthquake by 1.4–5.8 bar. Perhaps because the sample is small and the aftershocks are poorly located, we find no correlation of 1906 aftershock frequency or magnitude with the peak dynamic stress, although all aftershocks sustained a calculated dynamic stress of ≥3 bar. Just 20 km to the south of the 1911 epicenter, we find that surface creep of the Calaveras fault at Hollister paused for ~17 yr after 1906, about the expected delay for the calculated static stress drop imparted by the 1906 earthquake when San Andreas fault postseismic creep and viscoelastic relaxation are included. Thus, the 1911 earthquake may have been promoted by the transient dynamic stresses, while Calaveras fault creep 20 km to the south appears to have been inhibited by the static stress changes.

[Articles] A Viscoelastic Earthquake Simulator with Application to the San Francisco Bay Region

Pollitz, F. F. — 2009-06-07

Earthquake simulation on synthetic fault networks carries great potential for characterizing the statistical patterns of earthquake occurrence. I present an earthquake simulator based on elastic dislocation theory. It accounts for the effects of interseismic tectonic loading, static stress steps at the time of earthquakes, and postearthquake stress readjustment through viscoelastic relaxation of the lower crust and mantle. Earthquake rupture initiation and termination are determined with a Coulomb failure stress criterion and the static cascade model. The simulator is applied to interacting multifault systems: one, a synthetic two-fault network, and the other, a fault network representative of the San Francisco Bay region. The faults are discretized both along strike and along dip and can accommodate both strike slip and dip slip. Stress and seismicity functions are evaluated over 30,000 yr trial time periods, resulting in a detailed statistical characterization of the fault systems. Seismicity functions such as the coefficient of variation and a- and b-values exhibit systematic patterns with respect to simple model parameters. This suggests that reliable estimation of the controlling parameters of an earthquake simulator is a prerequisite to the interpretation of its output in terms of seismic hazard.

[Articles] Statistics of Earthquake Stress Drops on a Heterogeneous Fault in an Elastic Half-Space

Bailey, I. W., Ben-Zion, Y. — 2009-06-07

We investigate properties of earthquake stress drops in simulations of evolving seismicity and stress field on a heterogeneous fault. The model consists of an inherently discrete strike-slip fault surrounded by a 3D elastic half-space. We consider various spatial distributions of frictional properties and analyze results generated by 150–300 model years. In all cases, the self-organized heterogeneous initial stress distributions at the times of earthquake failure lead to stress drops that are systematically lower than those predicted for a homogeneous process. In particular, the large system-sized events have stress drops that are consistently ~25% of predictions based on the average fault strength. The type and amount of assumed spatial heterogeneity on the fault affect the stress-drop statistics of small earthquakes (M_L<5) more than those of the larger events. This produces a decrease in the range of stress drops as the earthquake magnitudes increase. The results can resolve the discrepancy between traditional estimates of stress drops and seismological observations. The general tendency for low stress drops of large events provides a rationale for reducing the statistical estimates of potential ground motion associated with large earthquakes.

[Articles] Earthquake Stress Drops and Inferred Fault Strength on the Hayward Fault, East San Francisco Bay, California

Hardebeck, J. L., Aron, A. — 2009-06-07

We study variations in earthquake stress drop with respect to depth, faulting regime, creeping versus locked fault behavior, and wall-rock geology. We use the P-wave displacement spectra from borehole seismic recordings of M 1.0–4.2 earthquakes in the east San Francisco Bay to estimate stress drop using a stack-and-invert empirical Green’s function method. The median stress drop is 8.7 MPa, and most stress drops are in the range between 0.4 and 130 MPa. An apparent correlation between stress drop and magnitude is entirely an artifact of the limited frequency band of 4–55 Hz. There is a trend of increasing stress drop with depth, with a median stress drop of ~5 MPa for 1–7 km depth, ~10 MPa for 7–13 km depth, and ~50 MPa deeper than 13 km. We use S/P amplitude ratios measured from the borehole records to better constrain the first-motion focal mechanisms. High stress drops are observed for a deep cluster of thrust-faulting earthquakes. The correlation of stress drops with depth and faulting regime implies that stress drop is related to the applied shear stress. We compare the spatial distribution of stress drops on the Hayward fault to a model of creeping versus locked behavior of the fault and find that high stress drops are concentrated around the major locked patch near Oakland. This also suggests a connection between stress drop and applied shear stress, as the locked patch may experience higher applied shear stress as a result of the difference in cumulative slip or the presence of higher-strength material. The stress drops do not directly correlate with the strength of the proposed wall-rock geology at depth, suggesting that the relationship between fault strength and the strength of the wall rock is complex.

[Articles] Differential Energy Radiation from Two Earthquakes in Japan with Identical Mw: The Kyushu 1996 and Tottori 2000 Earthquakes

Choy, G. L., Boatwright, J. — 2009-06-07

We examine two closely located earthquakes in Japan that had identical moment magnitudes M_w but significantly different energy magnitudes M_e. We use teleseismic data from the Global Seismograph Network and strong-motion data from the National Research Institute for Earth Science and Disaster Prevention’s K-Net to analyze the 19 October 1996 Kyushu earthquake (M_w 6.7, M_e 6.6) and the 6 October 2000 Tottori earthquake (M_w 6.7, M_e 7.4). To obtain regional estimates of radiated energy E_S we apply a spectral technique to regional (<200 km) waveforms that are dominated by S and Lg waves. For the thrust-fault Kyushu earthquake, we estimate an average regional attenuation Q(f)=230f^0.65. For the strike-slip Tottori earthquake, the average regional attenuation is Q(f)=180f^0.6. These attenuation functions are similar to those derived from studies of both California and Japan earthquakes. The regional estimate of E_S for the Kyushu earthquake, 3.8x10¹⁴ J, is significantly smaller than that for the Tottori earthquake, E_S 1.3x10¹⁵ J. These estimates correspond well with the teleseismic estimates of 3.9x10¹⁴ J and 1.8x10¹⁵ J, respectively. The apparent stress (_a=µE_S/M₀, with µ equal to rigidity) for the Kyushu earthquake is 4 times smaller than the apparent stress for the Tottori earthquake. In terms of the fault maturity model, the significantly greater release of energy by the strike-slip Tottori earthquake can be related to strong deformation in an immature intraplate setting. The relatively lower energy release of the thrust-fault Kyushu earthquake can be related to rupture on mature faults at a subduction environment. The consistence between teleseismic and regional estimates of E_S is particularly significant as teleseismic data for computing E_S are routinely available for all large earthquakes whereas often there are no near-field data.

[Articles] A Method for Rapid Determination of Moment Magnitude Mw for Moderate to Large Earthquakes from the Near-Field Spectra of Strong-Motion Records (MWSYNTH)

Delouis, B., Charlety, J., Vallee, M. — 2009-06-07

Seismic moment and the corresponding moment magnitude M_w are classically obtained from the spectrum of far-field body waves. Near-field records are generally not used for that purpose, particularly in the case of large earthquakes because different types of wave arrive simultaneously, preventing the definition of a simple relation between the seismic moment and the spectrum. We developed an original method to determine M_w from the displacement spectra of near-field records. The spectral amplitude at low frequency obtained from the real records is compared to that of synthetic records computed using kinematic rupture models scaled with M_w. Synthetic records are computed and averaged for various fault orientations and for epicentral distances ranging from 1 to 100 km. The initial portion of the spectrum affected by baseline shift in the acceleration records is automatically identified and removed by high-pass filtering using a cutoff frequency adapted to each station. The synthetic spectral values as a function of moment magnitude, epicentral distance, and filtering are computed only once and stored in tables. The spectral amplitudes of the real records are simply interpolated in the tables of synthetic data, allowing a fast determination of M_w. The method has been validated using 22 shallow earthquakes (depth<50 km) with magnitude ranging from 3.9 to 7.7. We show that a window of 80 sec of signal after the earthquake origin time provides robust values of M_w for the whole magnitude range considered here. Shorter time windows may be used but with M_w underestimated for large events. The method is well suited for near real-time fast determination of M_w.

[Articles] Comparison of Magnitude Estimates for New Zealand Earthquakes: Moment Magnitude, Local Magnitude, and Teleseismic Body-Wave Magnitude

Ristau, J. — 2009-06-07

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M_w>~3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude (M_L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M_w with M_L for New Zealand earthquakes. In addition to M_w and M_L, there are more than 330 teleseismic body-wave magnitude (m_b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M_w. These are used to examine the relationship between M_w, M_L, and m_b for New Zealand earthquakes. There is a clear distinction in the relationship of M_L to M_w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M_L to be fairly consistent with M_w, particularly for events with M_w>~4.5 and with . Deep focus earthquakes have M_L consistently larger than M_w (more than a full magnitude unit for some events) with . M_w and m_b are in fairly good agreement regardless of the depth, whereas m_b estimates are consistently smaller than M_L for deep events. This suggests that M_L is overestimated for deep focus events and that the large M_Ls are not the result of physical properties of the source.

[Articles] Source Parameters for the 28 April 2007 Mw 4.0 Earthquake in Folkestone, United Kingdom

Ottemoller, L., Baptie, B., Smith, N. J. P. — 2009-06-07

A moderate size earthquake (M_L 4.3, M_w 4.0) occurred in southeastern England on 28 April 2007. The earthquake was of some significance as it caused damage in the town of Folkestone and produced the largest peak horizontal ground acceleration (PGA, 0.1g) measured in the United Kingdom to date. It was followed by 12 aftershocks between M_L 0.8 and 1.7. The earthquake was the first of this size recorded by a significant number of newly installed broadband stations in the United Kingdom. The hypocenter of the event was at a depth of about 5 km beneath Folkestone, with an error ellipse indicating horizontal errors in a location of about 5 km. The depth was well constrained using a number of techniques, of which local travel-time inversion and teleseismic depth phase modelling are most reliable. A stress drop of 28.6 bars and a source radius of 0.5 km were determined from the analysis of displacement source spectra. We derived a near-surface attenuation factor =0.02 from the aftershock data that were used in the spectral analysis of the mainshock. Applying the horizontal to vertical (H/V) spectral ratio technique to microtremor data recorded at a station 2 km from the epicenter revealed site amplification at frequencies of 0.4 and 3.9 Hz. This amplification is likely to have contributed to the mainshock PGA of 0.1g measured at the same site. Similar site conditions may have been responsible for the damage in parts of Folkestone. The moment tensor computed from regional broadband data showed a strike-slip mechanism with a normal component and either right-lateral movement on a west-southwest–east-northeast-striking or left-lateral movement on a north-northwest–south-southeast-striking nodal plane. The north-northwest–south-southeast-striking nodal plane matches the trend of the main faults affecting the Kent coalfield and also possibly the Variscan front. It is thus possible that the causative fault was associated with the Variscan front, a major structural boundary at the northern limit of late Carboniferous folding and thrusting.

[Articles] First Application of the New IASPEI Teleseismic Magnitude Standards to Data of the China National Seismographic Network

Bormann, P., Liu, R., Xu, Z., Ren, K., Zhang, L., Wendt, S. — 2009-06-07

This article presents the results of the application of the new measurement standards for teleseismic magnitudes. They have been applied in parallel with traditional Chinese magnitude procedures to more than 14,000 digital broadband-velocity seismograms of 531 earthquakes in the magnitude range 4–9. The records were made between 2001 and 2007 at stations of the China National Seismographic Network (CNSN) and analyzed at the China Earthquake Network Center (CENC). The regression relations between different types of standard magnitudes, traditional Chinese magnitudes, and the respective values of m_b, M_S, and M_e published by the National Earthquake Information Center (NEIC), as well as Global Centroid Moment Tensor (GCMT) moment magnitudes M_w, are presented. The new broadband body-wave magnitude m_B(BB) is measured at dominant periods in the range 0.2<T<30 sec that on average increase exponentially with magnitude, as expected by seismic scaling laws. The broadband surface-wave magnitude M_S(BB) measures the maximum Rayleigh-wave amplitudes in a wide range of periods (3≤T<60 sec) and epicentral distances (2°≤≤160°), as originally proposed for the current International Association of Seismology and Physics of the Earth’s Interior (IASPEI) standard calibration function for M_S determination. The article analyzes and disproves with rich data several widespread prejudices or misunderstandings concerning the applicability, stability, and accuracy of magnitudes based on readings from unfiltered broadband records in general and the IASPEI M_S formula in particular. It is shown that the use of surface-wave amplitudes in a broad period range removes most of the systematic distance dependence that is observed when the IASPEI formula is restricted to periods around 20 sec. In the distance range between 2° and 103°, we have investigated the trend between M_S(BB) and M_S(20), which is the common surface-wave magnitude determined by the NEIC for periods between 18 and 22 sec and at teleseismic distances only. The two magnitudes agree rather well and have comparable measurement errors, even when M_S(BB) is measured at regional distances of less than 20°. On average, however, M_S(BB) is slightly larger than M_S(20) at distances less than 45° with a small and consistent distance-dependent trend in the residuals M_S(BB)-M_S(20) of -0.0029 magnitude units per degree in both the regional and teleseismic distance range. The analyzed regression relations further reveal the following: (1) the new standard for m_b delays saturation and yields about 0.5 units larger m_b values for great earthquakes; (2) broadband m_B(BB) moves the saturation limit for body-wave magnitudes further up to about 8.3. This makes m_B(BB) a good candidate for providing rapid magnitude estimates of strong earthquakes well ahead of M_w; (3) standard M_S(20) scales perfectly with the M_S published by the NEIC; (4) M_S(BB) reduces the underestimation of surface-wave magnitude by M_S(20) for weaker earthquakes and those recorded at regional distances for which the maximum surface-wave amplitudes occur mostly at periods well below 16 sec; and (5) new m_B(BB) and M_S(BB), that is, the body-wave and surface-wave magnitudes directly measured on velocity-broadband records, reproduce well the classical Gutenberg–Richter relation between m_B and M_S, which—together with the relation between seismic energy E_S and m_B—formed the basis for deriving the modern moment- and energy-magnitude scales M_w and M_e.

[Articles] Source Phenomenology Experiments with Borehole Explosions of Special Design in Israel

Gitterman, Y. — 2009-06-07

Two source phenomenology explosion experiments—decoupling and depth-of-burial (DOB)—were conducted by the Geophysical Institute of Israel (GII) in July 2006 and January 2007 in Negev, Israel. A special blast design and technology were utilized to form large 1240 kg (decoupling) and 4200 kg (DOB) near-spherical Ammonium Nitrate and fuel oil charges in cavities at different depths (26–63 m), in marls and phosphates, created beforehand by small shots in boreholes.

The main goal of the experiments was investigation of empirical relationships between the contained spherical source and energy/magnitude parameters of different seismic phases at near-source and regional distances.

Small decoupling factors for the heavily overdriven tests were estimated as 2.8 and 6.4 for charge/volume ratios 250 and 70 (kg/m³), respectively. Very high signal frequencies (40–90 Hz) accompanied by the highest peak accelerations were observed at near-source distances for the deep (63 m) decoupled explosion. A clear magnitude/energy reduction with increasing depth of contained sources was obtained at regional distances (similar to the 1997 Balapan DOB experiment), complemented by near-source recordings of higher frequencies and larger amplitude/energy for deeper charges. Observations of the Negev DOB experiment are remarkably consistent with the Mueller and Murphy (1971) model predictions of source spectra features. The same conversion point of spectra dominance at f_c~10 Hz for different depth shots, predicted by the model, was observed over a broad distance range (0.2–230 km).

The novel aspects of the experiments include (1) investigating seismic coupling in a source medium (soft sediments) that was not previously studied to a large extent, (2) using a homogeneous source medium for all the tests, (3) using nearly spherical explosive sources as opposed to the long cylindrical charges used in most previous large-scale high explosives tests, and (4) full containment of all explosions.

The experiments, designed to simulate nuclear sources, provide data and knowledge for improvement of nuclear test monitoring for low-yield explosions.

[Articles] Automatic S-Wave Picker for Local Earthquake Tomography

Diehl, T., Deichmann, N., Kissling, E., Husen, S. — 2009-06-07

High-resolution seismic tomography at local and regional scales requires large and consistent sets of arrival-time data. Algorithms combining accurate picking with an automated quality classification can be used for repicking waveforms and compiling large arrival-time data sets suitable for tomographic inversion. S-wave velocities represent a key parameter for petrological interpretation, improved hypocenter determination, as well as for seismic hazard models. In our approach, we combine three commonly used phase detection and picking methods in a robust S-wave picking procedure. Information from the different techniques provides an in situ estimate of timing uncertainty and of the reliability of the automatic phase identification. Automatic picks are compared against manually picked reference picks of selected earthquakes in the Alpine region. The average accuracy of automatic picks and their classification is comparable with the reference picks, although a higher number of picks is downgraded to lower quality classes by the automatic picker. In the production-mode, we apply the picker to a data set of 552 earthquakes in the Alps recorded at epicentral distances ≤150 km. The resulting data set includes about 2500 S phases with an upper error bound of 0.27 sec.

[Articles] Determination of Seismogenic Structures in Southeastern Sicily (Italy) by High-Precision Relative Relocation of Microearthquakes

Brancato, A., Hole, J. A., Gresta, S., Beale, J. N. — 2009-06-07

In December 1990, southeastern Sicily was struck by a moderate earthquake (M_L 5.4). The event and the subsequent aftershocks represent renewed activity for the Malta Escarpment fault system (MEFS) three centuries after the most recent of several large destructive historical earthquakes. An 11 yr period of quiescence was interrupted with a minor swarm in 2002 (M_max 3.6). These sequences and regional seismicity were relocated through 3D tomography. The resulting velocity model indicates a velocity contrast in the vicinity of MEFS. High-precision relative relocations were derived by the cross-spectral method for 18 events from the 1990 sequence and 10 earthquakes from the 2002 swarm. These swarms were aligned along fault segments striking east-northeast–west-southwest, conjugate to the north-northwest–south-southeast orientation of the MEFS. We conclude that the MEFS is fragmented in segments with different seismic behaviors, and that great stress accumulation along the main segment of the MEFS is less likely.

[Articles] Teleseismic Receiver Function and Surface-Wave Study of Velocity Structure beneath the Yanqing-Huailai Basin Northwest of Beijing

Zhou, R.-M., Stump, B. W., Herrmann, R. B., Yang, Z.-X., Chen, Y.-T. — 2009-06-07

Shear-wave velocities beneath the Yanqing-Huailai Basin, 90–140 km northwest of Beijing, are estimated from the joint inversion of surface-wave phase velocities and teleseismic receiver functions. The data set is from a temporary broadband seismic network supported by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) in the basin and includes 34 teleseismic events from 2003 to 2005.

Receiver functions from the teleseismic events are similar for the stations around the Yanqing-Huailai Basin and exhibit little variation with azimuth. The velocity models constrained by receiver functions and surface-wave dispersion curves are also similar. The resulting models reflect the low-velocity basin sediments to 2 km followed by a positive velocity gradient to 15 km with shear-wave velocity increasing from 2.0 to 3.55 km/sec. Evidence of a midcrust low-velocity layer starts at 15 km with a shear velocity decrease to 3.3 km/sec that extends to approximately 25 km. The total crustal thickness is 38–42 km with a smooth Moho transition to an upper-mantle shear velocity of 4.3 km/sec. The low-velocity zone is consistent with recent extension, geothermal activity, and earthquake locations above this depth.

The average shear velocity model for the basin has similarities to other regional and global models but provides more detailed structure in the uppermost and lower portions of the crust. The new model includes the effect of the sediments in the basin, the low-velocity layer, and the gradual Moho transition. Predicted P- and S-travel times are 1–3.5 sec slower than the previous models at regional distances.

[Articles] P- and S-Wave Receiver Function Images of Crustal Imbrication beneath the Cheyenne Belt in Southeast Wyoming

Hansen, S., Dueker, K. — 2009-06-07

We extend the receiver function deconvolution methodology of Bostock (2004) to S-wave receiver functions and develop a method of source function spectrum estimation to constrain the crustal structure across the Archean–Proterozoic Cheyenne belt suture in southeast Wyoming using data from a dense deployment of seismic stations. S-wave receiver functions are particularly useful because free-surface reverberations do not contaminated the direct S_dp arrivals, and the S-wave receiver function image is able to validate our P-wave receiver function image. P- and S-wave receiver function images and a teleseismic P-wave tomogram find a structure consistent with the imbrication of Proterozoic lower crust across the Cheyenne belt. Both P and S-wave receiver function images delineate a double Moho north of the Cheyenne belt: the Archean Moho is imaged at 41–43 km depth with a deeper velocity step at 60–62 km depth. South of the Cheyenne belt, the P-wave receiver function image finds the Proterozoic Moho dipping ~7° northwest consistent with observed back-azimuth dependent P_ms amplitudes. Given the lateral continuity with the northwest dipping Proterozoic Moho, the deeper velocity step of the double Moho is interpreted as the imbricated Proterozoic Moho. Modeling of P-wave receiver function amplitudes suggests a 6.4–7.4 km/sec velocity step across the shallower Archean Moho and a 7.4–7.9 km/sec velocity step across the deeper imbricated Proterozoic Moho. We speculate that imbrication of the Proterozoic lower crust was contemporaneous with the 1.76 Ga uplift and deformation of the 50 km-wide Palmer Canyon block immediately north of the Cheyenne belt exposed in the Laramie Mountains.

[Articles] 3D Scattering Image of Mt. Vesuvius

Tramelli, A., Del Pezzo, E., Fehler, M. C. — 2009-06-07

In this article we apply a passive scattering-imaging method, derived from the method developed by Nishigami (1991) to data from the coda of the local volcano-tectonic (VT) earthquakes of Mt. Vesuvius. This method provides the space distribution of the strong scatterers together with a rough estimate of their strength. In the development of our method we use a realistic raytracing calculated with a ray-bending approach in the 3D velocity model of Mt. Vesuvius structure obtained with travel-time inversion. The inversion procedure adopted for the scattering imaging in the present study is based on the conjugate gradient method (CGM). The volume under study is divided into cubic cells with different dimensions in a multiscale approach. We obtain the best resolution (900 m cubic cell size) in the central part of the volume under study (roughly in a radius of 4 km centered in the crater) within a depth of 5 km. We analyzed the coda signals after filtering in two frequency bands, the first centered at 12 Hz and the second at 18 Hz, where most of the seismic energy is concentrated. Results show that most of the strong scatterers are located in the depth range between the surface and 3000 m below the sea level, in correspondence with the crater axis where most of the seismicity occurs. Part of the scatterers are located in the zones characterized by the maximum velocity contrasts.

[Articles] Deconvolution of Three-Component Teleseismic Data from Southern Tibet Using the SVA Technique

Dasgupta, S., Nowack, R. L., Mitra, S. — 2009-06-07

In this study, the SV autocorrelation (SVA) technique of Dasgupta and Nowack (2006) is tested using three-component P-wave data from selected INDEPTH II and CDSN stations in southern Tibet. The SVA technique involves constructing an estimate of the source and distant Earth wavelet from the autocorrelation of the SV component and using this to deconvolve the data. The deconvolved vertical components are then used to infer the crustal P-velocity structure. Initial models for the inversion of the deconvolved vertical components were based on the S-velocity results obtained from the inversions of radial receiver functions by Mitra et al. (2005). The Moho depths obtained from inverting the vertical components deepen from the south to the north in the study area and are comparable to the depths obtained by Mitra et al. (2005) to the north and similar but somewhat shallower to the south. The crustal V_P/V_S ratios are obtained from the inverted V_S models from Mitra et al. (2005) and the inferred V_P speeds obtained from this study. Except for station BB18 with a somewhat higher V_P/V_S ratio, the other stations to the north in the Tethyan Himalayas and the southern Lhasa terrane have V_P/V_S ratios between 1.75 and 1.80 with a mean of 1.77 (a Poisson’s ratio of 0.265). The range of values could result from noise in the data as well as from lateral heterogeneity in the region with different piercing points at depth for the Ps and PpPp phases.

[Articles] Localized Boundary Integral Equation-Discrete Wavenumber Method for Simulating Wave Propagation in Irregular Multiple Layers, Part I: Theory

Zhou, H., Chen, X. — 2009-06-07

We present a new method of synthesizing seismograms for irregular multilayered problems. It is an extension of the local (lo) boundary integral equation (BIE) discrete wavenumber method (DWM) topography problem. Following similar procedures as those developed in solving the P-SV waves of topography problems, we first provide the formulation of Bouchon and Campillo’s BIE–DWM (Bouchon, 1985; Campillo and Bouchon, 1985) for the multilayered problem. By orthogonally decomposing the forces on irregular and flat parts of each interface and applying a discrete Fourier transform (DFT) we derive their relation. Finally, considering the continuity of displacement and traction on each interface, we get a linear equation only involving the unknown forces on irregular parts of interfaces and discuss its solution. In this algorithm the dimension of the lineal equation is decided by the sampling number on irregular parts of interfaces. Therefore, its computation efficiency increases dramatically, particularly for the problem in which the corrugated part of the layer is highly localized.

[Articles] Localized Boundary Integral Equation-Discrete Wavenumber Method for Simulating Wave Propagation in Irregular Multiple Layers, Part II: Validation and Application

Zhou, H., Chen, X. — 2009-06-07

In the present study, the localized boundary integral equation–discrete wavenumber method (loBIE–DWM) obtained in a companion article (Zhou and Chen, 2009) is validated. For SH waves our results are tested by comparing them with Cao’s (Cao, 2003), while for P-SV waves they are compared with Kawase’s (Kawase and Aki, 1989). The good agreement demonstrated that the method has the same accuracy as other methods. We subsequently used this method to synthesize wave propagation in two examples and discuss the effects of irregular interfaces. Finally, we synthesized the velocity records at the Baijiatuan station due to the Zhangbei earthquake to demonstrate the ability of the loBIE–DWM to handle the problem with the long epicenter distance. In order to help understand the formulation of the loBIE–DWM, the numerical implementation and efficiency analysis for a simple example are given.

[Short Notes] Is There a Basis for Preferring Characteristic Earthquakes over a Gutenberg-Richter Distribution in Probabilistic Earthquake Forecasting?

Parsons, T., Geist, E. L. — 2009-06-07

The idea that faults rupture in repeated, characteristic earthquakes is central to most probabilistic earthquake forecasts. The concept is elegant in its simplicity, and if the same event has repeated itself multiple times in the past, we might anticipate the next. In practice however, assembling a fault-segmented characteristic earthquake rupture model can grow into a complex task laden with unquantified uncertainty. We weigh the evidence that supports characteristic earthquakes against a potentially simpler model made from extrapolation of a Gutenberg–Richter magnitude-frequency law to individual fault zones. We find that the Gutenberg–Richter model satisfies key data constraints used for earthquake forecasting equally well as a characteristic model. Therefore, judicious use of instrumental and historical earthquake catalogs enables large-earthquake-rate calculations with quantifiable uncertainty that should get at least equal weighting in probabilistic forecasting.

[Short Notes] The Seismicity along the Dead Sea Fault during the Last 60,000 Years

2009-06-07

Evidence for unchanging slip rate and a Gutenberg–Richter relation for earthquake distribution along the Dead Sea fault during the past 60,000 yr are presented. The evidence comes from three different segments, approximately 100 km apart, and from three different timescales: prehistoric–paleoseismic, historical, and modern (instrumental) records. The paleoseismic data are based on two different methods. In the southern Arava Valley and the northern Jordan Valley segments, the amount of normal displacement along several faults is used, while in the Dead Sea basin the appearance of brecciated beds, which are considered as seismites, is used. We found that for the southern Arava Valley segment a constant dip-slip rate of 0.5 mm/yr can explain the cumulative normal slip during the past 45,000 yr. This suggests that normal faulting is only ~10% of the total left-lateral strike-slip motion. We also found that for all three segments, the paleoseismic and historical records of strong earthquakes lie on the linear extrapolation of the frequency–magnitude relation of the instrumental record. The calculated b-values for all three segments are between 0.85 and 1, similar to other major strike-slip faults in the world. It is concluded that the Gutenberg–Richter distribution is a stable mode in the tectonic setting of the Dead Sea fault during the past 60,000 yr.

[Short Notes] Arrival-Time Order Location Revisited

Rosenberger, A. — 2009-06-07

A fast method to estimate an earthquake epicentral region from the very first P-wave arrivals is relevant in the context of early warning and rapid response systems. Arrival-time order location (AOL) uses the sequence in which an array of sensors detects an event to constrain the epicentral region based on simple geometrical relations. No velocity model is needed. This article recasts AOL using the theory of spatial tessellations and describes a solution based on higher order Voronoi diagrams. The implementation can take advantage of modern computational geometry algorithms. The required calculations are fast and are suitable for a real-time system.

[Short Notes] Brittle Creep Damage as the Seismic Signature of Dyke Propagations within Basaltic Volcanoes

Traversa, P., Grasso, J.-R. — 2009-06-07

Contemporary to nine dyke intrusions on Piton de la Fournaise, Etna, and Miyakejima volcanoes, we recover stationary seismicity rate and energy release over time, whether the dyke reaches the surface or not. This generic seismicity pattern for the dyke propagation of low viscosity magma argues for the fluid driven crack propagation to be a scale independent stationary process. This prevents any prediction of the time to eruption during the dyke propagation phase using seismicity rate alone. The seismic signature of the volcano deformation triggered by dyke injections corresponds to brittle creep damage in a strain driven setting. Whether mechanical properties of host rock structure or geometrical effects influence this generic stationary response is not resolved by the seismic data. Because a few, if any, aftershocks are resolved contemporary to dyke intrusion, the seismicity is purely driven by the dyke dynamics, that is, a proxy for the dyke volumetric growth.

[Short Notes] Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

Adams, A., Brazier, R., Nyblade, A., Rodgers, A., Al-Amri, A. — 2009-06-07

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

[Errata] Erratum to Dynamic Stresses, Coulomb Failure, and Remote Triggering

Hill, D. P. — 2009-06-07

[Review Articles] Introduction to the Special Issue on Rotational Seismology and Engineering Applications

Lee, W. H. K, Celebi, M., Todorovska, M. I., Igel, H. — 2009-05-01

Rotational seismology is an emerging field for studying all aspects of rotational ground motions induced by earthquakes, explosions, and ambient vibrations. It is of interest to a wide range of geophysical disciplines, including strong-motion seismology, broadband seismology, earthquake engineering, earthquake physics, seismic instrumentation, seismic hazards, seismotectonics, and geodesy, as well as to physicists using Earth-based observatories for detecting gravitational waves generated by astronomical sources (predicted by Einstein in 1916). In this introduction to the BSSA special issue on rotational seismology and engineering applications, we will include (1) some background information, (2) a summary of the recent events that led to this special issue, and (3) an overview of its 51 papers—27 articles, 11 short notes, 4 reviews, 6 tutorials, and 3 supplementary articles. Our comments on these 51 papers are very brief and give just a hint of what the papers are about.

Papers in this special issue demonstrate that earthquake monitoring cannot be limited to measuring only the three components of translational motion. We also need to simultaneously measure the three components of rotational motion and the many components of strains. A golden opportunity to improve our understanding of earthquakes lies in the near field of large earthquakes (within about 25 km of the earthquake ruptures), where nonlinear rock and soil response influences ground motions in a complicated way.

[Review Articles] Review: Progress in Rotational Ground-Motion Observations from Explosions and Local Earthquakes in Taiwan

2009-05-01

Rotational motions generated by large earthquakes in the far field have been successfully measured, and observations agree well with the classical elasticity theory. However, recent rotational measurements in the near field of earthquakes in Japan and in Taiwan indicate that rotational ground motions are 10 to 100 times larger than expected from the classical elasticity theory. The near-field strong-motion records of the 1999 M_w 7.6 Chi-Chi, Taiwan, earthquake suggest that the ground motions along the 100 km rupture are complex. Some rather arbitrary baseline corrections are necessary in order to obtain reasonable displacement values from double integration of the acceleration data. Because rotational motions can contaminate acceleration observations due to the induced perturbation of the Earth’s gravitational field, we started a modest program to observe rotational ground motions in Taiwan.

Three papers have reported the rotational observations in Taiwan: (1) at the HGSD station (Liu et al., 2009), (2) at the N3 site from two TAiwan Integrated GEodynamics Research (TAIGER) explosions (Lin et al., 2009), and (3) at the Taiwan campus of the National Chung-Cheng University (NCCU) (Wu et al., 2009). In addition, Langston et al. (2009) reported the results of analyzing the TAIGER explosion data. As noted by several authors before, we found a linear relationship between peak rotational rate (PRR in mrad/sec) and peak ground acceleration (PGA in m/sec²) from local earthquakes in Taiwan, PRR=0.002+1.301 PGA, with a correlation coefficient of 0.988.

[Review Articles] Review: Rotations in Structural Response

Trifunac, M. D. — 2009-05-01

Measurements of point rotations in full-scale structures during earthquake excitation do not exist at present, but average rotations can be computed from pairs of parallel transducers. The examples presented illustrate rotations in the range from 10^-6 to 10^-3 rad and angular accelerations from 10^-4 to 10^-1 rad/sec². Measurements of rotations in the structures for excitation by microtremors are also described. It is argued that recording the rotational components of motion contributes significantly to the overall volume and quality of information. It is recommended that the development and deployment of instruments to measure rotational components of motion in free-field conditions and in full-scale structures will open a new frontier for advanced experimental identification of structural properties and for structural health monitoring.

[Review Articles] Review: Requirements for a Ground Rotation Sensor to Improve Advanced LIGO

Lantz, B., Schofield, R., O'Reilly, B., Clark, D. E., DeBra, D. — 2009-05-01

The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) project is to measure gravitational waves from astrophysical sources. The project has recently begun the construction of Advanced LIGO, a major upgrade to the existing detectors. The performance of the new detectors requires the optical component to be mounted on high performance seismic isolation and alignment systems. The performance and reliability of these isolation systems could be improved with accurate measurements of the tip–tilt rotations of the ground at the level of at 10 mHz to at 200 mHz. We briefly describe the LIGO project and explain the desire for rotation sensing of the ground. We then derive the performance requirements for a rotation sensor and show that it would improve the Advanced LIGO detector.

[Review Articles] Review: Accelerometer Development for Use in Gravitational Wave-Detection Interferometers

DeSalvo, R. — 2009-05-01

Accelerometers were and are being developed to instrument the seismic attenuation chains of gravitational wave (GW) interferometric detectors. The main requirements of these instruments are strong directionality (>10³ rejection of signal from orthogonal directions for feedback) and vacuum compatibility as well as high low-frequency sensitivity. Because of the directionality requirements, GW accelerometers tend to be more specialized for sensing of different degrees of freedom (horizontal, vertical, and tilt) than the classical geophysics seismometers but share several characteristics with these. The initial design of GW-dedicated accelerometers was inspired by geophysics instruments. Techniques developed for the GW field are now starting to spill over back into the geophysics field.

[Review Articles] Tutorial on Earthquake Rotational Effects: Historical Examples

Kozak, J. T. — 2009-05-01

Earthquake rotational effects have been observed for centuries. The first class of rotational seismic models includes two models defined by Mallet (1862) in the mid-nineteenth century based on the rotation of bodies to their underlying structures. These rotational effects satisfactorily explain observed surface rotations. In this short tutorial (based mostly on western literature), we will briefly discuss the historical aspects of earthquake rotational effects.

[Review Articles] Tutorial on Rotations in the Theories of Finite Deformation and Micropolar (Cosserat) Elasticity

Pujol, J. — 2009-05-01

Although earthquake source studies have had a great impact on tectonics studies, there are at least two important problems for which seismology seems unable to provide answers. One of them refers to the rotation about vertical axes of crustal blocks in continental areas of diffuse deformation. The other problem is the stress rotations observed after large earthquakes. In both cases there are a number of competing explanations but none is supported by hard evidence. These problems are unlikely to be solved by conventional seismology, but the situation may be different if rotation data are acquired. In the near field of large earthquakes the linearized theory may not apply or a different theory may be needed. In this tutorial we consider rotations from two different points of view: the classical nonlinear theory and a nonclassical linear theory. In the nonlinear theory the deformation tensor can be expressed as the product of two tensors, one corresponding to a rotation and the other to strain, applied sequentially. In contrast, in the linearized theory the deformation tensor is the sum of a rotation and a strain tensor and the order of their application is immaterial. A linear theory that includes rotations not considered by classical elasticity (linear or not) is the micropolar theory, which deals with materials with microstructure. This theory assigns to each point in space six degrees of freedom, three corresponding to position and three corresponding to rotations. The specification of a linear micropolar isotropic body requires six elastic moduli, two of which are the classical Lame’s parameters. Wave propagation in a micropolar medium is more complicated than in a linear elastic medium, with two coupled wave equations. The micropolar theory has been successful with media having periodic inner structures, but there is very little experimental work on solids with more complicated structure.

[Review Articles] Tutorial on New Developments in the Physics of Rotational Motions

Teisseyre, R. — 2009-05-01

We present a linear continuum theory incorporating asymmetric stress fields as well as symmetric strains and antisymmetric rotations. We discuss the related constitutive laws and balance equations. In this theory, the motion equation related to the balance of the antisymmetric part of stresses replaces that for the stress moments. Our theory proves that the rotation waves may exist even in a homogeneous elastic continuum.

Different kinds of extreme deformations are considered. The wave solutions, including the coaction of the rotation and twist fields, are presented and discussed. The dislocation density–stress relations are derived with the help of the symmetric and antisymmetric parts of stresses. The synchronization solution, rotation, and twist, shifted in phase by /2, are presented for a material in an advanced deformation state with granulation and microcracking. Some examples of the spin and twist motion records are reported that confirm this synchronization hypothesis.

[Review Articles] Tutorial on Surface Rotations from Wave Passage Effects: Stochastic Spectral Approach

Zembaty, Z. — 2009-05-01

This article presents a concise review of the methods to obtain spectral densities of the rotational components of seismic ground motion from the spectral densities of both the translational components and wave propagation parameters. The rotational components are obtained by decomposing ground motion at the site into body and surface wave contributions with random amplitudes. To obtain rotation the resulting stochastic fields of body and surface waves are differentiated with respect to spatial coordinates. Assumption of plane waves radiating from a point source leads to two rotational components: rocking around a horizontal axis perpendicular to the source-site direction and torsion around a vertical axis. Construction of the rocking acceleration spectral density from P-, SV-, and Rayleigh-wave contributions as well as torsional spectral density from SH and Love waves (in terms of translational spectral densities and wave parameters) are discussed in detail. A short numerical analysis illustrates the proposed approach. A shift of the rotational spectra into higher frequencies compared to respective translational spectra is observed.

[Review Articles] Tutorial on Gravitational Pendulum Theory Applied to Seismic Sensing of Translation and Rotation

Peters, R. D. — 2009-05-01

Following a treatment of the simple pendulum provided in Appendix A, a rigorous derivation is given first for the response of an idealized rigid compound pendulum to external accelerations distributed through a broad range of frequencies. It is afterward shown that the same pendulum can be an effective sensor of rotation, if the axis is positioned close to the center of mass.

[Review Articles] Review Article: Tutorial on Measuring Rotations Using Multipendulum Systems

Graizer, V. — 2009-05-01

This article considers a classical approach of using a combination of pendulums to measure rotations. The idea of using two identical pendulums installed on different sides of the same axis of rotation for separate measurement of rotational and translational motion was apparently first suggested by Golitzin (1912). It was implemented by Kharin and Simonov (1969) in an instrument designed to record strong ground motion (VBPP—a seismograph of large translational motions and rotations). Unfortunately, difficulty in building identical mechanical systems resulted in unreliable measurements of the rotational component. We modified Golitzin’s idea by using the same configuration of pendulums (a two-pendulum system) without the requirement that the pendulums be identical (Graizer et al., 1989). Instead of building two identical pendulums, one needs to calibrate the instrument to obtain the natural parameters of each pendulum and apply postprocessing to separate the rotational and translational motions. The two-pendulum system for separate measurements of large amplitude rotations was implemented at the end of the 1980s at the Institute of the Physics of the Earth in Moscow, Russia, using commercially available pendulum instruments. The system was tested using a basic shake table and later successfully applied to measurements in the near field of two large underground nuclear explosions. In this article I updated and generalized the approach to measuring translational and large amplitude rotational motion formulated in previous publications (Graizer, 1989; Graizer et al., 1989). Numerical testing demonstrated that using a combination of pendulums for measuring rotations may be limited for recording relatively large amplitudes of rotations of the order of 10^-4 and higher for the two-pendulum system of about 100 cm size.

[Review Articles] Suggested Notation Conventions for Rotational Seismology

Evans, J. R., International Working Group on Rotational Seismology — 2009-05-01

We note substantial inconsistency among authors discussing rotational motions observed with inertial seismic sensors (and much more so in the broader topic of rotational phenomena). Working from physics and other precedents, we propose standard terminology and a preferred reference frame for inertial sensors (Fig. 1) that may be consistently used in discussions of both finite and infinitesimal observed rotational and translational motions in seismology and earthquake engineering. The scope of this article is limited to observations because there are significant differences in the analysis of finite and infinitesimal rotations, though such discussions should remain compatible with those presented here where possible. We recommend the general use of the notation conventions presented in this tutorial, and we recommend that any deviations or alternatives be explicitly defined.

[Review Articles] Suggested Readings in Continuum Mechanics and Earthquake Seismology

Grekova, E. F., Lee, W. H. K. — 2009-05-01

Rotational seismology is a newly emerging field of interest to scientists from widely differing disciplines. We compile here some suggested readings for those wishing to become familiar with disciplines relevant to rotational seismology and its engineering applications. These readings are not exhaustive but contain a number of basic references with occasional annotations. We concentrate on two areas: continuum mechanics and earthquake seismology. Continuum mechanics is the branch of mechanics that deals with the deformation and motion of materials, including various elasticity theories. Although classical elasticity works well for studying earthquakes in the far field, there are alternate elasticity theories that may be more appropriate for studying earthquakes in the near field. The readings in continuum mechanics are intended for seismologists and include (1) fundamental textbooks, (2) intermediate to advanced books, (3) heteromodular theories, and (4) Cosserat theory. A brief introduction to continuum mechanics for seismologists is included as an appendix. In addition, because we wish to attract workers in other disciplines to rotational seismology, we include a short list of suggested readings in earthquake seismology.

[Review Article] A Glossary for Rotational Seismology

Lee, W. H. K. — 2009-05-01

Some common terms for rotational seismology are compiled from contributions by many authors, including R. DeSalvo, J. R. Evans, E. F. Grekova, C. R. Hutt, H. Igel, C. A. Langston, B. Lantz, E. Majewski, R. Nigbor, J. Pujol, P. Spudich, R. Teisseyre, M. D. Trifunac, R. J. Twiss, and Z. Zembaty. In addition, some glossary terms about earthquakes are excerpted from Aki and Lee (2003), Lee and Wu (2009), and others for the benefit of readers who are not seismologists. It is a first attempt to compile a glossary for rotational seismology (including some terms for continuum mechanics), and it is by no means complete. Several iterations will be required to reach a satisfactory glossary, and this compilation is just a start.

[Articles] Single-Couple Component of Far-Field Radiation from Dynamical Fractures

Knopoff, L., Chen, Y.-T. — 2009-05-01

We reexamine two canons of the seismological literature, that elastic displacements in the far field are proportional to slip velocities on the dynamical fault surface, and that dynamical in-plane slip on an earthquake fault has a double-couple body force equivalent. We show that if faulting takes place on a fault of finite thickness, and there is a strength-weakening zone near the advancing crack tip, there is an additional single-couple term in the body force equivalence and additional terms in the far-field displacement, which are proportional to the time rate of increase of stress drop in the advancing weakening zone. We also show that the single-couple equivalent does not violate principles of Newtonian mechanics because the torque imbalance in the single couple is counterbalanced by rotations within the fault zone; the crack therefore radiates torque waves and a rotational deformation field.

[Articles] An Asymmetric Micropolar Moment Tensor Derived from a Discrete-Block Model for a Rotating Granular Substructure

Twiss, R. J. — 2009-05-01

I define an asymmetric moment tensor for individual slip events and for an average over multiple slip events using a discrete rigid-block model to account for the brittle deformation of a granular material. For the brittle crust, the grains are taken to be fault-bounded blocks. Permanent deformation accumulates by slip events on the boundaries of the blocks. The deformation is described by two independent motions: the local relative motion of the block centroids and the local rigid rotation of the blocks about their centroids. Averaging each of these local motions over multiple slip events in a volume defines both the macrodeformation, which consists of the macrostrain and macrorotation, and the microrotation. An asymmetric local micropolar moment tensor and an asymmetric micropolar moment-density tensor are defined from the local and the averaged motions, respectively.

The model shows (1) the symmetric part of the micropolar moment tensors depends on the constant-volume local shear strain of the block centroids or its averaged equivalent, the macrostrain; (2) the antisymmetric part depends on an objective quantity defined as the difference between the rotational component associated with the centroid deformation and the local block rotation, or their averaged equivalents the macrorotation and the microrotation; and (3) the symmetric and antisymmetric parts of the micropolar moment-density tensor can be inferred up to a scalar magnitude by a micropolar inversion of standard seismic focal mechanisms.

Three field tests show consistency with the theory, but definitive tests are thwarted by insufficient quantitative information or insufficient resolution of the available data.

[Articles] Fundamental Deformations in Asymmetric Continuum

Teisseyre, R., Gorski, M. — 2009-05-01

The complexity of processes in a seismic source zone has inspired us to reconsider a class of basic motions and deformations in an asymmetric continuum, including simple motions (translation and rotation, named spin) and simple deformations. Simple deformations include the axial nuclei, that is, point extension/compression, and the shear nuclei related to string-string point deformations. The point deformation is further redefined as another kind of rotational motion, called twist, representing oscillations of the main shear axes and their amplitudes. Some remarks are added on the recording systems used to measure these motions and deformations.

[Articles] Spinors and Twistors in the Description of Rotational Seismic Waves and Spin and Twist Solitons

Majewski, E. — 2009-05-01

A noncommutative (anti-) self-dual Yang–Mills theory as a source of multisoliton solutions of nonlinear wave equations was applied to the description of rotational seismic waves that are excited in the earthquake source. Spinors and twistors are used to describe spin and twist solitons branching off dispersion curves for rotational seismic waves. Complex physical structures are adopted to describe spin and twist effects resulting from the presence of translational and rotational defects in elastic rocks. A seismic space is also assumed to have a complex structure. An earthquake source zone is modeled by a set of equations for interacting fields that is mathematically similar to the noncommutative (anti-) self-dual Yang–Mills equations. Some similarities between dislocations and strings are emphasized, for example, those that exist between surface defects and D-branes in string theories. Dislocations and disclinations are treated as sources for seismic spin and twist fields. By symmetry reduction various soliton equations for seismic spin and twist solitons can be obtained from the set of earthquake source zone equations, which is similar to the noncommutative (anti-) self-dual Yang–Mills equations by symmetry reduction.

[Articles] Numerical Simulation of Ground Rotations along 2D Topographical Profiles under the Incidence of Elastic Plane Waves

2009-05-01

The surface displacement field along a topographical profile of an elastic half-space subjected to the incidence of elastic waves can be computed using different numerical methods. The method of fundamental solutions (MFS) is one of such techniques in which the diffracted field is constructed by means of a representation in terms of the Green’s functions for discrete forces located outside the domain of interest. From the enforcement of boundary conditions, such forces can be computed; thus, the ground motion can be calculated. One important advantage of MFS over boundary integral techniques is that singularities are avoided. The computation of ground-motion rotations implies the application of the rotational operator to the displacement field. This can be done using either numerical derivatives or analytical expressions to compute the rotational Green’s tensor. We validate the method using exact analytical solutions in terms of both displacement and rotation, which are known for simple geometries. To demonstrate the accuracy for generic geometries, we compare results against those obtained using the spectral-element method. We compute surface rotations for incoming plane waves (P, SV, and Rayleigh) near a topographical profile. We point out the effects of topography on rotational ground motion in both frequency and time domains.

[Articles] Source and Basin Effects on Rotational Ground Motions: Comparison with Translations

Wang, H., Igel, H., Gallovic, F., Cochard, A. — 2009-05-01

The recent availability of accurate observations of rotational ground motions has reopened interest in understanding near-fault characteristics of such motions in the context of strong ground motion and earthquake engineering. In order to investigate source and structure-dependent variations of rotational motions, we simulate several M 7 earthquakes with varying source scenarios on the Newport–Inglewood (NI) fault embedded in the 3D Los Angeles basin using a finite-difference method in the frequency range up to 0.5 Hz. We use a precalculated database with several hundred numerical Green’s functions for a discretized model of the NI fault that allows arbitrary finite-fault scenarios to be synthesized by superposition. We investigate source and basin effects on the rotational part of ground motion (namely maximum peak ground rotation rates and their variations) and compare them with the corresponding values of translational motion. Our main conclusions are: (1) the pure strike-slip source mechanism leads to larger rotation rates around the vertical axis than around the horizontal ones; (2) variation of hypocenter introduces more scatter on ground rotation rate than variations of slip history; (3) the coprocessing of translation and rotation recordings might reveal information on local velocity structure as indicated by plane-wave theory; and (4) the attenuation of accelerations (horizontal components) and rotation rate (vertical component) with distance from the fault are very similar, suggesting that similar expressions (as a function of distance) as for the peak accelerations can be adopted for the peak rotation rates when determining their attenuation relations.

[Articles] Research and Development Status of a New Rotational Seismometer Based on the Flux Pinning Effect of a Superconductor

Takamori, A., Araya, A., Otake, Y., Ishidoshiro, K., Ando, M. — 2009-05-01

Rotational motions induced by earthquakes are expected to contain unique geophysical information not obtainable from translational motions, such as the distribution of slip velocity near the earthquake source. However their observations have not been performed as actively as those of translational motions, mainly because of technical difficulties. Rotational seismometers with sufficient resolution (better than 10^-10 rad/sec/Hz^1/2 over 0.1 mHz to 10 Hz) and reliability at reasonable cost are needed to establish broad observation networks for rotational seismology. To achieve this goal, we are developing a seismometer with a new design based on a proof mass levitated by a magnetic suspension that uses the flux pinning effect of a superconductor to make it freely rotatable without introducing unwanted parasitic resonances and other complexities. Prototype systems were built and tested to assess the feasibility of the technologies used in the new seismometer, and their advantages and capabilities have been successfully demonstrated. The design of the new seismometer together with the status of the development and future plans are presented in this article.

[Articles] Performance Characteristics of a Rotational Seismometer for Near-Field and Engineering Applications

Cowsik, R., Madziwa-Nussinov, T., Wagoner, K., Wiens, D., Wysession, M. — 2009-05-01

Based on the concept that torsion balances act as good filters for rotational oscillations at frequencies above their natural frequency of oscillations, we have fabricated a simple prototype to serve as a rotational seismometer for near-field and engineering studies. This instrument displays a nearly flat response at frequencies above 10 mHz, and the preliminary data taken with this instrument show a sensitivity of 5x10^-6(/0.01 Hz) rad sec^-1Hz^-1/2. Marginal improvements of the fabrication and housing will be needed to fully cover the sensitivities recommended by a U.S. Geological Survey (USGS) panel for studies at these frequencies.

[Articles] Ring Laser Measurements of Ground Rotations for Seismology

2009-05-01

Since the discovery of the wave nature of light, optical interferometry has assumed an important place in high precision metrology. This is mostly due to the inherent high sensor resolution for operational wavelengths in the vicinity of several hundred nanometers. In this context, interferometers in the Michelson configuration are most prominently used in gravitational wave antennas, such as the large projects VIRGO, LIGO, TAMA, and GEO600. In the Sagnac configuration they are used for high resolution rotation monitoring such as the precise observation of Earth rotation. Modern large-scale ring lasers reach a sensitivity for the measurement of rotation of 1 prad/sec (with approximately 1 hr of averaging). Because of the comparatively short wavelengths employed, optical interferometers are extremely sensitive to small mechanical perturbations of the entire apparatus. These can be caused by deformations, thermal or mechanical stress, and instabilities in the alignment of the optical components at the level of about /100. Ring lasers suitable for geophysical applications require a sensor resolution in the range of 10^-8 rad/sec and below. This demands a scale factor of the instrument that is only achievable with mechanical dimensions of the interferometer on the order of about 1 m² or larger. At the same time the necessary mechanical rigidity of the entire instrument has to be on the order of 5 nm. Currently, this has only been achieved with monolithic ring lasers made from blocks of Zerodur and installed in a temperature stabilized underground environment. However if long-term sensor stability is not required, compromises can be made and, in particular for studies of regional seismic events, it becomes feasible to build a heterolithic rotation sensor in a simpler and much cheaper way. Here, we report the design and first results from the GEOsensor, which has been specifically constructed for studies in rotational seismology. The sensor is operated at the Piñon Flat Seismological Observatory in Southern California.

[Articles] Perspectives for Ring Laser Gyroscopes in Low-Frequency Seismology

Widmer-Schnidrig, R., Zurn, W. — 2009-05-01

Variations in the ring laser signal of the rotating Earth are caused by local rotations and by north–south tilts. Analytic expressions for the computation of synthetic seismograms for ring lasers based on normal mode summation are derived. We find that horizontal ring lasers are predominantly sensitive to torsional motion. Synthetic Sagnac signals are computed for recent earthquakes and for spherical Earth models in order to establish the general characteristics and amplitude of the expected ring laser signals. In the normal mode band north–south tilts typically amount to less than 10% of the total signal for a horizontal ring laser. The fact that no free oscillation spectra have so far been reported from ring lasers is consistent with amplitudes of our synthetic Sagnac signals and current noise levels of these sensors.

[Articles] The Application of Fiber Optic Gyroscopes for the Measurement of Rotations in Structural Engineering

Schreiber, K. U., Velikoseltsev, A., Carr, A. J., Franco-Anaya, R. — 2009-05-01

The effects of rotations have been neglected in studies on the seismic properties of civil engineering structures in the past. This was mainly because their influence was thought to be small and there were no suitable sensors available to measure the system response of buildings to rotations properly. Only the effects of torsions caused by asymmetries in buildings, where the center of stiffness differs from the center of mass, are known from differential measurements of accelerometers. Different types of inertial rotation sensors exploiting the Sagnac effect have now reached the necessary sensitivity to be used for the investigation of rotational excitations in buildings. Because large ring lasers (Schreiber et al., 2003, 2004) have successfully recorded signals of earthquake induced rotations from teleseismic events (Igel et al., 2005), it is now time to study the behavior of buildings with respect to rotations. Fiber optic gyroscopes (FOGs) are commonly used for applications in inertial navigation. They are exploiting the Sagnac effect in a passive optical interferometer design in order to measure rotations with high precision. For that reason, these gyros can measure absolute rotations and do not require a specific frame of reference. Because the concept of operation is entirely based on optical signals, there are no mechanical moving parts inside the sensor, so the transfer function is constant and the system works over a very wide range of excitation frequencies (10^-3 Hz<f_FOG<2 kHz). Furthermore, one can obtain a well-defined reference to north from an FOG, which provides the additional advantage of using these sensors for the long term monitoring of structural stability. In this article we report initial measurements with an FOG on a shake table as well as results from in situ applications in very tall structures.

[Articles] Laboratory and Field Testing of Commercial Rotational Seismometers

Nigbor, R. L., Evans, J. R., Hutt, C. R. — 2009-05-01

There are a small number of commercially available sensors to measure rotational motion in the frequency and amplitude ranges appropriate for earthquake motions on the ground and in structures. However, the performance of these rotational seismometers has not been rigorously and independently tested and characterized for earthquake monitoring purposes as is done for translational strong- and weak-motion seismometers. Quantities such as sensitivity, frequency response, resolution, and linearity are needed for the understanding of recorded rotational data. To address this need, we, with assistance from colleagues in the United States and Taiwan, have been developing performance test methodologies and equipment for rotational seismometers. In this article the performance testing methodologies are applied to samples of a commonly used commercial rotational seismometer, the eentec model R-1. Several examples were obtained for various test sequences in 2006, 2007, and 2008. Performance testing of these sensors consisted of measuring: (1) sensitivity and frequency response; (2) clip level; (3) self noise and resolution; and (4) cross-axis sensitivity, both rotational and translational. These sensor-specific results will assist in understanding the performance envelope of the R-1 rotational seismometer, and the test methodologies can be applied to other rotational seismometers.

[Articles] Observing Rotational and Translational Ground Motions at the HGSD Station in Taiwan from 2007 to 2008

Liu, C.-C., Huang, B.-S., Lee, W. H. K., Lin, C.-J. — 2009-05-01

Because of a lack of suitable instruments, rotational ground motions have not been observed until the last decade. Rotational measurements in the near field of earthquakes in Japan (Takeo, 1998) indicate that rotational ground motions are many times larger than expected from the classical elasticity theory. After failing to obtain useful rotational ground motions (using similar rotational sensors as Takeo did), we deployed a far more sensitive rotational velocity sensor (R-1) at the HGSD station in eastern Taiwan. From 7 December 2004 to 12 November 2006, several hundreds of earthquakes were recorded during our Phase 1 operation. This was mostly a learning exercise to solve field operation problems; Phase 1 operations ended when our two R-1 sensors ceased to operate. A K2+R1 instrument was deployed in the spring of 2007 to start our Phase 2 operation. From 8 May 2007 to 17 February 2008, we observed 52 local earthquakes with good rotational velocity signals (with signal-to-noise ratio >~5), together with excellent translational acceleration signals (with signal-to-noise ratio >~10). Unfortunately, field operation was interrupted due to flooding of the HGSD station site in mid-February 2008; we just resumed normal operation in June 2008.

This article reports our observations of rotational and translational ground motions made at the HGSD station so far. We concentrate on describing our instrumentation and the data obtained from 52 local earthquakes during our Phase 2 operation and present some very preliminary results.

[Articles] Recording Rotational and Translational Ground Motions of Two TAIGER Explosions in Northeastern Taiwan on 4 March 2008

Lin, C.-J., Liu, C.-C., Lee, W. H. K. — 2009-05-01

Two explosions were set off on 4 March 2008 at the N3 explosion site in northeastern Taiwan. The code name for the first shot with 3000 kg explosives is N3P and that for the second shot with 750 kg explosives is N3. To record these two explosions, 8 triaxial rotational sensors, 13 triaxial accelerometers, and 12 six-channel, 24 bit dataloggers with Global Positioning System receivers were deployed to continuously record several hours before and after the explosions. These instruments were installed at about 250 m (1 station), 500 m (11 stations), and 600 m (1 station) from the explosions. The 11 stations form a center array with station spacing of about 5 m.

Except for one rotational sensor, onscale records were obtained. Although the N3P shot used four times larger amounts of explosives than those used for the N3 shot, the peak ground translational acceleration and rotational velocity at the 13 station sites from the N3P shot are only about 1.5 times larger than those for the N3 shot. We also observed large variations (by tens of percent) of translational accelerations and rotational velocities at the center array with station spacing of about 5 m. The largest peak rotational velocity was observed for the x component: 2.74 and 1.75 mrad/sec at a distance of 254 m from the N3P and N3 shots, respectively.

The main purpose of this article is to document our recordings of rotational and translation motions from two explosions in Taiwan and to release the data online for open access. The translational acceleration data from this experiment have been analyzed by Langston et al. (2009), and we plan to submit an article with analysis of the rotational velocity data in the future.

[Articles] Tilt Motions Recorded at Two WISE Sites for the 2003 Tokachi-Oki Earthquake (M 8.3)

Kinoshita, S., Ishikawa, H., Satoh, T. — 2009-05-01

Broadband velocity seismographs installed at the Ohtsu water level observatory and Monsizu stations of the Warning Information System of Earthquake (WISE) network recorded long-period pulses produced by tilt motions resulting from the 2003 Tokachi-Oki earthquake (M 8.3) of 25 September 2003. The long-period pulse signal is represented using a low-pass-filtered expansion composed of intrinsic mode functions. By eliminating the leading phases, such as a direct P phase that appears before the onset of tilt motion from the expansion, the transient signal produced by the tilt motion is approximately obtained. A reverse filter, which is defined by the inverse of the frequency characteristics of the broadband velocity seismometer for the acceleration input, is applied to the transient signal to estimate the tilt motion. The estimated permanent acceleration of tilt motion at the two sites ranges from 0.6 to 6.5 cm/sec². The river embankment adjacent to the Ohtsu water level observatory site collapsed due to liquefaction at the bottom of the embankment. The Monsizu site was constructed at a depth of 2 m in soft soil floodplain deposits. These two broadband velocity seismograms involving the tilt motion reveal that the tilt motion is mainly generated under the influence of the collapsed soil structure or of the deformation of the soil deposits. These observations indicate that the broadband velocity seismograph is more sensitive to tilt motion than acceleration-type strong-motion seismographs.

[Articles] The Effect of Torsional Ground Motion on Structural Response: Code Recommendation for Accidental Eccentricity

Ghayamghamian, M. R., Nouri, G. R., Igel, H., Tobita, T. — 2009-05-01

In this article, data were collected from the Chiba dense array, which consists of 44 accelerometers with interstation spacing in the range of 5–300 m that are employed to estimate the torsional ground motion. The geodetic method was used to estimate torsional motions from the translational records in the Chiba dense array. The translational and computed torsional motions were then applied to the building models with different structural characteristics to evaluate the effectiveness of the accidental eccentricity levels proposed in various design codes. The results of analysis suggest that the 5% accidental eccentricity is on the safe side for most periods of interest in engineering practice. However, in the case of stiff structures (with periods shorter than 0.3 sec), an increase of up to four times in building displacement is observed by including the torsional excitation. Furthermore, we found that the accidental eccentricity coefficient increased up to 0.6 at periods shorter than 0.3 sec, which is 12 times larger than what is proposed by the codes.

[Articles] Study of Rotational Ground Motion in the Near-Field Region

2009-05-01

During the nineteenth and twentieth centuries observational seismologists recorded primarily the earthquake-induced translational wave field, while the rotational motion still remains poorly observed and investigated. We aim to further understand the rotational ground motion and its relation to the translational wave field, with a special emphasis on the near field, a few wavelengths away from the hypocenter, where damage related to rotational motion might need to be considered. A broad picture of the available values of rotational amplitudes and their variability is obtained by gathering most of the published data on strong rotational motion. To obtain a more detailed picture we perform a large scale 3D numerical study of a strike-slip event in the Grenoble valley where a combination of topographic, source, and site effects produces a realistic wave field. We analyzed the synthetic dataset in terms of the rotational and translational peak amplitudes and their dependence on two effects: nonlinear soil behavior and source directivity. On a soft soil deposit, we observe peak ground rotation of 1 mrad and the peak ground rotation rate of 10 mrad/sec, for an M_w 6.0 event. Those values show a strong dependence on the hypocenter location, the local site conditions, and the topographical features, inducing a variability of almost one order of magnitude in a range of distances of 20 km. Finally, we compare our numerical results in terms of peak ground velocity (PGV) versus peak ground rotation (PG) with field data obtained at similar scenarios (e.g., Parkfield) by array techniques to investigate the relation between translational and rotational amplitudes expected in the near field for shallow, medium-sized earthquakes. Results of our numerical simulation fit reasonably well with those observed in past studies. Furthermore, the spatial variations of the PGV/PG ratio show a trend, which is correlated with the velocity structure of the model under study.

[Articles] Seismic-Wave Strain, Rotation, and Gradiometry for the 4 March 2008 TAIGER Explosions

Langston, C. A., Lee, W. H. K., Lin, C. J., Liu, C. C. — 2009-05-01

Acceleration spatial gradients, horizontal strains, and horizontal rotation were computed using strong-motion array data from the 4 March 2008 TAIGER explosions in northeastern Taiwan and used in conjunction with the original three component acceleration data to perform a gradiometric analysis of the strong ground motion wave train. The analysis yields a complex, frequency-dependent view of the nature of seismic-wave propagation over short propagation distances that imply significant lateral velocity changes in structure. Areal strain and rotation about the vertical axis have equal amplitudes and suggest significant wave scattering within the confines of the river valley where the experiment was performed and/or significant departure from an axisymmetric explosion source. Gradiometry shows that the P wave arrives at the array 35° off-azimuth clockwise from the straight-line path and appears to have been refracted from the northern side of the valley. Large, slowly propagating secondary surface waves initially arrive 45° counterclockwise from the straight-line path but later arrivals are seen to propagate in all directions, including back toward the explosion source. A frequency-dependent radiation pattern for the triple-borehole explosion in comparison to the single-borehole explosion explains the differences in the maximum amplitudes between the sources seen in the acceleration data. The use of seismic strain and rotation with standard particle motion wave fields at a single location allows for a direct view of seismic-wave propagation that illuminates the true nature of the seismogram.

[Articles] Sensitivity Densities for Rotational Ground-Motion Measurements

Fichtner, A., Igel, H. — 2009-05-01

We derive and analyze sensitivity densities for two quantities derived from rotational ground-motion measurements: the rms (root-mean-square) amplitude A of the rotation seismogram and the apparent shear-wave speed , where A_v denotes the rms amplitude of the velocity seismogram. In the case of a plane S wave in a homogeneous and isotropic medium, β_a coincides with the true shear-wave speed β. Based on analytical and numerical examples, we demonstrate that the β_a kernels attain large absolute values only in the vicinity of the receiver but not in the vicinity of the source. This effect is pronounced in the case of both body S waves and surface waves (Love + Rayleigh). Moreover, the β_a kernels are dominated by the higher Fresnel zones while reaching only small absolute values in the first Fresnel zone. This implies (1) that measurements of β_a are to the first order independent of the Earth structure near the source, (2) that such measurements may be used for one-station local shear-wave speed tomography, and (3) that comparatively low-frequency signals can be used in order to invert for small-scale structures. The sensitivity densities corresponding to the rotation amplitude measurement A resemble those for the velocity amplitude measurements A_v. It is, therefore, the combination of A with A_v, and not one of them alone, that is likely to provide additional constraints on the Earth’s structure near the receiver.

[Articles] Observations and Modeling of Rotational Signals in the P Coda: Constraints on Crustal Scattering

2009-05-01

In addition to three classical components (vertical, north–south, and east–west) of ground translations recorded by a broadband seismometer, a component of earthquake induced rotational ground motions around the vertical axis is consistently measured by a ring laser sensor located in Wettzell, southeast Germany. Significant rotations around the vertical axis in the P coda of teleseismic signals are either directly visible or can be inferred through the investigation of cross correlation between the transverse component (the component that is perpendicular to the great circle connecting the earthquake and the seismometer) of translation acceleration and ring laser rotation rate. Theoretically, in spherically symmetric isotropic media, we should not observe rotational signals around the vertical axis before the onset of SH waves. Possible causes for the observed rotations in the P coda are: (1) tilt–ring laser coupling, (2) anisotropy, (3) topographic scattering, and (4) P-SH scattering in the crust. Here we show that P-SH scattering in the 3D random crust can explain the observations and allow us to constrain crustal scattering properties.

[Articles] Interpretation of Broadband Ocean-Bottom Seismometer Horizontal Data Seismic Background Noise

Pillet, R., Deschamps, A., Legrand, D., Virieux, J., Bethoux, N., Yates, B. — 2009-05-01

The seismic background noise observed on the OBS (ocean-bottom seismometer) recordings is well known to be strong. With the recent appearance of broadband OBS, the background noise in the extended frequency range is even more significant. By taking into account the various degrees of freedom of the seismic sensors, it has been shown that the seismometers are not only translational sensors but also rotational ones with a sensitivity that depends on the installation. The strong similarity of the background noise recorded on the two horizontal traces of the broadband Hippocampe OBS data deployed during the Rosmarin program is interpreted as a rotational motion around the vertical axis. This torsion motion is so important that it dominates the other noise seismic contributions like translations and tilt (rotational motion around a horizontal axis) and represents more than 90% of the background noise signal at the periods between 5 and 50 sec.

Use of the difference between the two horizontal traces makes it possible to reduce the background noise made up primarily of this torsion motion and thus to enhance the displacement signal but also strongly modify it. An effort should be made to better control the installation of the sensors on the seafloor. A better solution would be to simultaneously record rotations at the same position as that of the seismometer. Such data will permit the correction of the seismic traces of the OBS data and strongly reduce the background noise.

[Articles] About the Nonunique Sensitivity of Pendulum Seismometers to Translational, Angular, and Centripetal Acceleration

Forbriger, T. — 2009-05-01

The displacement of each part of a seismometer’s frame is identical for a purely translational motion. However, in the presence of rotary motion the different parts of a seismometer’s frame will undergo different displacements. The definition of the sensitivity of the seismometer then requires the selection of a reference location on the seismometer’s frame to which the sensitivity is attributed. This location does not necessarily coincide with the hinge and can be selected arbitrarily. The appropriate choice is to attribute the output signal to the location of the point mass of the equivalent simple pendulum (or reduced pendulum), which usually lies within the seismometer’s casing. Rotations of the sensor about this location produce no output signal due to angular or centripetal acceleration. The sensor then appears sensitive to linear acceleration only.

[Articles] The Effects of Tilt on Interferometric Rotation Sensors

Pham, N. D., Igel, H., Wassermann, J., Cochard, A., Schreiber, U. — 2009-05-01

Ring-laser rotation sensors are contaminated by rotations around horizontal axes (also called tilts) through the vector product between the local normal direction and the vector of the rotation rate composed of Earth’s rotation and local ground rotations. In this study, we investigate theoretically this cross-axis sensitivity and estimate the effects based on magnitude–amplitude relations to be expected for observations of local earthquakes and teleseismic events. We investigate tilt-ring-laser coupling for rotational motions in the P coda of several past earthquakes using tilt motions derived from observed translations. The results show that compared to the corresponding vertical rotation rate tilt-ring-laser coupling is negligible for observations of teleseismic events and for the applicable range of the local magnitude scale.

[Articles] The Response to Complex Ground Motions of Seismometers with Galperin Sensor Configuration

Graizer, V. — 2009-05-01

Most seismological instruments recording ground motion use three sensors oriented north, east, and upward. In this cardinal configuration horizontal and vertical sensors differ in their construction because of the gravitational acceleration affecting the vertical sensor. An alternative sensor arrangement was first introduced by Galperin (1955) for petroleum exploration. In this arrangement three identical sensors are also positioned orthogonally to each other but are tilted at the same angle of 54.7° to the vertical axis (an orthogonal triaxial system of coordinates balanced on its corner). Records obtained using this sensor configuration must be rotated into an Earth referenced cardinal X, Y, Z coordinate system for most analyses. A number of recent seismological instruments (e.g., STS-2 and Trillium seismometers) use Galperin sensor configuration. In most seismological studies it is assumed that the rotational components of earthquake ground motion are small enough to be neglected. However, examples of significant rotational components have been noted (e.g., Bouchon and Aki, 1982; Graizer, 1991; Takeo, 1998; Huang, 2003; Zahradnik and Plesinger, 2005; Cochard et al., 2006; Graizer, 2006a; Schreiber et al., 2006; Spudich and Fletcher, 2008). The response of pendulums when installed in a cardinal configuration to input motions that include rotations has been studied in a number of publications (Golitzin, 1912; Rodgers, 1968; Wong and Trifunac, 1977; Graizer, 1991; Todorovska, 1998; Trifunac and Todorovska, 2001; Graizer, 2005, 2006b; Graizer and Kalkan, 2008). This article considers the response to input motions of pendulums in a Galperin sensor configuration as well as the resulting cardinal orientation system response. Given the benefits of identical designs for all three sensors in a Galperin configuration, this geometry may be useful for strong-motion measurements as well. The disadvantage of this sensor configuration is that if any of the sensors is not working properly or there are misalignments of sensor axes, then all three cardinal components are degraded.

[Articles] Empirical Scaling of Rotational Spectra of Strong Earthquake Ground Motion

Lee, V. W., Trifunac, M. D. — 2009-05-01

Many empirical scaling equations have been developed for scaling Fourier spectrum amplitudes of strong earthquake accelerations and for generation of artificial strong-motion accelerograms for the translational, torsional, and rocking components of strong motion. It has also been shown that rotational components of strong motion significantly contribute to the overall response of structures; however, little progress has been made in the development and deployment of strong-motion instruments to measure rotations. This article presents a simple approximate algorithm for generating torsional and rocking Fourier spectral amplitudes from the corresponding translational motions. The method can be used to generate torsion and rocking spectra from the translational Fourier spectra of actual records. Inverse Fourier transform can be used to generate torsional and rocking time histories.

[Articles] Transient and Permanent Rotations in a Shear Layer Excited by Strong Earthquake Pulses

Gicev, V., Trifunac, M. D. — 2009-05-01

Nonlinear waves in a bilinear soil layer are described for excitation by vertically arriving S-wave pulses of strong ground motion. Conditions that lead to the nonlinear deformation are described in terms of amplitudes and wavelengths of incident pulses. It is shown that the layer can fail during the first passage of the incident wave (during a time shorter than the travel time through the layer). Peak amplitudes of (1) transient rotations, of (2) permanent rotations (strains), and of (3) the peak ductility in the layer are described in terms of the dimensionless amplitudes of incident pulses and the places of their occurrence in the layer. Even a simple model like this (one-dimensional propagation, simple shape of incident pulse, bilinear stress-strain soil model) leads to very complicated response. The results presented offer only a glimpse at the complexity in a realistic setting.

[Articles] Response Spectra for Near-Source, Differential, and Rotational Strong Ground Motion

Jalali, R. S., Trifunac, M. D. — 2009-05-01

It is shown that the pseudorelative spectral velocity (PSV) of an equivalent oscillator, which can be represented by a single degree-of-freedom system for excitation by synchronous horizontal excitation, can be extended to describe the PSV spectra of the same oscillator when excited by simultaneous action of horizontal, vertical, and rocking components of strong ground motion. At short periods, the new spectra are governed by differential ground motion and peak ground velocity, and they depend on the transit time of the waves along the length of the structure and on the oscillator frequency. At long periods, PSV spectral amplitudes tend toward an asymptote with amplitude proportional to the maximum rocking angle of ground motion.

[Short Notes] Waves in Linear Elastic Media with Microrotations, Part 1: Isotropic Full Cosserat Model

Kulesh, M. — 2009-05-01

In this article, we consider a problem of the surface elastic wave propagation within the framework of the isotropic Cosserat continuum. The medium deformation in this model is described not only by the displacement vector but also by a kinematically independent rotation vector. We discuss the general solution of equations of motion. This solution describes the following wave types: longitudinal and transverse bulk waves, Rayleigh wave, surface transverse wave in a half-space as well as Lamb wave and transverse wave in a thin layer. Within the framework of Cosserat continuum, both the Rayleigh and surface transverse waves in a half-space are dispersive. The transverse wave in a thin layer and the surface transverse wave in a half-space do not have any analogies in the classical elasticity theory.

[Short Notes] Waves in Linear Elastic Media with Microrotations, Part 2: Isotropic Reduced Cosserat Model

Grekova, E. F., Kulesh, M. A., Herman, G. C. — 2009-05-01

We consider wave propagation in soils and rocks modeled as an isotropic linear elastic reduced Cosserat continuum to take into account the proper rotational dynamics of heterogeneities contained in media. In such a medium, translations and rotations are kinematically independent, the stress tensor is nonsymmetric, and the couple stresses are zero. We consider plane wave propagation, construct the Green’s function for the harmonic point source in the 3D unbounded medium, and study the Rayleigh-type wave. The compression wave for the isotropic case is the same as in the classical medium. The shear wave is coupled with rotation and differs both from the classical case and from the case of the full Cosserat continuum. There are forbidden bands of frequencies where some waves do not propagate, localization phenomena are possible, and strongly dispersive behavior is observed near these bands. For the Rayleigh wave, there is also a cutoff wavenumber for one of the dispersion branches.

[Short Notes] Rotational Motions of Seismic Surface Waves in a Laterally Heterogeneous Earth

Ferreira, A. M. G., Igel, H. — 2009-05-01

A number of recent studies have analyzed seismic rotational data using a relationship between transverse acceleration and rotation rate derived for a homogeneous full space. In this study we explore this relationship further theoretically by presenting a full ray theory (FRT) method to simulate rotational motions of fundamental mode seismic surface waves in smooth, laterally heterogeneous Earth models. In the ray picture of wave propagation the vertical component of the rotational rate motion of fundamental mode Love waves is obtained by dividing the transverse component of ground acceleration by the Love-wave local phase velocity beneath the seismic recording station. We illustrate the method with examples of theoretical calculations of T40 sec rotational rate ground motions of fundamental Love waves using the crust model CRUST2.0 combined with the mantle model S20RTS for the 25 September 2003 M 8.1 Tokachi-oki earthquake, Japan. FRT rotation synthetics match complete calculations using the spectral-element method very well and fit real data reasonably well. Furthermore, we show that the effect of realistic local structure beneath receivers on rotational motions is strong enough to be observable. FRT calculations could potentially help to determine Love-wave local dispersion curves and, thus, to estimate the 1D local shear velocity structure beneath seismic stations from point measurements of rotational rate and acceleration ground motions.

[Short Notes] Design of a Relatively Inexpensive Ring Laser Seismic Detector

Dunn, R. W., Mahdi, H. H., Al-Shukri, H. J. — 2009-05-01

During the last decade the ability of large ring lasers to measure seismic induced ground rotation has demonstrated their potential for making contributions to the field of seismology. Unfortunately, ring lasers like many optical instruments tend to be expensive. In this technical note, we examine some design options for deploying a relatively low cost ring laser ground rotation sensor. A review of the design parameters, common to all active ring lasers, is followed by an examination of a specific relatively low cost approach currently employed in a ring laser that has been operating outside Conway, Arkansas. The article concludes with a discussion of some earthquakes detected by this instrument.

[Short Notes] Strong-Motion Fluid Rotation Seismograph

Jedlicka, P., Buben, J., Kozak, J. — 2009-05-01

A new type of rotation seismometer is proposed using a fluid ring-shaped sensor, where the inertial mass is a liquid moving in the tube attached to a pier to measure rotational movements. Rotational components of seismic waves, which are strong enough to twist heavy objects on the Earth’s surface in the epicenter zone of an earthquake, decrease rapidly with distance. These weak rotational components may easily vanish in standard seismograms displaying translational displacements. In the proposed fluid, seismometer translational displacements are mutually compensated so that weak rotational vibration components become easily identifiable and recognizable. The technical and physical parameters of such a fluid ring device were tested and evaluated. The proposed fluid seismometer was primarily designed for observing rotational movements in the near field of earthquakes. However, it may also be used for monitoring undesirable rotational effects in large engineering works.

[Short Notes] Performance Test of a Commercial Rotational Motions Sensor

Wassermann, J., Lehndorfer, S., Igel, H., Schreiber, U. — 2009-05-01

The application of rotational motion sensors has only recently proven to give new ways of measuring seismic-wave-field properties when comparing the recorded data with seismograms of collocated traditional seismometers. The data in these test cases were produced using either sophisticated and thus expensive ring laser technology or cumbersome seismic array techniques including some restrictive assumption about the wave field. In this article, we want to test the performance of one of the first medium-priced commercial rotational motion sensor (eentec R1) by comparing its output with the aforementioned classical array-derived rotational motions. The data set consists of seismic array and rotational motion measurements that were performed during a demolition blast of a 50 m high building in the city of Munich (Germany). In addition to the simple comparison of the outputs, we want to classify the performance of the two methods by comparing derived wave-field properties with the result of classical frequency-wavenumber (f-k) array analysis. The results of this experiment demonstrate that, when using an array technique for estimating rotational motions, much effort in site selection, array design, and a priori knowledge of subsurface conditions is needed. It also becomes evident that the performance of an array and its estimated quantities strongly depends on the number of deployed seismic stations. Given the uncertainties in both the array-derived measurements and the rotation sensor transfer function, it is difficult to quantify the accuracy of the rotation sensor data, which indicates the need for further extensive laboratory and field testing.

[Short Notes] Rotational Motions Observed during an Earthquake Swarm in April 1998 Offshore Ito, Japan

Takeo, M. — 2009-05-01

We observed six components of ground rotational and translational motions in a near-field region during an earthquake swarm in April 1998 offshore Ito, Izu Peninsula, Japan. To check the reliability of our observation using a MotionPack sensor, we installed an 8301F sensor, which is an inertial angular displacement sensor with a higher sensitivity than MotionPack. The noise level of the MotionPack is much higher than that of the 8301F, but the waveforms of these signal components are quite similar to each other, suggesting that the MotionPack records rotational rate correctly but with higher noise. Our observation made known a linear correlation between the maximum rotational displacements around the vertical axis and the maximum translational velocities. The waveforms of rotational motion around the vertical axis are analogous to those of translational horizontal velocities, suggesting the linear correlation between maximum values. These rotational motions are much larger than those calculated by array data at the San Andreas fault. This large discrepancy might be explained by one or more of the following effects: the difference in the spatial scale of rotational motion by single gyro measurement and by array observation, the effect of the precipitous topography at the station of the offshore Ito swarm, and the difference of the degree of maturation between the San Andreas fault and the swarm regions of offshore Ito.

[Short Notes] Array Deployment to Observe Rotational and Translational Ground Motions along the Meishan Fault, Taiwan: A Progress Report

Wu, C.-F., Lee, W. H. K., Huang, H.-C. — 2009-05-01

Measurements in the near field of earthquakes in Japan (Takeo, 1998) and in Taiwan (Huang et al., 2006; Liu et al., 2009) indicate that rotational ground motions are many times larger than expected from the classical elasticity theory. The Central Weather Bureau (CWB) deployed four instrumentation sets on the campus of the National Chung-Cheng University, Chia-yi, Taiwan, in order to study in detail earthquakes in the near field for both seismology and earthquake engineering. Both rotational and translational ground motions are being monitored along the active Meishan fault, where a major earthquake occurred in 1906, more than a century ago. The deployed instruments are: (1) a 32-element seismic array in the free field, (2) a 32-element seismic array in a building, (3) a six-channel unit with a broadband seismometer and an accelerometer, and (4) a six-channel unit with an accelerometer and an external rotational velocity sensor.

This short note is a progress report on this array deployment. Although 24 local earthquakes were recorded by one or more of the four instrumentation sets from 12 December 2007 to 3 July 2008, we are still in the process of upgrading equipment and improving field operations.

[Short Notes] Rotational Earthquake Effects in the United Kingdom

Sargeant, S. L., Musson, R. M. W. — 2009-05-01

The United Kingdom is an area of low to moderate seismicity, and damaging earthquakes are uncommon. However, even in the limited record of damage from historical British earthquakes, a number of instances can be found of rotational effects on parts of structures, primarily chimneys or the tops of spires. We have assembled all the instances we know of from the United Kingdom record and present them here with illustrations and extracts from the original reports. It is not possible to determine whether these are the effects of true rotational motion or the effects of translatory shaking. Interestingly, this problem was considered in some detail by field investigators as long ago as the 1880s.

[Short Notes] Software for Inference of Dynamic Ground Strains and Rotations and Their Errors from Short Baseline Array Observations of Ground Motions

Spudich, P., Fletcher, J. B. — 2009-05-01

In two previous articles we presented a formulation for inferring the strains and rotations of the ground beneath a seismic array having a finite footprint. In this article we derive expressions for the error covariance matrices of the inferred strains and rotations, and we present software for the calculation of ground strains, rotations, and their variances from short baseline array ground-motion data.

[Short Notes] Rotational Seismic Load Definition in Eurocode 8, Part 6, for Slender Tower-Shaped Structures

Zembaty, Z. — 2009-05-01

This note describes the rotational seismic load definition as included in Part 6 of Eurocode 8 (EC8.6, 2005). The Eurocode 8, Part 6 (EC8.6, 2005), definition of the rotational ground-motion component depends upon the structural subsoil compliance, which is controlled by the shear-wave velocity in the top 30 m of ground. A comparison of the effects of the rocking ground motion and the horizontal ground motion on the response of a 160 m reinforced concrete chimney shows that for the Eurocode 8, Part 6 (EC8.6, 2005), definition of the rotational seismic ground motion, the rocking excitations contribute significantly to the overall response of the structure. The engineering code formulas for the rocking component of ground motion, however, should be calibrated and reconciled with the results of the latest empirical research.

[Articles] San Andreas Fault Rupture Scenarios from Multiple Paleoseismic Records: Stringing Pearls

Biasi, G. P., Weldon, R. J. — 2009-03-19

We present a new method to objectively combine paleoseismic event data from multiple sites into rupture scenarios and apply it to the southern San Andreas fault (SSAF) of California. First, a pool is constructed of all ruptures between sites allowed by fault geometry and available event age probability distribution functions (PDFs). Scenarios constructed by drawing from this pool are evaluated by the average quality of agreement event dating evidence, by the degree of misfit of cumulative displacement over all ruptures compared to a prediction from the fault slip rate and elapsed time, and by the number of events in the scenario. Three slip-rate models from the 2008 Working Group on California Earthquake Probabilities (WGCEP) were considered. Scenarios with full fault length ruptures tend to be inconsistent with low slip rates through the San Bernardino and San Gorgonio sections of the SSAF. Favorable scenarios tend to include 1857-like ruptures for three of the most recent five prehistoric ruptures in the northern half of the SSAF from Carrizo to approximately Wrightwood. They also include one to two ruptures that involve the southern half of the SSAF, but most earthquakes there appear shorter and exhibit less consistency from event to event. By combining paleoseismic data into ensembles of scenarios and selecting viable scenarios using external constraints, our method provides rupture histories useful for seismic hazard assessment without having to first settle which event at a site correlates with those at adjoining sites. This opens a way for paleoseismic data to be used with greater power to understand the seismic hazard posed by faults like the southern San Andreas.

[Articles] New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California

2009-03-19

High-resolution seismic compressed high intensity Radar pulse (CHIRP) data and piston cores acquired in Fallen Leaf Lake (FLL) and Lake Tahoe provide new paleoseismic constraints on the West Tahoe–Dollar Point fault (WTDPF), the westernmost normal fault in the Lake Tahoe Basin, California. Paleoearthquake records along three sections of the WTDPF are investigated to determine the magnitude and recency of coseismic slip. CHIRP profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event (MRE) on the WTDPF. Three faults are imaged beneath FLL, and the maximum vertical offset observed across the primary trace of the WTDPF is ~3.7 m. Coregistered piston cores in FLL recovered sediment and organic material above and below the MRE horizon. Radiocarbon dating of organic material constrained the age of the MRE to be between 3.6 and 4.9 k.y. B.P., with a preferred age of 4.1–4.5 k.y. B.P. In Lake Tahoe near Rubicon Point, approximately 2.0 m of vertical offset is observed across the WTDPF. Based on nearby core data, the timing of this offset occurred between B.P., which is consistent with the MRE age in FLL. Offset of Tioga-aged glacial deposits provides a long-term record of vertical deformation on the WTDPF since B.P., yielding a slip rate of 0.4–0.8 mm/yr . In summary, the slip rate and earthquake potential along the WTDPF is comparable to the nearby Genoa fault, making it the most active and potentially hazardous fault in the Lake Tahoe Basin.

[Articles] A Probabilistic Method for the Magnitude Estimation of a Historical Damaging Earthquake Using Structural Fragility Functions

Ryu, H., Kim, J. K., Baker, J. W. — 2009-03-19

A method is proposed here to estimate the magnitude of a historical earthquake by using fragility functions and written descriptions of damage. Probabilistic descriptions are used to describe the distribution of potential earthquake events, the resulting intensity of ground shaking at the site, and the distribution of resulting damage to structures. This information is then combined using Bayes’ theorem to compute the posterior distribution of the magnitude that caused a past damaging event. To validate the proposed method, the magnitude of the Northridge earthquake that occurred on 17 January 1994 is estimated. As an application example, the magnitude of a Korean earthquake that occurred in 1613 is estimated. Bins of input ground motions are created by a spectral matching method using an attenuation relationship of Korea, and probability-of-collapse estimates are obtained by performing incremental dynamic analysis (IDA). A basic formulation is presented and then extended to take the correlation of collapse capacity between structures, the effect of aging on structural response, and the site effects into account. Sensitivity analyses are performed to determine the importance of assumptions regarding the number of historically damaged buildings, the distributions of plausible magnitudes and distances, and the choice of attenuation relationship. The proposed method provides a comprehensive and straightforward procedure for magnitude estimation that can incorporate all relevant uncertainties.

[Articles] Attenuation of Macroseismic Intensity: A New Relation for the Marmara Sea Region, Northwest Turkey

Sorensen, M. B., Stromeyer, D., Grunthal, G. — 2009-03-19

Prediction equations for macroseismic intensity are the backbone of seismic hazard assessment, of source parameter estimation, and of shake map generation in cases where an output in terms of intensity is desired. This is especially required when a direct relation to the damage associated with ground shaking is of interest or if ground shaking estimates will be used for informing nonseismologists such as emergency response teams or the general public. In the current study we derive ground-motion prediction equations for macroseismic intensity valid for the Marmara Sea region, northwest Turkey. The relations have a physical basis and are easy to implement for the user. In one relation, the finite extent of the fault rupture is accounted for by defining distance as the Joyner–Boore distance leading to the relation where M_w is the moment magnitude, R_JB is the Joyner–Boore distance, and h is the hypocentral depth. Furthermore, a relation based on the epicentral distance (R_epi) is derived for application in cases where the extent of the fault plane is unknown: The relations are valid for the ranges 5≤I≤10, 5.9≤M_w≤7.4, and R≤350 km. It is shown that inclusion of the rupture dimensions leads to an improvement in the ability of the relation to fit observations in the near field for large earthquakes. Comparison to already existing intensity prediction equations for the region shows that the new relations provide better estimates of the macroseismic intensity distribution, especially in the region near the rupturing fault plane.

[Articles] Attenuation Relationship of Macroseismic Intensities in Central Europe

Stromeyer, D., Grunthal, G. — 2009-03-19

Two intensity-attenuation models for central Europe are presented in terms of mean relations and probability distributions of their variability. They are based on 14,714 intensity data points (IDP) of 31 earthquakes located in Germany, France, the Netherlands, and the Czech Republic. The attenuation relations are derived from crustal events with moment magnitudes M_w from 2.4 to 5.7, source depths h from 1 to 20 km, and epicentral distances R up to 400 km. Two different regression techniques are applied to derive the appropriate parameters a and b of the attenuation model . A combined weighted least-squares regression utilizing intensities and distances of the IDP in a direct way provides a=2.80 and b=0.0013. A chi-square regression method for mean intensity-class distances estimated by intensity-level binning gives a=2.95 and b=0.0025. The variabilities in the derived models can be described by normal distributions centered at the mean relation with a standard deviation of about 0.7 intensity units. This specification of attenuation variability is one of the advantages compared with former relations, making both attenuation models especially suitable to be applied in seismic hazard assessments. The two independent models can be regarded as an epistemic uncertainty in logic tree based hazard calculations.

[Articles] Regional Seismic Hazard Posed by the Mentawai Segment of the Sumatran Megathrust

Megawati, K., Pan, T.-C. — 2009-03-19

Several lines of evidence have indicated that the Mentawai segment of the Sumatran megathrust is very likely to rupture within the next few decades. The present study is to investigate seismic hazard and risk levels at major cities in Sumatra, Java, Singapore, and the Malay Peninsula caused by the potential giant earthquakes. Three scenarios are considered. The first one is an M_w 8.6 earthquake rupturing the 280 km segment that has been locked since 1797; in the second scenario, rupture occurs along a 600 km segment covering the combined rupture areas of the 1797 and 1833 historical events, producing an M_w 9.0 earthquake; and the third scenario has the same rupture area as the second scenario but with doubled slip amplitude, resulting in an M_w 9.2 earthquake. Simulation results indicate that ground motions produced by the hypothetical scenarios are strong enough to cause yielding to medium- and high-rise buildings in many major cities in Sumatra. It is vital to ensure that the overall strength, stiffness, and integrity of the structures are maintained throughout the entire duration of shaking. However, the ductile detailing in current practice is formulated based on an assumption that ground motions would last from 20 to 40 sec. This has not been tested for longer durations of 3–5 min, expected from giant earthquakes. In Singapore and Kuala Lumpur, only medium- and high-rise buildings, especially those located on soft-soil sites, are at risk. Given that seismic design has not been required in either city, and thus the resulting structures are relatively brittle, it is crucial to investigate their performance under moderate-amplitude, long-duration, ground motions. The present study also points out that shifting the response spectrum toward a longer period range becomes significant for sites located far from potential seismic sources, which should be carefully considered in formulation of future seismic codes.

[Articles] Effect of Time Dependence on Probabilistic Seismic-Hazard Maps and Deaggregation for the Central Apennines, Italy

Akinci, A., Galadini, F., Pantosti, D., Petersen, M., Malagnini, L., Perkins, D. — 2009-03-19

We produce probabilistic seismic-hazard assessments for the central Apennines, Italy, using time-dependent models that are characterized using a Brownian passage time recurrence model. Using aperiodicity parameters, of 0.3, 0.5, and 0.7, we examine the sensitivity of the probabilistic ground motion and its deaggregation to these parameters. For the seismic source model we incorporate both smoothed historical seismicity over the area and geological information on faults. We use the maximum magnitude model for the fault sources together with a uniform probability of rupture along the fault (floating fault model) to model fictitious faults to account for earthquakes that cannot be correlated with known geologic structural segmentation.

We show maps for peak ground acceleration (PGA) and 1.0 Hz spectral acceleration (SA₁) on rock having 10% probability of exceedence in 50 yr. We produce maps to compare the separate contributions of smoothed seismicity and fault components. In addition we construct maps that show sensitivity of the hazard for different parameters and the Poisson model.

For the Poisson model the addition of fault sources to the smoothed seismicity raises the hazard by 50% at locations where the smoothed seismicity contributes the highest hazard and up to 100% at locations where the hazard from smoothed seismicity is low. For the strongest aperiodicity parameter (smallest ), the hazard may further increase 60%–80% or more or may decrease by as much as 20% depending on the recency of the last event on the fault that dominates the hazard at a given site.

In order to present the most likely earthquake magnitude and/or the most likely source-site distance for scenario studies, we deaggregate the seismic hazard for SA₁ and PGA for two important cities (Rome and L’Aquila). For PGA both locations show the predominance of local sources having magnitudes of about 5.3 and 6.5, respectively. For SA₁ at a site in Rome, there is significant contribution from local smoothed seismicity and an additional contribution from the more distant Apennine faults having magnitude around 6.8. For L’Aquila the predominant sources remain local.

In order to show the variety of impact of different values, we also obtained deaggregations for another three sites. In general, as decreases (periodicity increases), the deaggregation indicates that the hazard is highest near faults with the highest earthquakes rates. This effect is strongest for the long-period (1 sec) ground motions.

[Articles] Seismic Interferometry of a Soil-Structure Interaction Model with Coupled Horizontal and Rocking Response

Todorovska, M. I. — 2009-03-19

This article presents a system identification analysis of a soil-structure interaction model with coupled horizontal and rocking response based on a combination of Fourier analysis, wave travel-time analysis, and a relationship between fixed-base, rigid-body, and system frequencies. The study provides insight into the coupling of the structural and soil vibrations useful for interpretation of seismic recordings in structures. The structural model captures one-dimensional shear-wave propagation in the structure. The analysis shows that the system functions with respect to foundation horizontal motion are those of the coupled soil-structure system, which differs from conclusions of earlier studies based on a model without foundation rocking. The energy of the system vibrational response is concentrated around the frequencies of vibration of the system, which depend on the properties of the structure, soil, and foundation. The analysis shows that the structural fundamental fixed-base (uncoupled) frequency f₁ is related to the wave travel time (from the base to the top) by f₁=1/(4) and that accurate measurement of , unaffected by soil-structure interaction, can be obtained from impulse response functions, provided that the data are sufficiently broadband. This is an important result for structural health monitoring because it shows that structural parameters unaffected by soil-structure interaction (, as well as f₁ for structures deforming primarily in shear) can be estimated from seismic monitoring data with minimum instrumentation (two horizontal sensors, one at the base and one at the top). This extends the usability of old strong-motion data in buildings, most of which have not been extensively instrumented, and lessons that can be learned for development and validation of structural health monitoring methodologies. The presented results correspond to a model of the north–south response of the Millikan Library in Pasadena, California, which has become a classical case study for soil-structure interaction.

[Articles] Soil-Structure System Identification of Millikan Library North-South Response during Four Earthquakes (1970-2002): What Caused the Observed Wandering of the System Frequencies?

Todorovska, M. I. — 2009-03-19

An application is presented of a recently proposed system identification method for buildings that takes into account the effects of soil-structure interaction and the coupling of the horizontal and rocking motions of the foundation. The method gives the uncoupled structural fixed-base frequency f₁ and rigid-body rocking frequency f_R using data only from two horizontal sensors (at base and roof). The fixed-base frequency is estimated from the wave travel time through the structure, the apparent system frequency from Fourier analysis, and the rocking frequency from a relation between these three. The case study is Millikan Library in Pasadena, California. Results are shown for four earthquakes between 1970 and 2002. The method makes it possible to quantify the degree to which the observed changes (wandering) of its resonant frequencies have been due to changes in the structure alone. The results show that (1) both f₁ and f_R are amplitude dependent, (2) significant permanent reduction of frequency occurred over the years, ~22% for f₁ and 11% for the apparent frequency f_1,app, mostly caused by the San Fernando earthquake of 1971, while (3) the changes of f_R have been amplitude dependent and recoverable. (4) During the San Fernando earthquake, both f₁ and f_R dropped, respectively, by ~24% and ~18%, resulting in 21% drop of f_1,app. (5) After this earthquake, the changes in the observed resonant frequencies, which are those of the system, have been due to a much larger degree (4–5 times) to changes of f_R than to changes of f₁. (6) The small permanent changes in f₁ that appear to have occurred after the San Fernando earthquake cannot be deciphered with certainty because of the small number of earthquake records available for this analysis since 1971. Records from the period 1988 to 2002, when released, can be used in future to refine and verify these trends.

[Articles] Mixture Models for Improved Short-Term Earthquake Forecasting

Rhoades, D. A., Gerstenberger, M. C. — 2009-03-19

The short-term earthquake probability (STEP) forecasting model applies the Omori–Utsu aftershock-decay relation and the Gutenberg–Richter frequency-magnitude relation to clusters of earthquakes. It is mainly intended to forecast aftershock activity and depends on a time-invariant background model to forecast most of the major earthquakes. On the other hand, the long-range earthquake forecasting model EEPAS (every earthquake a precursor according to scale) exploits the precursory scale increase phenomenon and associated predictive scaling relations to forecast the major earthquakes months, years, or decades in advance, depending on magnitude. Both models are shown to be more informative than time-invariant models of seismicity. By forming a mixture of the two, we aim to create an even more informative short-term forecasting model. Using the Advanced National Seismic System catalog of California over the period 1984–2004, the optimal mixture model for forecasting earthquakes with M≥5.0 is a convex linear combination consisting of 0.42 of the EEPAS forecast and 0.58 of the STEP forecast. This mixture gives an average probability gain of more than 2 compared to each of the individual models. Several different mixture models will be submitted to the CSEP Testing Center at the Southern California Earthquake Center to ascertain whether or not this result is borne out by real-time tests of the models against future earthquakes.

[Articles] Uncertainties in Strong Ground-Motion Prediction with Finite-Fault Synthetic Seismograms: An Application to the 1984 M 5.7 Gubbio, Central Italy, Earthquake

Ameri, G., Gallovic, F., Pacor, F., Emolo, A. — 2009-03-19

This study investigates the engineering applicability of two conceptually different finite-fault simulation techniques. We focus our attention on two important aspects: first to quantify the capability of the methods to reproduce the observed ground-motion parameters (peaks and integral quantities); second to quantify the dependence of the strong-motion parameters on the variability in the large-scale kinematic definition of the source (i.e., position of the nucleation point, value of the rupture velocity, and distribution of the final slip on the fault).

We applied an approximated simulation technique, the deterministic-stochastic method and a broadband technique, the hybrid-integral-composite method, to model the 1984 M_w 5.7 Gubbio, central Italy, earthquake, at five accelerometric stations. We first optimize the position of the nucleation point and the value of the rupture velocity for three different final slip distributions on the fault by minimizing an error function in terms of acceleration response spectra in the frequency band from 1 to 9 Hz. We found that the best model is given by a rupture propagating at about 2.65 km/sec from a hypocenter located approximately at the center of the fault. In the second part of the article we calculate more than 2400 scenarios varying the kinematic source parameters. At the five sites we compute the residuals distributions for the various strong-motion parameters and show that their standard deviations depend on the source parameterization adopted by the two techniques. Furthermore, we show that Arias Intensity (AI) and significant duration are characterized by the largest and smallest standard deviation, respectively. Housner Intensity is better modeled and less affected by uncertainties in the source kinematic parameters than AI. The fact that the uncertainties in the kinematic model affects the variability of different ground-motion parameters in different ways has to be taken into account when performing hazard assessment and earthquake engineering studies for future events.

[Articles] A Constant Stress-Drop Model for Producing Broadband Synthetic Seismograms: Comparison with the Next Generation Attenuation Relations

Frankel, A. — 2009-03-19

Broadband (0.1–20 Hz) synthetic seismograms for finite-fault sources were produced for a model where stress drop is constant with seismic moment to see if they can match the magnitude dependence and distance decay of response spectral amplitudes found in the Next Generation Attenuation (NGA) relations recently developed from strong-motion data of crustal earthquakes in tectonically active regions. The broadband synthetics were constructed for earthquakes of M 5.5, 6.5, and 7.5 by combining deterministic synthetics for plane-layered models at low frequencies with stochastic synthetics at high frequencies. The stochastic portion used a source model where the Brune stress drop of 100 bars is constant with seismic moment. The deterministic synthetics were calculated using an average slip velocity, and hence, dynamic stress drop, on the fault that is uniform with magnitude. One novel aspect of this procedure is that the transition frequency between the deterministic and stochastic portions varied with magnitude, so that the transition frequency is inversely related to the rise time of slip on the fault. The spectral accelerations at 0.2, 1.0, and 3.0 sec periods from the synthetics generally agreed with those from the set of NGA relations for M 5.5–7.5 for distances of 2–100 km. At distances of 100–200 km some of the NGA relations for 0.2 sec spectral acceleration were substantially larger than the values of the synthetics for M 7.5 and M 6.5 earthquakes because these relations do not have a term accounting for Q. At 3 and 5 sec periods, the synthetics for M 7.5 earthquakes generally had larger spectral accelerations than the NGA relations, although there was large scatter in the results from the synthetics. The synthetics showed a sag in response spectra at close-in distances for M 5.5 between 0.3 and 0.7 sec that is not predicted from the NGA relations.

[Articles] Effects of Realistic Surface Topography on Seismic Ground Motion in the Yangminshan Region of Taiwan Based Upon the Spectral-Element Method and LiDAR DTM

Lee, S.-J., Chan, Y.-C., Komatitsch, D., Huang, B.-S., Tromp, J. — 2009-03-19

We combine light detection and ranging (LiDAR) digital terrain model (DTM) data and an improved mesh implementation to investigate the effects of high-resolution surface topography on seismic ground motion based upon the spectral-element method. In general, topography increases the amplitude of shaking at mountain tops and ridges, whereas valleys usually have reduced ground motion, as has been observed in both records from past earthquakes and numerical simulations. However, the effects of realistic topography on ground motion have not often been clearly characterized in numerical simulations, especially the seismic response of the true ground surface. Here, we use LiDAR DTM data, which provide two-meter resolution at the free surface, and a spectral-element method to simulate three-dimensional (3D) seismic-wave propagation in the Yangminshan region in Taiwan, incorporating the effects of realistic topography. A smoothed topographic map is employed beneath the model surface in order to decrease mesh distortions due to steep ground surfaces. Numerical simulations show that seismic shaking in mountainous areas is strongly affected by topography and source frequency content. The amplification of ground motion mainly occurs at the tops of hills and ridges whilst the valleys and flat-topped hills experience lower levels of ground shaking. Interaction between small-scale topographic features and high-frequency surface waves can produce unusually strong shaking. We demonstrate that topographic variations can change peak ground acceleration (PGA) values by ±50% in mountainous areas, and the relative change in PGA between a valley and a ridge can be as high as a factor of 2 compared to a flat surface response. This suggests that high-resolution, realistic topographic features should be taken into account in seismic hazard analysis, especially for densely populated mountainous areas.

[Articles] Combination of Acceleration-Sensor and Broadband Velocity-Sensor Recordings for Attenuation Studies: The Case of the 8 January 2006 Kythera Intermediate-Depth Earthquake

2009-03-19

On 8 January 2006, an intermediate-depth earthquake occurred at the western part of the Hellenic trench close to the island of Kythera (southern Greece). This is the first intermediate-depth earthquake in the broader Aegean area that has produced such an extensive set of useful recordings, as it was recorded by the main permanent seismological networks and numerous acceleration sensors operating in Greece, as well as by EGELADOS, a large-scale temporary amphibian broadband seismological network deployed in the southern Aegean area.

An effort to combine all the available data (broadband velocity and acceleration sensor) was made to study the properties of ground-motion attenuation of this earthquake. The combination of both types of data revealed interesting properties of the earthquake wave field, which would remain hidden if only one type of data was used. Moreover, the data have been used for a validation of existing peak ground-motion empirical prediction relations and the preliminary study of the very inhomogeneous attenuation pattern of the southern Aegean intermediate-depth events at both near- and far-source distances.

[Articles] Modeling of Strong-Motion Data in Northeastern India: Q, Stress Drop, and Site Amplification

Raghukanth, S. T. G., Nadh Somala, S. — 2009-03-19

In this article point source seismological model is used to model the available strong-motion accelerograms in northeastern India. Key seismic parameters such as Quality factor (Q-value), kappa factor (₀), site amplification, and stress drop are derived from the strong-motion data. A total of 111 three-component records coming from seven earthquakes with magnitudes ranging from M_w 5.2 to 7.2 are used in obtaining the source and path parameters. The Q-value of the Indo–Burma tectonic domain, which is a subduction zone, is obtained as 431f^0.7. For the Bengal basin–Shillong plateau region the Quality factor is estimated as 224f^0.93. The kappa factors for vertical and horizontal components are obtained as 0.013 and 0.033 for soft rock type sites. For firm ground sites ₀ is estimated as 0.025 for vertical and 0.041 for horizontal components, respectively. The site amplification functions for soft rock and firm ground conditions are found from the horizontal-to-vertical ratio. The obtained stress drops of the seven events lie between 128 and 282 bars. The source parameters obtained in this study provide a useful framework for working out ground-motion relations in northeastern India.

[Articles] Inversion of Seismogram Envelopes Using a Multiple Isotropic Scattering Model in Garhwal Himalaya

Padhy, S. — 2009-03-19

We estimated intrinsic attenuation , scattering attenuation , site amplification Z, and source energy S from inversion of three-component coda envelopes of the 1999 Chamoli earthquake of India for central frequencies 1.5, 3, 6, 12, and 24 Hz. The multiple isotropic scattering of S waves was numerically simulated by using a Monte Carlo method based on the radiative transfer theory. Isotropic sources and acoustic scattering in a full space were assumed. Adapting a grid search for scattering coefficient g and least-squares inversion for intrinsic attenuation parameter b, and source energy S, we inverted the observed envelopes of ten aftershocks (M_L≥3.5) in 1–24 Hz.

Our results reveal that both and are weakly frequency dependent with the power-law forms of (0.006±0.004)f^{-(0.89±0.33)} and (0.003±0.0005)f^{-(0.84±0.08)}, respectively. High scattering loss can be interpreted to be due to the presence of large lateral velocity heterogeneities in the crust. The total attenuation Q^-1 decreases with frequency, taking the power-law form of (0.009±0.003)f^{-(0.87±0.19)}. The mean free path ranges from 30 to 300 km, with an average value of 100 km, and the intrinsic attenuation parameter b ranges from 0.01 to 0.05 sec^-1, with an average value of 0.03 sec^-1. Our estimates of source energy are in good agreement with the values obtained assuming an ²-source model. Site effects estimated using the fixed values of S_k, b, and g exhibit less scatter, ranging from 0.73 to 2.54 with no significant frequency dependence consistent with the rock sites.

[Articles] Site Amplifications Observed in the Gubbio Basin, Central Italy: Hints for Lateral Propagation Effects

2009-03-19

In this study we investigate the site amplification effects observed in the Gubbio plain, central Italy. The recordings of 140 local earthquakes (1.2≤Ml≤4.7) observed by two linear arrays installed along the longitudinal and perpendicular basin axis are analyzed both in the time and frequency domains to determine the spatial variation of the local site effects in the basin. The time series recorded in the Gubbio plain are characterized by locally generated surface waves, which cause a significant increase in both ground-motion duration and amplitude with respect to a nearby reference station on a rock site. In the middle of the basin where the sedimentary cover is thickest (ca 600 m), the peak ground velocity is amplified on average by a factor of 5 with respect to the reference station installed on rock, and the duration is increased by a factor of about 2. The analyses in the frequency domain show that the spectral energy of the basin-generated waves is mainly distributed over the range 0.4–2 Hz and affects both horizontal and vertical components. The site responses estimated by applying two reference site methods show that the overall shape of the amplification functions is similar for the sites located inside the basin with a difference in the amplification levels, which can reach values as high as 30 in the middle of the valley. Amplifications up to a factor of 10 affect the vertical component of ground motion over a broad frequency range for nearly all basin sites with the maximum amplifications occurring at around 1 Hz. Finally, although the horizontal-to-vertical spectral ratios were found to provide a reliable estimation of the fundamental resonance frequency of the site, they fail to estimate the shape of the site transfer function.

[Articles] VS30 Estimates Using Constrained H/V Measurements

Castellaro, S., Mulargia, F. — 2009-03-19

Most seismic codes adopt the average shear-wave velocity as a key parameter in the first 30 m of subsoil (V_S30). Estimates of V_S30 are therefore required for both large- and small-scale seismic microzonation. We propose a technique to measure the V_S30 based on the horizontal to vertical spectral ratio (H/V) of microtremor recorded at a single station. The H/V is fitted with a synthetic curve using the independently known thickness of a superficial layer of the subsoil as a constraint. The proposed procedure consists of three steps: (1) identify the depth of a shallow stratigraphic horizon from independent geotechnical data, (2) identify its corresponding H/V marker, and (3) use it as a constraint to fit the experimental H/V curve with the theoretical one. The synthetic H/V curve is calculated by assuming a stratified one-dimensional (1D) soil model and a tremor wave field from distant random sources that are composed of Rayleigh and Love waves. This technique has been validated on different geological settings following a triple check procedure: (1) the theoretical Rayleigh-wave phase velocity dispersion curves calculated for the model derived from the H/V fit were compared to the experimental curves measured with the 2D extended spatial autocorrelation (ESAC) method and the 1D refraction microtremor (ReMi) array surveys; (2) the V_S profiles and (3) the V_S30 estimates obtained by the proposed technique were compared to those obtained by using the latter array techniques. Finally, the inferred stratigraphy was compared to the geological model. The proposed technique is not meant to provide accurate V_S profiles but has the potential to provide V_S30 estimates that are coherent with those measured through ESAC and ReMi and to satisfy law requirements. Furthermore, the H/V is found to be capable in detecting deviations from 1D subsoil geometry over the length of a few meters, the correctness of which was confirmed by direct drilling.

[Articles] A Method to Estimate the Phase Velocities of Microtremors Using a Time-Frequency Analysis and Its Applications

Morikawa, H., Udagawa, S. — 2009-03-19

The phase velocities of Rayleigh waves provide basic information for estimating the shear-wave velocity structure of the ground. Many methods have been proposed for this purpose; however, it is difficult to estimate phase velocities directly from the data found in microtremor records on the differences in phase angles between two sites. The reason for this difficulty is that the sources of microtremors are not stable in either time or space. In order to counteract this problem, therefore, we consider the nonstationary characteristic of microtremors in short-time range and propose a method that can find the instantaneously corresponding portions of microtremor records at two different sites and that can estimate the apparent phase velocities between the two sites. We use the technique of a time-frequency analysis for which the Hilbert–Huang Transform is adopted. After we confirm the applicability of the proposed method based on some numerical calculations, the method is applied to some actual data on microtremors and reasonable results are obtained.

[Articles] Regional Multistation Discriminants: Magnitude, Distance, and Amplitude Corrections, and Sources of Error

Anderson, D. N., Walter, W. R., Fagan, D. K., Mercier, T. M., Taylor, S. R. — 2009-03-19

Magnitude, distance, and amplitude corrections (MDAC) made to observed regional amplitudes are necessary so that what remains in the corrected amplitude is mostly information about the seismic source type. Corrected amplitudes can be used in ratios to discriminate between earthquakes and explosions. However, source effects remain that cannot easily be determined and applied as amplitude corrections, such as those due to depth, focal mechanism, local material property, and apparent stress variability. We develop a mathematical model to capture these near-source effects as random (unknown), giving an error partition of three sources: model inadequacy, station noise, and amplitude correlation. This mathematical model is the basis for a general multistation regional discriminant formulation. The standard error of the discriminant includes the variances of model inadequacy and station noise, along with amplitude correlation in its formulation. The developed methods are demonstrated for a collection of Nevada test site (NTS) events observed at regional stations (see Fig. 1). Importantly, the proposed formulation includes all corrected amplitude information through the construction of multistation discriminants. In contrast, previous studies have only computed discriminants from single stations having both P and S amplitudes. The proposed multistation approach has similarities to the well-established m_b versus M_s discriminant and represents a new paradigm for the regional discrimination problem.

[Articles] S-Wave Generation by Underground Explosions: Implications from Observed Frequency-Dependent Source Scaling

Murphy, J. R., Barker, B. W., Sultanov, D. D., Kuznetsov, O. P. — 2009-03-19

A systematic analysis of the frequency-dependent source scaling of regional phase data observed from underground nuclear explosions conducted at the former Soviet Semipalatinsk Test Site, the Chinese Lop Nor Test Site, the Russian Novaya Zemlya Test Site, and the U. S. Nevada Test Site has been conducted in an attempt to derive improved, quantitative constraints on proposed physical mechanisms for S-wave generation by explosion sources. The source scaling results for all these testing areas have been found to be remarkably consistent indicating that the observed Sn and Lg spectra scale with explosion yield in a manner that is very comparable to that of the corresponding direct Pn spectra, which differs significantly only over narrow frequency bands defined by differences in the P- and S-wave source corner frequencies. More specifically it has been found that the observed frequency-dependent source scaling of S/P spectral ratios at each of these test sites is very consistent with the simple phenomenological model proposed by Fisk et al. (2005), in which the S-wave source is obtained from the corresponding Mueller/Murphy P-wave source by scaling the corner frequency by the S/P velocity ratio of the source medium. While these results have not yet led to the identification of a specific physical mechanism for S-wave generation by explosions, they do provide very strong constraints that must be satisfied by any plausible proposed physical mechanism.

[Articles] Explosion Coupling in Frozen and Unfrozen Rock: Experimental Data Collection and Analysis

Bonner, J. L., Leidig, M. R., Sammis, C., Martin, R. J. — 2009-03-19

Nuclear monitoring agencies often use seismic amplitudes to estimate the yields of underground nuclear tests. Any emplacement phenomena that can alter those amplitudes and lead to bias in estimated yields must be considered in the analysis. One condition that might cause such a bias is detonation in frozen rock. Laboratory analyses (Mellor, 1971; Miller and Florence, 1991) have shown that frozen rock has faster seismic velocity and greater compressive strength than unfrozen rock. This increased strength is hypothesized to reduce the seismically estimated yield of an explosion in frozen rock.

To test this hypothesis, we conducted the Frozen Rock Experiment (FRE), a series of explosions in frozen and unfrozen rock, in central Alaska during August 2006. Over 120 seismic instruments were deployed to record six detonations—three in frozen and three in unfrozen-dry media—at a wide range of distances and azimuths. The data acquired show that the frozen test site explosions had significantly larger amplitudes for all phases (P, S, and surface waves) above 8–10 Hz. These data confirm that the frozen rock medium was stronger and resulted in a smaller seismic source radius for the explosions, thus increasing the corner frequency when compared to the unfrozen rock explosions. Between 3 and 9 Hz, the unfrozen shots produced slightly larger S and surface waves resulting in different P/S spectral ratio plots for the frozen and unfrozen shots, possibly affecting regional phase discrimination. We show that the observed amplitude differences for these shots can be effectively modeled using the Mueller-Murphy (1971) explosion source and the in situ P- and S-wave velocities for the two test sites. Differences in the velocities at the frozen and unfrozen rock test sites are caused by minor metamorphic facies changes, saturated versus dry conditions, and the presence of ice in the pores and fractures at the frozen test site. Extrapolation of the results of this study to synthetic nuclear explosions suggests there may not be significant coupling differences between explosions in frozen and unfrozen hard rock.

[Articles] Moment Tensor Inversions of Icequakes on Gornergletscher, Switzerland

2009-03-19

We have determined seismic source mechanisms for shallow and intermediate-depth icequake clusters recorded on the glacier Gornergletscher, Switzerland, during the summers of 2004 and 2006. The selected seismic events are part of a large data set of over 80,000 seismic events acquired with a dense seismic network deployed in order to study the yearly rapid drainage of Gornersee lake, a nearby ice-marginal lake. Using simple frequency and distance scaling and Green’s functions for a homogeneous half-space, we calculated moment tensor solutions for icequakes with M_w-1.5 using a full-waveform inversion method usually applied to moderate seismic events (M_w>4) recorded at local to regional distances (50–700 km). Inversions from typical shallow events are shown to represent tensile crack openings. This explains well the dominating Rayleigh waves and compressive first motions observed at all recording seismograms. As these characteristics can be observed in most icequake signals, we believe that the vast majority of icequakes recorded in the 2 yr is due to tensile faulting, most likely caused by surface crevasse openings. We also identified a shallow cluster with somewhat atypical waveforms in that they show less dominant Rayleigh waves and quadrantal radiation patterns of first motions. Their moment tensors are dominated by a large double-couple component, which is strong evidence for shear faulting. Although less than a dozen such icequakes have been identified, this is a substantial result as it shows that shear faulting in glacier ice is generally possible even in the absence of extreme flow changes such as during glacier surges. A third source of icequakes was located at 100 m depth. These sources can be represented by tensile crack openings. Because of the high-hydrostatic pressure within the ice at these depths, these events are most likely related to the presence of water lenses that reduce the effective stress to allow for tensile faulting.

[Short Notes] Constant Stress Drop from Small to Great Earthquakes in Magnitude-Area Scaling

Shaw, B. E. — 2009-03-19

Earthquakes span a tremendous range of scales, more than 5 orders of magnitude in length. Are earthquakes fundamentally the same across this huge range of scales, or are the great earthquakes somehow different from the small ones? We show that a robust scaling law seen in small earthquakes, with stress drops being independent of earthquake size, indeed holds for great earthquakes as well. The simplest hypothesis, that earthquake stress drops are constant from the smallest to the largest events, combined with a more thorough treatment of the geometrical effects of the finite seismogenic layer depth gives a new magnitude-area scaling that matches the data well and matches the data better over the whole magnitude range than the currently used scaling laws, which have nonconstant stress-drop scaling. This has significant implications for earthquake physics and for seismic hazard estimates.

[Short Notes] Fault Slip Velocities Inferred from the Spectra of Ground Motions

Anil-Bayrak, N. A., Beresnev, I. A. — 2009-03-19

There is much practical need in obtaining independent information about earthquake-source dynamic properties directly from observable data. One such dynamic parameter, the peak slip velocity during earthquake rupture, can be calculated from the corner frequencies of the source spectra, on the assumption of the validity of the ²-source model. To obtain the source terms, observed Fourier spectra should be corrected for the site and path effects. Small-to-moderate earthquakes in Japan recorded on multiple rock sites are well suited for the application of this methodology. The results indicate that the maximum slip velocity of the selected earthquakes ranged from approximately 0.2 to 0.6 m/sec. Direct observation-based determinations of this type provide valuable physical information about the in-situ faulting processes that can be used for constraining dynamics theories of faulting or in ground-motion prediction.

[Short Notes] Omori-Utsu Law c-Values Associated with Recent Moderate Earthquakes in Japan

Enescu, B., Mori, J., Miyazawa, M., Kano, Y. — 2009-03-19

We investigate the early aftershock activity associated with four moderate earthquakes (M_w 6.6–6.7) that occurred recently in Japan. For each aftershock sequence, we examine continuous high-pass filtered seismograms recorded at seismic stations nearby the main fault to identify as many early events as possible. The magnitude of these events is calibrated using aftershocks that are listed in the earthquake catalog of Japan Meteorological Agency (JMA). The analysis of the aftershock decay rates reveals a power-law time dependence with a scaling exponent close to 1.0 that starts from about one minute from the mainshock. Our results demonstrate that the c-value of the Omori–Utsu law is very small, although a lower bound is not established due to completeness problems in the first minute after the mainshock and statistical fluctuations.

[Short Notes] Evidence for Great Tsunamigenic Earthquakes (M 8.6) along the Mexican Subduction Zone

Suarez, G., Albini, P. — 2009-03-19

A very large earthquake took place on 28 March 1787 along the Mexican subduction zone. A unique characteristic of the 1787 event is the large tsunami reported at various coastal locations in southern Mexico. The segment of the coast affected by the strong tsunami coincides with locations where high felt intensities (modified Mercalli intensity [MMI] >VIII) were reported. Assuming, as is generally the case for large and great earthquakes in Mexico, that the fault rupture of the 1787 earthquake encompasses the areas where intensities greater than VIII were reported and where a strong tsunami invaded land, the estimated length of the rupture is approximately 450 km long. This fault length would correspond to an earthquake of approximately M_w 8.6. In the instrumental record there is no evidence of earthquakes of this magnitude in the Middle American subduction zone. This evidence indicates that the rupture area of the 1787 earthquake was at least three times longer that those normally observed for earthquakes that take place in the Mexican subduction zone. The 1787 earthquake appears to have ruptured a long fault segment that more recently broke in earthquakes of more moderate magnitude (M_w 7–8) and relatively short recurrence times of about 30 to 40 yr. Examples of this mode of variable rupture length where great but infrequent earthquakes rupture the fault areas of relatively smaller and more frequent earthquakes have been observed in the Sumatra–Andaman and Colombia–Ecuador plate boundaries, among others.

[Short Notes] A New Trigger Criterion for Improved Real-Time Performance of Onsite Earthquake Early Warning in Southern California

Bose, M., Hauksson, E., Solanki, K., Kanamori, H., Wu, Y.-M., Heaton, T. H. — 2009-03-19

We have implemented and tested an algorithm for onsite earthquake early warning (EEW) in California using the infrastructure of the Southern California Seismic Network (SCSN). The algorithm relies on two parameters derived from the initial 3 sec of P waveform data at a single seismic sensor: period parameter _c and high-pass filtered displacement amplitude P_d. Previous studies have determined empirical relationships between _c and the moment magnitude M_w of an earthquake, and between P_d and the peak ground velocity (PGV) at the site of observation. In 2007, seven local earthquakes in southern California with 4.0≤M_L≤4.7 have triggered the calculation of M_w and PGV by the EEW algorithm. While the mean values of estimated parameters were in the expected range, the scatter was large, in particular for the smallest events. During the same time period the EEW algorithm produced a large number of false triggers due to low trigger thresholds. To improve the real-time performance of the onsite approach, we have developed a new trigger criterion that is based on combinations of observed _c and P_d values. This new criterion removes 97% of previous false triggers and leads to a significant reduction of the scatter in magnitude estimates for small earthquakes.

[Short Notes] An Upper Bound on Rg to Lg Scattering Using Modal Energy Conservation

Stevens, J. L., Xu, H., Baker, G. E. — 2009-03-19

We use conservation of energy to place an upper bound on Lg generated by explosions through Rg scattering. We assume that all energy in Rg scatters very rapidly into the higher mode surface waves that make up Lg. We consider Rg generated directly by the explosion source and by a collocated but possibly shallower compensated linear vector dipole (CLVD) source. Rg scattering may be a viable mechanism for generating Lg at low frequencies (<1 Hz), but at higher frequencies Lg generated directly by the CLVD or explosion source is comparable to or greater than the upper bound on Lg generated by Rg scattering. For shallow sources at frequencies up to 2–3 Hz the direct CLVD waves may be smaller than the direct explosion or the upper bound limit on the scattered waves. Either the direct waves or scattering from the spherical explosion Rg will dominate over scattering from the CLVD generated Rg at all frequencies.

[Short Notes] Improved Statistical Processing for Common-Conversion-Point Stacked Receiver Functions

Leahy, G. M., Collins, J. A. — 2009-03-19

The interpretation of teleseismic receiver functions (RFs) is typically limited by poor constraints on the uncertainty of amplitudes of converted phases. In continental regions these problems are overcome by stacking large amounts of data. In oceanic regions, however, data quality is notoriously noisy and the number of events are often limited by significantly shorter station deployment times. Estimates of RF pulse amplitude uncertainty are, therefore, necessary to allow an analyst to differentiate between real structure in the Earth and random correlations in ambient noise. Here we combine a common-conversion-point stacking technique with multiple-taper correlation RF estimates, which allow frequency-domain weighting. We then compute jackknife statistics to estimate local uncertainties in RF amplitude. We apply this technique to a continental station in Arabia (RAYN) and to the ocean island station at Raratonga, Cook Islands (RAR) in order to compare our method to results obtained via other techniques. The structure we recover matches previous crustal studies at both stations and provides new interpretations of conversions in the upper mantle. At single stations this technique works well to resolve crust and mantle structure up to a depth of 100 km. Geographical dispersion of ray paths at greater depths decreases the number of events per bin and, therefore, increases the uncertainty in converted amplitude. The ability to stack crossing ray paths implies that this method will be well suited to the analysis of data from seismic arrays.

[Short Notes] Interperiod Dependence of Ground-Motion Prediction Equations: A Copula Perspective

Goda, K., Atkinson, G. M. — 2009-03-19

The conventional approach for developing empirical prediction equations of multivariate ground-motion parameters (e.g., response spectra at a set of vibration periods) is based on regression analysis of the individual parameters; their dependence is subsequently characterized by evaluating the linear correlation coefficient of the logarithm of the ground-motion parameters that are corrected by using a ground-motion prediction equation. A copula technique, which offers a flexible way of describing nonlinear dependence among multivariate data in isolation from their marginal distribution functions, can be used to validate this two-step approach. This new perspective on the multivariate aspects of the development of ground-motion prediction equations is explored through analysis of the strong ground-motion data associated with the Boore and Atkinson (2008) Pacific Earthquake Engineering Research–Next Generation Attenuation of Ground Motions (PEER-NGA) relation. The analysis results demonstrate that multivariate ground-motion parameters can be marginally modeled as a lognormal variate, and their interperiod dependence can be captured by using the normal copula. This finding validates the conventional two-step approach.

[Short Notes] Effect of Spatial Correlation on Estimated Ground-Motion Prediction Equations

Hong, H. P., Zhang, Y., Goda, K. — 2009-03-19

The consideration of spatially correlated seismic hazard could be of importance for seismic risk assessment. The estimation of this correlation for the peak ground acceleration and the pseudospectral acceleration has been reported in the literature, although it is not presented as a necessary ingredient for developing ground-motion prediction equations (GMPEs). In the present study, we show that spatial correlation can be incorporated in the existing regression algorithms given by Joyner and Boore for assessing a GMPE. The modified algorithms can be used to estimate both GMPE coefficients and spatial correlation model parameters simultaneously. In particular, they are used to investigate the influence of spatial correlation on GMPEs and to assess parameters of an empirical spatial correlation model by considering a set of 592 California records. Analysis results indicate that the effects of incorporating spatial correlation on the estimated GMPEs are insignificant, and that spatial correlation parameters obtained using the modified algorithm are similar to those estimated based on statistical analysis of regression residuals.

[Short Notes] On the Use of High-Resolution Topographic Data as a Proxy for Seismic Site Conditions (VS30)

Allen, T. I., Wald, D. J. — 2009-03-19

An alternative method has recently been proposed for evaluating global seismic site conditions, or the average shear velocity to 30 m depth (V_S30), from the Shuttle Radar Topography Mission (SRTM) 30 arcsec digital elevation models (DEMs). The basic premise of the method is that the topographic slope can be used as a reliable proxy for V_S30 in the absence of geologically and geotechnically based site-condition maps through correlations between V_S30 measurements and topographic gradient. Here we evaluate the use of higher-resolution (3 and 9 arcsec) DEMs to examine whether we are able to resolve V_S30 in more detail than can be achieved using the lower-resolution SRTM data. High-quality DEMs at resolutions greater than 30 arcsec are not uniformly available at the global scale. However, in many regions where such data exist, they may be employed to resolve finer-scale variations in topographic gradient, and consequently, V_S30. We use the U.S. Geological Survey Earth Resources Observation and Science (EROS) Data Center’s National Elevation Dataset (NED) to investigate the use of high-resolution DEMs for estimating V_S30 in several regions across the United States, including the San Francisco Bay area in California, Los Angeles, California, and St. Louis, Missouri. We compare these results with an example from Taipei, Taiwan, that uses 9 arcsec SRTM data, which are globally available.

The use of higher-resolution NED data recovers finer-scale variations in topographic gradient, which better correlate to geological and geomorphic features, in particular, at the transition between hills and basins, warranting their use over 30 arcsec SRTM data where available. However, statistical analyses indicate little to no improvement over lower-resolution topography when compared to V_S30 measurements, suggesting that some topographic smoothing may provide more stable V_S30 estimates. Furthermore, we find that elevation variability in canopy-based SRTM measurements at resolutions greater than 30 arcsec are too large to resolve reliable slopes, particularly in low-gradient sedimentary basins.

[Articles] Estimating Earthquake Magnitude with Early Arrivals: A Test Using Dynamic and Kinematic Models

Murphy, S., Nielsen, S. — 2009-01-21

Recent studies on seismological data indicate that earthquake magnitude scales with either the dominant period or the peak amplitude in the seismogram’s first few seconds. At first sight, this may indicate that the earthquake’s final size is somehow related to the way rupture starts. One working hypothesis is that strong radiation from the initial phase of rupture is indicative of a triggering asperity releasing a consistent amount of elastic energy, with the potential to drive the fracture to large extents. We tested this concept with a number of numerical simulations, but within the models investigated, scaling was found only for ruptures extending up to about four times the size of the initial asperity; at larger distances the correlation was lost. Alternatively, a careful kinematic analysis of the earthquake source radiation shows that the initial signal recorded at any station does not necessarily correspond to the rupture initiation but may represent an extended portion of the radiating source. Using the concept of isochrones, we show that the apparent scaling may be explained by a simple kinematic model respecting causality, up to a given magnitude threshold where the scaling relation saturates. The saturation level is in agreement with that observed in some, but not all, of the real seismicity catalogs.

[Articles] Developing Local to Near-Regional Explosion and Earthquake Discriminants

Zeiler, C., Velasco, A. A. — 2009-01-21

We develop local to near-regional (0–200 km) amplitude discriminants using earthquakes, single-fired explosions, and delayed-fired explosions. We use the Source Phenomenology Experiment (SPE) broadband data, collected across the Colorado Plateau during the summer of 2003, which recorded explosions in hard- and soft-rock mines. We optimized a local surface wave magnitude scale derived from a stable regional surface wave magnitude (Russell, 2006) for the sources. However, the magnitude of the source and the distance to the station limit the surface wave energy, which can cause instability in the surface wave magnitudes. Magnitude scales for the local phases (Pg, Lg/Sg, and Rg) were also tested and compared. When an azimuthal station correction was applied to each magnitude scale, there was an average reduction of standard deviation by 16%–58%. Regional and teleseismic discrimination techniques were optimized for local distance, and our results show that lithology of the source is a critical component in discriminating events at local distances. We also found that magnitude and amplitude ratios are effectively used to discriminate between small earthquakes and explosions at local distances, with each performing the best in the hard-rock lithology. Furthermore, the ratio techniques perform equally in both lithologies if multiple stations are used to establish the ratio values; we find that an average of four or five stations will generally have a 90% or better correct identification for both types of ratios. Finally, we find that the source lithology, local site effects, and large-scale geologic features control most of the variability in the amplitude measurements.

[Articles] Characteristics of Chemical Explosive Sources from Time-Dependent Moment Tensors

Yang, X., Bonner, J. L. — 2009-01-21

Using a frequency-domain linear inversion technique and near-source broadband data, we inverted for the time-dependent source moment tensors of eight chemical explosions detonated in an open-pit coal mine during the Source Phenomenology Experiments (SPE) conducted by a consortium of U.S. research institutions to investigate a suite of explosive-source related problems. The moment tensors of the explosions from the inversion are dominated by their isotropic components regardless of variations between explosions in source size, confinement condition, and whether the explosion was on a bench and collapsed the vertical face of the bench. The percentage of isotropic moment-tensor component ranges from 96% to 98% for largest part of the source-time histories. Source-configuration variations result in differences that are most apparent in long-period moment-tensor spectra reflecting possible secondary source effects such as cylindrical source shape, spall, and compensated linear vector dipole (CLVD). Unconfined explosions show more oscillatory diagonal moment-tensor component time histories than confined and partially confined explosions possibly due to stronger free-surface effects such as material cast. Compared with pit explosions, deviatoric components of moment tensors of the two bench explosions are of higher amplitudes. There is a discernible long-period (<5 Hz) signal on one of the off-diagonal components, which could be related to the presence of the bench face in the source region and the horizontal material cast by the explosions. Although off-diagonal moment-tensor components comprise a small portion of the moment tensor, they are capable of generating a disproportionally large amount of shear waves.

[Articles] Spatiotemporal Complexity of Continental Intraplate Seismicity: Insights from Geodynamic Modeling and Implications for Seismic Hazard Estimation

Li, Q., Liu, M., Stein, S. — 2009-01-21

Continental intraplate seismicity seems often episodic, clustered, and migrating. The observed seismicity shows both spatial clustering in seismic zones and scattering across large plate interiors, temporal clustering followed by long periods of quiescence, and migration of seismicity from one seismic zone to another. Here, we explore the complex spatiotemporal patterns of intraplate seismicity using a 3D visco-elasto-plastic finite-element model. The model simulates tectonic loading, crustal failure in earthquakes, and coseismic and postseismic stress evolution. For a laterally homogeneous lithosphere with randomly prespecified perturbations of crustal strength, the model predicts various spatiotemporal patterns of seismicity at different timescales: spatial clustering in narrow belts and scattering across large regions over hundreds of years, connected seismic belts over thousands of years, and widely scattered seismicity over tens of thousands of years. The orientation of seismic belts coincides with the optimal failure directions associated with the assumed tectonic loading. Stress triggering and migration cause spatiotemporal clustering of earthquakes. When weak zones are included in the model the predicted seismicity initiates within the weak zones but then extends far beyond them. If a fault zone is weakened following a large earthquake, repeated large earthquakes can occur on the same fault zone even in the absence of strong tectonic loading. These complex spatiotemporal patterns of intraplate seismicity predicted in this simple model suggest that assessment of earthquake hazard based on the limited historic record may be biased toward overestimating the hazard in regions of recent large earthquakes and underestimating the hazard where seismicity has been low during the historic record.

[Articles] Recurrence of Large Earthquakes in Magmatic Continental Rifts: Insights from a Paleoseismic Study along the Laikipia-Marmanet Fault, Subukia Valley, Kenya Rift

Zielke, O., Strecker, M. R. — 2009-01-21

The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the generation of large-magnitude earthquakes. On 6 January 1928, the M_S 6.9 Subukia earthquake occurred on the Laikipia–Marmanet fault, the eastern rift-bounding structure of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rift’s seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the ruptured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain.

We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes. We also made a total station survey of the site topography and mapped the site geology.

We show that the Laikipia–Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, including the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of 7.5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U=0.09–0.15 mm/yr, constraining the scarp age to 50–85 ka. Assuming an average fault dip of 55°–75°, the preferred uplift rate (0.15 mm/yr) accommodates approximately 10%–20% of the recent rate of extension (0.5 mm/yr) across the Kenya rift.

[Articles] Fragmentary Evidence of Great-Earthquake Subsidence during Holocene Emergence, Valdivia Estuary, South Central Chile

Nelson, A. R., Kashima, K., Bradley, L.-A. — 2009-01-21

A reconnaissance of Holocene stratigraphy beneath fringing marshes of the Valdivia estuary, where an M 9.5 earthquake caused 1–2 m of regional coseismic subsidence in 1960, shows only fragmentary evidence of prehistoric coseismic subsidence. In most of the 150 hand-driven cores that were examined, a distinct unconformity separates 0.5–1.5 m of late Holocene tidal and floodplain mud, peat, and sand from underlying middle Holocene subtidal mud and sand. At the Las Coloradas site, where stratigraphy is best preserved, two A horizons of marsh and meadow soils abruptly overlain by sand and mud probably record coseismic subsidence shortly followed by tsunamis. The amount of subsidence during the earthquakes proved difficult to reconstruct with a diatom transfer function because of differences between modern and fossil diatom assemblages. Maximum ages on macrofossils from the two A horizons at the Las Coloradas site of 1.7–1.3 ka and 2.7–1.7 ka allow correlation of the younger horizon with either of two of six -dated A horizons buried by tsunami sand or post-tsunami tidal sand 200 km to the south at Maullín, and with a lake-wide mass wasting event in Lago Puyehue, 100 km to the southeast. Tidal records of prehistoric coseismic subsidence at Valdivia are scarce because of a sea-level fall of 3–8 m over the past 6000 years, erosion of marsh and meadow soils during subsidence-induced flooding of the estuary, and largely complete land-level recovery during cycles of coseismic subsidence and postseismic uplift.

[Articles] The Mw 7.7 Tocopilla Earthquake of 14 November 2007 at the Southern Edge of the Northern Chile Seismic Gap: Rupture in the Deep Part of the Coupled Plate Interface

Delouis, B., Pardo, M., Legrand, D., Monfret, T. — 2009-01-21

The slip distribution of the M_w 7.7 Tocopilla earthquake was obtained from the joint inversion of teleseismic and strong-motion data. Rupture occurred as underthrusting at the base of the seismically coupled plate interface, mainly between 35 and 50 km depth. From the hypocenter, located below the coast 25 km south of the town of Tocopilla, the rupture propagated 50 km northward and 100 km southward. Overall, the slip distribution was dominated by two slip patches, one near the hypocenter and the other 70 km to the south where slip reached its maximum value (3 m). An additional branch of moderate slip propagated at shallower depth toward the west near the northern tip of the Mejillones peninsula. Rupture velocity remained close to 2.8 km/sec, with a total rupture duration of 45 sec. The first 2 weeks of aftershocks located with a local seismic network display a strong correlation with the slip distribution. The 2007 rupture ended below the Mejillones peninsula, where the 1995 Antofagasta rupture also ended (Ruegg et al., 1996; Delouis et al., 1997; Pritchard et al., 2006). This corroborates the role of barrier played by this structure. The downdip end of the seismically coupled zone at 50 km depth, evidenced by previous studies for the 1995 event, is also confirmed. The 2007 Tocopilla earthquake contributed only moderately to the rupturing of the great northern Chile seismic gap, which still has the capacity for generating a much larger underthrusting event.

[Articles] Seismotectonic Model of the Kangra-Chamba Sector of Northwest Himalaya: Constraints from Joint Hypocenter Determination and Focal Mechanism

Kumar, N., Sharma, J., Arora, B. R., Mukhopadhyay, S. — 2009-01-21

The existing seismological network in the Kangra–Chamba sector has been upgraded with 12 three-component digital seismometers to obtain new insight on the nature and sources of continued clustered seismicity in this part of northwest Himalaya. A combination of travel-time–distance plots and travel-time inversion of P and S phases have been used to derive a 1D velocity model for the region. The minimum 1D velocity model divides the average 44 km thick crust into four layers. The top ~10 km thick layer represents the metamorphosed sediments of the Chamba nappe that dominates the surface geology of the study area. Suggestion of a thin low-velocity layer at 15 km depth possibly marks the detachment zone separating the downgoing Indian plate from the overriding wedge. The improved locations of epicenters show close clustering of seismic events immediately northeast of the epicenter of the 1905 Kangra earthquake, while away from this zone the seismicity in the Chamba sector has more even distribution. In the later sector, space-depth distribution of hypocenters suggests that strain resulting from the ongoing collision of the Indian plate with Asia is being consumed by reverse-fault movement on the Chamba thrust. The clustered seismicity in the Kangra sector has three distinct source regions and mechanisms: (1) southward displacement of the thick Chamba nappe sheet over the Panjal imbricate zone along the Panjal thrust accounts for the seismicity at shallow depths of less than 7 km, (2) the nucleation of strains where the northeast dipping main boundary thrust (MBT) merges with the detachment plane produces focused seismicity near this junction, and (3) the seismicity in a small pocket below the plane of detachment appears to be a consequence of stresses generated at the base of the northeast dipping detachment plane by the transverse structure.

[Articles] Seismicity at the Eastern End of the 1944 Tonankai Earthquake Rupture Area

Obana, K., Kodaira, S., Kaneda, Y. — 2009-01-21

The rupture areas of the large interplate thrust earthquakes along the Nankai trough, offshore southwestern Japan, are divided into several segments. The 1944 Tonankai earthquake ruptured one of the segments off of the Kii Peninsula. In 2005, we conducted an ocean-bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake. Little seismic activity was observed on the plate interface; most earthquake activity was within the subducting and overriding plates. Aftershocks of the 2004 off Kii Peninsula earthquake, which was an intraplate earthquake in the subducting Philippine Sea plate, were mainly located in the subducting oceanic crust and uppermost mantle. However, several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge, with a focal mechanism indicating deformation by a subducting seamount. The earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 Tonankai earthquake. In contrast, few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake. Difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling, which, in turn, caused the observed differences in the deformation of the sedimentary wedge. The spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the Nankai trough.

[Articles] Source Rupture Process of the 2004 Chuetsu, Mid-Niigata Prefecture, Japan, Earthquake Inferred from Waveform Inversion with Dense Strong-Motion Data

Asano, K., Iwata, T. — 2009-01-21

The kinematic source rupture process of the 2004 Chuetsu, mid-Niigata prefecture, Japan, earthquake, is estimated from strong-motion data by the linear waveform inversion method. In order to develop appropriate Green’s functions, one-dimensional velocity structure models for each station are constructed by modeling the aftershock waveforms. The estimated top of the bedrock is deeper at the western side of the fault and relatively shallow at the eastern side. This tendency coincides with other investigations of seismic tomography and microtremor array observations. The obtained source model shows large slips in the vicinity of the hypocenter. The reverse slipping rupture originated from the deeper part of the fault and propagated toward the up-dip and southwest directions. The near-surface slip is small. Two tests are demonstrated to check the stability of the obtained source model. The first test examines the effects of the number of available stations on the solution, and the second test examines how the difference in the target waveform type affects the solution. The number of stations affects the variation of slip amount, and more than 12 stations appear to be sufficient to obtain a stable solution in this case. The difference in the target wave type in the data set does not significantly affect the solution if the number of stations is sufficient and the Green’s functions are well calibrated. The model obtained using calibrated velocity structure models shows clear image of high-slip area compared to the model obtained using a single velocity structure model.

[Articles] Array Back-Projection Imaging of the 2007 Niigataken Chuetsu-oki Earthquake Striking the World's Largest Nuclear Power Plant

Honda, R., Aoi, S. — 2009-01-21

The 2007 Niigataken Chuetsu-oki earthquake occurred near the Kashiwazaki–Kariwa nuclear power plant in Japan, the largest in the world. The strong motions were recorded by seven seismometers installed at the foundation slab (base-mat) of the plant and exceeded the design level of the ground motion for the plant. The strong motion observed by the seismographs in and around the plant show high coherency with three significant pulses. In order to understand the cause of these pulses, the rupture process of the earthquake was estimated using these seismograms. The seismograph network was taken into account as a dense array and semblance-enhanced waveform stacking was performed. By projecting the power of the stacked waveforms onto the fault plane, the asperities that generated significant pulses were successfully separated. The first and third pulses were generated at the hypocenter and the southwest edge of the rupture zone, respectively. The rupture propagated toward the southwest and terminated offshore from the power plant. The overall pattern of the imaged asperities coincides well with the slip distribution determined by conventional waveform inversions.

[Articles] Investigation of Fault Mechanisms during Geothermal Reservoir Stimulation Experiments in the Cooper Basin, Australia

Baisch, S., Voros, R., Weidler, R., Wyborn, D. — 2009-01-21

Subsequent to an initial reservoir stimulation in 2003, a second long-term fluid-injection experiment was conducted in the Cooper Basin, Australia, in 2005 to further enhance and enlarge the existent geothermal reservoir. Approximately 25,000 m³ of water were injected into the granitic crust at 4250 m depth, thereby inducing about 16,000 seismic events, which were recorded by a local eight-station network. Event magnitudes calibrated by recordings of the permanent network of Geosciences Australia are in the range between M_L-1.2 and 2.9 with seismic moments between 10⁹ and 10¹³ N m. Hypocenter locations could be determined for 8886 events. The spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation, which is further extended in the course of restimulation.

Early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well. The immediate vicinity of the injection well remains seismically quiet during restimulation. These effects can be explained by a simple Kaiser effect model, where the induced seismicity is controlled by the in situ fluid pressure increase.

Compound fault-plane solutions based on P-phase polarities indicate a similar fault mechanism for nearly all events. The dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution. A small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone. Cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly, accumulating up to several centimeters of shear displacement.

[Articles] An Application of Relative Moment Tensor Inversion to the 26 December 2003 Mw 6.6 Iran-Bam Earthquake

Shomali, Z. H., Roberts, R. G. — 2009-01-21

A relative moment tensor inversion technique is used to retrieve the focal mechanisms of teleseismic earthquakes. The observed data consists of amplitude spectra of the direct P phase on vertical and the direct S phase on rotated horizontal components. The effect of propagation paths is minimized using relative amplitude spectra of close-lying earthquakes recorded by common stations. The inversion is carried out for six components of the moment tensors. The focal mechanisms are determined using a linear weighted least-squares approach (signed spectral moment). The method is applied to seven earthquakes that occurred close to the 26 December 2003 M_w 6.6 Iran-Bam earthquake and were recorded at regional–teleseismic distances with good azimuthal coverage. Most focal mechanisms obtained in this study show a predominantly right-lateral strike-slip component, similar to those obtained by other independent estimates. The application of the relative moment tensor inversion using spectral amplitudes combined with polarity data proves to be flexible and effective in estimating focal mechanisms of teleseismic data and the method is suitable for routine implementation in seismic networks.

[Articles] Seismic Location Bias and Uncertainty in the Presence of Correlated and Non-Gaussian Travel-Time Errors

Bondar, I., McLaughlin, K. — 2009-01-21

Correlated travel-time errors bias both seismic location and location uncertainty estimates. Methodologies are introduced to model travel-time correlation structures and to account for them in standard location algorithms. A robust method based on copula theory is used for estimating variogram models for travel-time error. Generic, transportable variogram models are presented for Pn and teleseismic P phases. A linearized iterative location algorithm is introduced that takes into account the correlated travel-time error structure represented by the covariance matrix generated from the generic variogram models for a specific network. We show that ignoring the correlated error structure leads to rapidly deteriorating error ellipse coverage with increasingly correlated networks. We demonstrate through the relocation of ground truth (GT) events that taking into account the correlated model error structure significantly improves error ellipse coverage and, for unbalanced networks, reduces location bias. Monte Carlo experiments show that the deteriorating effect of non-Gaussian error distributions, albeit consistent and nonnegligible, is of secondary importance compared to the penalty paid for ignoring the correlation structure in travel-time errors.

[Articles] LOTOS Code for Local Earthquake Tomographic Inversion: Benchmarks for Testing Tomographic Algorithms

Koulakov, I. — 2009-01-21

We present the LOTOS-07 code for performing local earthquake tomographic inversion, which is freely available (see the Data and Resources section for the Web site). The initial data for the code are the arrival times from local seismicity and coordinates of the stations. It does not require any information about the sources. The calculations start from absolute location of sources and estimates of an optimal 1D-velocity model. Then the sources are relocated simultaneously with the 3D-velocity distribution during iterative coupled tomographic inversions. The code allows results to be compared based on node or cell parameterizations. The synthetic dataset used for testing the code is based on source–receiver configurations from a real experiment in Costa Rica. The travel times for this dataset are computed by 3D tracing through a rather complicated synthetic model and are perturbed with realistic noise. We also present a series of synthetic datasets with unknown sources and velocity models (see the Data and Resources section for the Web site) that can be used as blind benchmarks for testing different tomographic algorithms. We encourage other users of tomography algorithms to join the program on creating benchmarks that can be used to check existing codes.

[Articles] Constraining Event Depths and Crustal Velocities Using Regional Depth Phases

Stroujkova, A. — 2009-01-21

A method for determining average crustal seismic velocities above a cluster of shallow seismic events using depth phases observed at regional and teleseismic arrays has been developed. The method uses travel-time differences between the direct and the surface-reflected phases to simultaneously find the event depths and the best-fitting seismic velocities in the hypocentral area. Simplicity of the method makes it suitable for rapid data analysis as it requires a relatively small amount of data and may provide useful information about the seismic velocities in remote regions with shallow seismicity. However, the method requires accurate depth phase identification, which is often a difficult task.

The technique has been applied to find above-source seismic velocities and depths of 14 clustered events from the Xiuyan (1999–2000) sequence in China. The estimated depths of the events are between 5 and 8 km. The obtained velocities are and , which corresponds to a slightly elevated V_P/V_S ratio of 1.86±0.12 as compared to the value predicted from IASP91 (V_P/V_S=1.72; see Kennett and Engdahl, 1991).

[Articles] Denoising of Seismograms Using the S Transform

Parolai, S. — 2009-01-21

The growing number of urban seismological experiments within the framework of seismic hazard studies has increased the necessity of effective tools for denoising seismograms. Because of the frequency dependency of seismic noise and the nonstationarity of the recorded signal, tools that can effectively take into account the frequency-time variation of the seismic recordings are more suitable for fulfilling this task. The S transform is an invertible time-frequency spectral localization technique that combines elements of wavelet transforms and short-time Fourier transforms. In this study, a customized thresholding technique is applied to the S-transform coefficients for obtaining an optimally (in the sense of the maximum increase of the signal-to-noise ratio with a minimal loss of information) denoised seismogram. Tests performed with synthetic data allow us to calibrate the optimal denoising procedure parameters and show the effectiveness of the proposed method when compared with standard filtering techniques. The application of a combination of denoising with time-frequency filtering on real data shows the potential of the method for extracting lower amplitude dispersive arrivals from seismograms.

[Articles] Source Spectra and Site Response from S Waves of Intermediate-Depth Vrancea, Romania, Earthquakes

Oth, A., Parolai, S., Bindi, D., Wenzel, F. — 2009-01-21

Seismograms from 55 intermediate-depth Vrancea earthquakes (M 4.0–7.1) recorded at 43 stations of an accelerometric network in Romania are used to derive source spectra and site amplification functions from S waves in the frequency range 0.5–20 Hz with the generalized inversion technique (GIT) (Castro et al., 1990). Attenuation is taken into account using the nonparametric attenuation functions derived by Oth et al. (2008) from the same dataset, and the attenuation-corrected data are then split into source and site contributions. The source spectra follow the ^-2 model (Brune, 1970, 1971) with high corner frequencies and a related Brune stress drop of the order of 100 MPa. The site amplification functions are separately determined for both horizontal and vertical components. Contrary to widespread expectation, the vertical component shows significant amplification effects at high frequencies. The H/Z ratios determined from the GIT results compare well with H/V ratios computed directly from the S-wave window of the accelerograms (Lermo and Chávez-García, 1993). The basic assumption for the determination of site effects from H/V ratios is that the vertical component is not or only little affected by site effects. For Vrancea earthquakes, this assumption is incorrect; consequently, site effects should not be estimated from H/V ratios. The reason for this peculiar fact is the geometry of intermediate-depth seismicity that leads to almost vertical ray paths beneath the stations.

[Articles] Large Effects of Moho Reflections (SmS) on Peak Ground Motion in Northwestern Taiwan

Liu, K.-S., Tsai, Y.-B. — 2009-01-21

A total of 336 three-component strong-motion recordings from the M_w 6.35 Niu Dou earthquake of 25 June 1995 at a focal depth of 39.9 km in northern Taiwan are used to study the effects on strong ground motion due to Moho reflection of S waves. The residuals of both horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) recorded from the earthquake are analyzed. The results confirm that many Class E soft soil stations in the Taipei Basin and the Ilan Plain had the expected large amplification of about 1.7 and 1.5 times, respectively, the predicted median PGA values. Surprisingly, a large group of Class C or D dense and stiff soil sites in Taoyuan (TCU007), Lungtan (TCU013), Guanshi (TCU021), Hsinchu (TCU095), and Miaoli (TCU047) areas in northwestern Taiwan had unusually large amplification of about 3.4–8.1 and 1.7–3.3 times the predicted median PGA and PGV values, respectively. They are interpreted in terms of focusing and interference between SmS waves reflected from the horizontal and inclined portions of an east-dipping Moho discontinuity in this area. This interpretation is supported by the close agreement between the expected amplitudes and arrival times of the largest shear waves with the observed data. Our results suggest that when a damaging earthquake occurs near an inclined Moho boundary, the reflected SmS waves can result in significantly amplified ground motions at distances beginning about 50 km. The exact distance range will depend on the thickness of the crust and the dip angle of the Moho boundary.

[Articles] Calibration of an ML Scale in the Alborz Region, Northern Iran

Askari, R., Ghods, A., Sobouti, F. — 2009-01-21

We have calculated empirical attenuation curves for the local magnitude scale (M_L) in the Alborz region of northern Iran. The maximum trace amplitudes derived from synthetic Wood–Anderson seismograms of 1290 records of 59 events in the distance range of 8.5–550 km recorded by the short-period Iranian Seismic Telemetry Network (ISTN) were inverted for the attenuation curve, magnitudes, and station corrections. The earthquakes ranged from M_L3.25 to 5.65 and were recorded at 26 stations in the region. We used both nonparametric and parametric least-squares methods for inversion. The resulting parametric equation is logA₀=-1.1725log(R)-0.0021R-0.4450. The two methods yielded very similar results and are very close to Richter’s original attenuation curve for southern California. The station corrections vary between -0.42 and +0.47 suggesting that local site effects may have a strong influence on the amplitudes.

[Articles] Ground-Motion Prediction Equation for SI Based on Spectral Acceleration Equations

Bradley, B. A., Cubrinovski, M., MacRae, G. A., Dhakal, R. P. — 2009-01-21

Spectrum intensity (SI), defined as the integral of the pseudospectral velocity of a ground motion from 0.1 to 2.5 sec, has recently been shown to be an intensity measure that efficiently predicts the seismic response of both liquefiable and nonliquefiable soil deposits as well as the seismic demands on pile foundations embedded in such deposits. In order for such an intensity measure to be used in performance-based assessment and design, ground-motion prediction relations are required to develop ground-motion hazard curves in terms of SI for various sites. As such relationships developed specifically for SI are sparse, we propose the development of a relationship based on current ground-motion prediction relations for spectral acceleration, which are available in most regions of seismic activity. Comparison with a direct prediction equation for SI provides a validation of the proposed approach. It is illustrated that SI is an intensity measure with a good predictability, thereby further promoting its attractiveness for use in reliability-based seismic response analysis.

[Articles] Near-Fault Earthquake Ground-Motion Simulation in the Grenoble Valley by a High-Performance Spectral Element Code

Stupazzini, M., Paolucci, R., Igel, H. — 2009-01-21

Near-fault effects are known to produce specific features of earthquake ground motion (such as long-period velocity pulses and directivity) that cannot be predicted by numerical approaches involving vertical plane wave propagation in one-dimensional (1D) soil models that are used as a standard in engineering applications. Coupling near-fault conditions with site effects induced by complex geological structures (such as deep alluvial basins or steep topographic irregularities) further contributes to the complexity of earthquake ground motion and to the difficulty to provide reliable predictions without making use of large-size 3D numerical simulations. In this article, we present a parametric study of the seismic response of the Grenoble Valley, France (due to an M_w 6 seismic source at some 10 km epicentral distance from the urban area) that was carried out in the framework of an international benchmark for earthquake ground-motion prediction. The spectral element code GeoELSE for seismic-wave propagation analyses in 3D heterogeneous media, in the linear and nonlinear range, was used for this purpose; full advantage was taken of its implementation on parallel computer architectures. After introducing GeoELSE and its parallel performance, and after introducing some of its validation benchmarks, the spatial variability of the seismic response of the Grenoble Valley is quantitatively investigated taking into account two effects: (i) the hypocenter location and (ii) the nonlinear soil behavior through a nonlinear viscoelastic soil model. Finally, numerical results are compared with available data and attenuation relationships of peak values of ground motion in the near-fault region. Based on the results of this work, the unfavorable interaction between fault rupture, radiation mechanism, and complex geological conditions may give rise to large values of peak ground velocity (exceeding 1 m/sec) even in low-to-moderate seismicity areas; it may therefore considerably increase the level of seismic risk, especially in highly populated and industrially active regions, such as the Alpine valleys.

[Articles] A Wavelet-Optimized Adaptive Grid Method for Finite-Difference Simulation of Wave Propagation

Pei, Z.-L., Fu, L.-Y., Yu, G.-X., Zhang, L.-X. — 2009-01-21

To accurately and efficiently model the complex spatiotemporal localization of wave propagation in heterogeneous media, we present a wavelet-optimized gridding scheme for the staggered-grid finite-difference (FD) numerical method. An average-interpolating (AI) wavelet transform with its wavelet coefficients obtained from linear combinations of samples, rather than from integrals, enables the gridding scheme constructed in the physical space to determine where to refine/coarsen the grid adapted to the complex spatial localization of problems and the relatively irregular sampling of seismic signals. The AI wavelet is a perfect transform with its wavelet coefficients decaying rapidly with increasing resolution. The wavelet scale parameter is related directly to the level of meshes. We can reconstruct the solution with good accuracy using a finite number of terms of wavelet expansion under some threshold sets for refinement/coarsening in a low- or high- degree of regularity. The adaptive meshes, in response to the heterogeneities of media and the resolution scales of wave fields, lead not only to a great saving of computation time and memory but also to an enabling method for solving problems with high accuracy in some local complex zones. Numerical examples demonstrate that the proposed wavelet-optimized adaptive FD scheme is efficient and accurate for wave propagation simulation.

[Articles] Effects of Topography on Seismic-Wave Propagation: An Example from Northern Taiwan

Lee, S.-J., Komatitsch, D., Huang, B.-S., Tromp, J. — 2009-01-21

Topography influences ground motion and, in general, increases the amplitude of shaking at mountain tops and ridges, whereas valleys have reduced ground motions, as is observed from data recorded during and after real earthquakes and from numerical simulations. However, recent publications have focused mainly on the implications for ground motion in the mountainous regions themselves, whereas the impact on surrounding low-lying areas has received less attention. Here, we develop a new spectral-element mesh implementation to accommodate realistic topography as well as the complex shape of the Taipei sedimentary basin, which is located close to the Central Mountain Range in northern Taiwan. Spectral-element numerical simulations indicate that high-resolution topography can change peak ground velocity (PGV) values in mountainous areas by ±50% compared to a half-space response. We further demonstrate that large-scale topography can affect the propagation of seismic waves in nearby areas. For example, if a shallow earthquake occurs in the I-Lan region of Taiwan, the Central Mountain Range will significantly scatter the surface waves and will in turn reduce the amplitude of ground motion in the Taipei basin. However, as the hypocenter moves deeper, topography scatters body waves, which subsequently propagate as surface waves into the basin. These waves continue to interact with the basin and the surrounding mountains, finally resulting in complex amplification patterns in Taipei City, with an overall PGV increase of more than 50%. For realistic subduction zone earthquake scenarios off the northeast coast of Taiwan, the effects of topography on ground motion in both the mountains and the Taipei basin vary and depend on the rupture process. The complex interactions that can occur between mountains and surrounding areas, especially sedimentary basins, illustrate the fact that topography should be taken into account when assessing seismic hazard.

[Articles] Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Dutta, U., Sen, M. K., Biswas, N., Yang, Z. — 2009-01-21

This study deals with shallow sedimentary structure of the Anchorage basin in Alaska. For this purpose, inversion of site response [SR(f)] data in the frequency range 0.5–11.0 Hz from various sites of the basin has been performed using the simulated annealing method to compute subsurface layer thickness, shear-wave velocity (β), density, and shear-wave quality factor. The one-dimensional (1D) models for the aforementioned parameters were obtained with preset bounds on the basis of available geological information such that the L-2 norm error between the observed and computed site response attained a global minimum. Next, the spatial distribution of the important parameter β was obtained by interpolating values yielded by the 1D models. The results indicate the presence of three distinct velocity zones as the source of spatial variation of SR(f) in the Anchorage basin. In the uppermost part of the basin, the β values of fine-grain Quaternary sediments mainly lie in the range of 180–500 m/sec with thickness varying from 15 to 50 m. This formation overlies relatively thick (80–200 m) coarse-grain Quaternary sediments with β values in the range of 600–900 m/sec. These two Quaternary units are, in turn, overlain on Tertiary sediments with β>1000 m/sec located at depths of 100 and 250 m, respectively, in the central and western side along the Knik Arm parts of the basin. The important implication of the result is that the sources of spatial variation of SR(f) in the Anchorage basin for the frequency band 0.5–11 Hz, besides in the uppermost 30 m, are found to be deeper than this depth. Thus, use of commonly considered geological formations in the depth intervals from 0 to 30 m for the ground-motion interpretation will likely yield erroneous results in the Anchorage basin.

[Articles] Comparison of Site Classification from VS30, VS10, and HVSR in Italy

Gallipoli, M. R., Mucciarelli, M. — 2009-01-21

The aim of this work is to verify if (1) the average of shear-wave velocity from the surface to 30 m depth (V_S30) is a good proxy of site amplification in a country with complex geology like Italy, (2) the grouping of V_S30 in different soil classes and relevant spectra in the Italian seismic codes is adequate, and (3) shallow shear-wave profiles (V_S10) could be a more economical tool for site classification. To answer the first two questions, we examined 40 estimates of V_S30 derived from microzonation projects we performed in Italy (27 obtained with downhole measurements, plus 13 velocity profiles obtained with surface techniques). In all the sites, we installed seismic instrumentation to record earthquakes and to estimate site response using horizontal to vertical spectral ratios (HVSR). The comparison between HVSR and V_S30 showed in about one-third of the sites that V_S30 is not a good proxy of observed amplification effects if the site does not have a monotonically increasing velocity profile. The reason V_S30 does not provide satisfactory estimates in Italy is linked to peculiar geological settings that are widespread in the country. We then compared the observed amplifications from earthquake HVSR data with the ones provided by the Italian seismic code, noting a substantial underestimation by the code, a somewhat unsettling situation because HVSR is usually considered to be a lower bound for amplification estimated with other techniques.

Finally, we studied 45 V_S downhole profiles to 30 m depth performed also at sites where earthquake recordings are not available. On this data set, we noticed that V_S10 could predict site classification with the same performances of V_S30. We consider alternative soil classification schemes that include soil frequency besides the velocity profile. In this two-parameter approach, V_S10 could be substituted for V_S30.

[Articles] Time-Lapse Changes of Seismic Velocity in the Shallow Ground Caused by Strong Ground Motion Shock of the 2000 Western-Tottori Earthquake, Japan, as Revealed from Coda Deconvolution Analysis

Sawazaki, K., Sato, H., Nakahara, H., Nishimura, T. — 2009-01-21

A large earthquake shock often drops the seismic velocity of the shallow ground. However, it is not clear whether the dropped velocity recovers shortly after the earthquake shock or not. The purpose of this article is to report the time-lapse changes of seismic velocity in the shallow ground after the 2000 Western-Tottori earthquake, Japan. We deconvolve the coda record of small earthquakes registered on the ground surface by that registered at the 100 m depth in a borehole at a station that experienced a strong shock from the mainshock. Because coda waves are mostly composed of randomly scattered S waves, deconvolution of the two coda records enables us to obtain a robust image of the ground structure. Assuming that the shear modulus was reduced at the depth of 0–11 m, we estimate the shear modulus change in each time period after the mainshock by fitting synthetic coda deconvolution to the observed one from 1 to 16 Hz. As a result, the shear modulus dropped to 52% of the value obtained before the mainshock a few minutes after the strong earthquake shock. This caused a decrease in the S-wave velocity of 30% and an increase in S-wave travel time of 17 msec. The shear modulus continued to recover for over 1 yr following the logarithm of the lapse time. It recovered to 69%, 83%, 87%, and 97% of the value obtained before the mainshock in the periods of 0 to 1 week, 1 week to 1 month, 1 month to 1 yr, and 1 to 4 yr after the mainshock, respectively.

[Articles] Scenario Liquefaction Hazard Maps of Santa Clara Valley, Northern California

Holzer, T. L., Noce, T. E., Bennett, M. J. — 2009-01-21

Maps showing the probability of surface manifestations of liquefaction in the northern Santa Clara Valley were prepared with liquefaction probability curves. These curves were based on complementary cumulative frequency distributions of the liquefaction potential index (LPI) for surficial geologic units in the study area. LPI values were computed with extensive cone penetration test soundings. Maps were developed for three earthquake scenarios, an M 7.8 event on the San Andreas fault comparable to the 1906 event, an M 6.7 event on the Hayward fault comparable to the 1868 event, and an M 6.9 event on the Calaveras fault. Ground motions were estimated with the Boore and Atkinson (2008) attenuation relation. Liquefaction is predicted for all three events in young Holocene levee deposits along the major creeks. Liquefaction probabilities are highest for the M 7.8 earthquake, ranging from 0.33 to 0.37 if a 1.5 m deep water table is assumed, and from 0.10 to 0.14, if a 5 m deep water table is assumed. Liquefaction probabilities of the other surficial geologic units are less than 0.05. Probabilities for the scenario earthquakes are generally consistent with observations during historical earthquakes.

[Short Notes] Pseudotachylytes: Rarely Generated, Rarely Preserved, or Rarely Reported?

Kirkpatrick, J. D., Shipton, Z. K., Persano, C. — 2009-01-21

Pseudotachylyte is the only fault rock that is known to form exclusively at seismic slip rates, so it is unique in preserving direct evidence of the dynamic processes in action during earthquakes. It is commonly assumed that pseudotachylyte is rare, and debate has centered on whether it is rarely generated or commonly generated but rarely preserved. We present field and electron microscope observations of eight new pseudotachylytes from faults in the Sierra Nevada that have previously been the focus of many detailed studies of fault growth and mechanics. These pseudotachylytes range from being abundant and easy to recognize in outcrop to being impossible to identify without microscope observations. Our data show that pseudotachylytes are much more common in the Sierra Nevada than has previously been reported. We suggest that pseudotachylytes may be present within many fault zones but remain unreported primarily due to difficulty in identifying very thin or reworked pseudotachylytes in the field; and therefore the use of these fault rocks to interpret dynamic earthquake processes must be revisited.

[Short Notes] Quantifying Natural Fault Geometry: Statistics of Splay Fault Angles

Ando, R., Shaw, B. E., Scholz, C. H. — 2009-01-21

We propose a new approach to quantifying fault system geometry, using an objective fit of the fault geometry to a test function, specifically here a fault branch. Fitting a Y-shaped object using a cost function to dextral faults in California, we find a number of significant results arising from use of a a systematic, objective, quantitative approach. (1) The largest angle of the branch structure is generally very close to 180°, implying that the branch is a splay fault off the primary through-going fault. (2) The distribution of the smallest angle, the splay angle, has a peak near ±17°, symmetric about the primary fault. (3) These features appear independent of scale. These results are not yet explained by any theory, and they pose new questions and constraints for the physics of fault system formation and behavior.

[Short Notes] Estimation of the Maximum Earthquake Magnitude from the Geothermal Gradient

Kudo, T., Tanaka, T., Furumoto, M. — 2009-01-21

The thickness of the brittle layer in the crust, which is related to the geothermal structure, should determine the maximum fault size or the maximum earthquake magnitude. A relationship between the maximum magnitude and the geothermal gradient is investigated. Using two catalogs of large events (M≥6.5) in the Japanese Islands, recent events from 1927 to 2005 and events from 1500 to 1927, we obtain the maximum magnitude M_max as a function of the geothermal gradient T_g (in degrees per kilometer): M_max=-1.67logT_g+9.93 and M_max=-1.67logT_g+10.43 for waiting durations of about 100 and 500 yr, respectively.

[Short Notes] Differences in Attenuation of Ground Motion Perpendicular to the Mexican Subduction Zone between Colima and Guerrero: An Explanation Based on Numerical Modeling

Cruz-Jimenez, H., Chavez-Garcia, F. J., Furumura, T. — 2009-01-21

A recent article (Tejeda-Jácome and Chávez-García, 2007) showed that significant differences in seismic ground-motion attenuation perpendicular to the Pacific coast exist between Guerrero and Colima, in western Mexico. For hypocentral distances larger than 100 km, larger ground motions are predicted using a model derived from local data in the northern section of the subduction zone (Colima) relative to ground-motion models derived using data from the southern section (Guerrero). In this article we consider two possible explanations for the differences. The first possible explanation is differences in the geometry of the subducting slab between Colima and Guerrero. The second is the varying shallow structure of the overriding plate due to the presence of the trans-Mexican volcanic belt (TMVB), oblique to the Middle American trench. We use 2D numerical modeling for P-SV waves to investigate which of the two factors can explain the differences in the observed attenuation. Our results show that it is the presence of the TMVB, closer to the coast in Colima, that better explains the differences.

[Short Notes] High Crustal Seismic Attenuation in Ladakh-Karakoram

Rai, S. S., Ashish, , Padhi, A., Sarma, P. R. — 2009-01-21

An analysis of the Lg wave has been attempted in order to study the seismic attenuation Q₀ along a profile from the southern edge of the northwestern Himalaya to Ladakh and Karakoram using broadband waveform recordings of regional earthquakes. The lateral variability in seismic attenuation is derived from the inversion of 23 two-station Q₀ measurements using a differential evolution global optimization scheme. The Q₀ value decreases northwards from ~700 in Himalaya and ~400 beneath the Indus Zangbo suture (IZS) to ~70 in Ladakh–Karakoram. This suggests an efficient transmission of seismic waves beneath the Himalaya and the IZS and high attenuation under Ladakh–Karakoram. The Q₀ values for northwestern Himalaya and the IZS are significantly (>50%) higher compared to their counterpart at 90° E, while Ladakh–Karakoram and southern Tibet show comparable Q₀ (~70). The high attenuation zone (low Q₀) is correlatable with increase in electrical conductivity and decrease in the midcrustal S velocity, suggesting the possible presence of aqueous fluid/partial melt in the Ladakh crust.

[Short Notes] Pn Travel-Time Tomography of the Paleo-Continental-Collision and Rifting Zone around Korea and Japan

Hong, T.-K., Kang, T.-S. — 2009-01-21

A recent dense deployment of seismic stations in South Korea and Japan allows regional seismic imaging of the far-east Asian region that experienced continental collisions and riftings. We perform seismic imaging based on a mantle-lid P-wave (Pn) travel-time tomography to exploit the tectonic imprints in the lithosphere. The average Pn velocity in the region is estimated to be 7.95±0.03 km/sec. The inverted Pn velocities illuminate the tectonic structures. High velocities of ~8.15 km/sec are observed in the Precambrian massif regions, while low velocities of ~7.8 km/sec are associated with the fold belt and sedimentary basin regions in the southern Korean Peninsula. The Pn velocity is estimated to be low in the backarc basins, including the Ulleung and the Yamato basins, while it is estimated to be higher on the continental fragments, including the South Korea plateau and the Oki bank. The high-velocity structures along the Japanese Islands support the separation of the Japanese province from the Eurasia plate. The high-velocity anomaly along the east coast of the Korean Peninsula around the Hupo bank suggests intrusion and solidification of high-density material in the lower crust and mantle lid.

[Short Notes] Tomographic Pn Velocity and Anisotropy Structure in the Central and Eastern United States

Zhang, Q., Sandvol, E., Liu, M. — 2009-01-21

A total of 19,166 Pn phase readings from the International Seismological Centre (ISC) and the National Earthquake Information Center (NEIC) catalogs as well as hand-picked arrivals from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center were inverted to map the velocity and anisotropy structure of the lithospheric mantle in the central and eastern United States (CEUS). Our Pn tomographic model shows a broad region of very fast velocity under the North American craton (the northern CEUS) and significant lateral variations within the rest of the CEUS. The surface locations of the major intraplate seismic zones are near the edges of high-velocity anomalies, which is consistent with the notion that stress accumulation and hence focused deformation are likely to occur at the rheological boundaries around the rigid lithospheric root. However, the ancient rifts show no clear correlation to the low-velocity anomalies in the lithospheric mantle. Our Pn anisotropic model shows a complex pattern of fast directions with an overall north–south trend in the CEUS that may reflect the preserved fabrics of the cratonic lithosphere. Nonetheless, high Pn anisotropy seems to wrap around the high-velocity blocks, which may indicate local deformation around the rigid blocks.

[Short Notes] Structure of the Upper Crust in Japan from S-Wave Attenuation Tomography

2009-01-21

Seismic attenuation (Q-value) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms. We apply the attenuation tomography method of Pei et al. (2006) using M_L amplitude data to estimate attenuation within the upper crust in Japan. More than 60,000 Sg-wave maximum amplitude readings from 5559 events, recorded by 971 stations, were selected from the dense High-Sensitivity Seismography Network (Hi-net) under the condition that epicentral distance is less than 2º and event depth is less than 10 km. The lateral S-wave Q variations of the upper crust at 1 Hz in Japan were obtained. The results indicate that low Q-values exist in the central Japanese islands, with almost the same distribution as volcanoes, while high Q-values exist mainly between the front of volcanoes and the Japanese east coast. In addition, a low Q was found between the eastern coast and the subducted trench. Most large crustal earthquakes occur in or around zones of low Q or the boundaries between areas of low and high Q, which will be very helpful in estimating the risk of large earthquakes.

[Short Notes] Coseismic Change and Recovery of Scattering Environment in the Crust after the 1995 Hyogo-ken Nanbu Earthquake, Japan

Sugaya, K., Hiramatsu, Y., Furumoto, M., Katao, H. — 2009-01-21

We observe a unique temporal variation in crustal heterogeneity from the analysis of coda Q^-1 for 14 years in the Tamba region, northeast to the rupture zone of the 1995 Hyogo-ken Nanbu earthquake (M_JMA 7.3) in southwest Japan. The values of at lower frequencies (1.5–4.0 Hz) that increased coseismically due to the static stress change decreased back to the preevent values in about two years. No such variations are found at higher frequencies (5.0–24.0 Hz). We confirm that no tectonic events that cause a significant stress change occurred during the recovery period. The time required for the recovery of the scattering environment observed here, such as the number density of cracks and cracks opened by the stress change, is consistent with those of previous studies focused on the brittle shallower crust. This suggests a possibility that a similar mechanism of the recovery operates in both the brittle and the ductile parts of the crust.

[Short Notes] Lg-Coda Methods Applied to Nevada Test Site Events: Spectral Peaking and Yield Estimation

Murphy, K. R., Mayeda, K., Walter, W. R. — 2009-01-21

We have applied the regional S-wave coda calibration technique of Mayeda et al. (2003) to earthquake data in and around the Nevada Test Site (NTS) using four regional broadband stations from the Lawrence Livermore National Laboratory (LLNL) seismic network. We applied the same path and site corrections to tamped nuclear explosion data and averaged the source spectra over the four stations. Narrowband coda amplitudes from the spectra were then regressed against inferred yield based on the regional m_b(Pn) magnitude of Denny et al. (1987), along with the yield formulation of Vergino and Mensing (1990). We find the following: (1) the coda-derived spectra show a peak that is dependent upon emplacement depth, not event size; (2) source size estimates are stable for the coda and show a dependence upon the near-source strength and gas porosity; (3) for explosions with the same m_b(Pn) or inferred yield, those in weaker material have lower coda amplitudes at 1–3 Hz.

[Short Notes] The F-Detector Revisited: An Improved Strategy for Signal Detection at Seismic and Infrasound Arrays

Arrowsmith, S. J., Whitaker, R., Katz, C., Hayward, C. — 2009-01-21

This short article explores and extends the adaptive detection algorithm recently developed by Arrowsmith, Whitaker, et al. (2008). In particular, this article highlights its application for seismic data, compares results for colocated seismic and infrasonic data, and assesses detector performance through comparison with analyst picks. We assess the adaptive detector by generating receiver-operating characteristic (ROC) curves, illustrating the trade-off between detection probability and false-alarm probability, and comparing the results with the conventional F-detector. The results show that the adaptive detector performs much better than the conventional detector for both seismic and infrasound data by maintaining high detection probabilities while significantly decreasing false-alarm probabilities, illustrating that correlated noise is ubiquitous for both types of data. The effect of the adaptation window is illustrated and shown to be especially important for infrasound data where diurnal variations in ambient noise levels are pronounced. A window choice of 1 hr (i.e., significantly less than 24 hr) is shown to be adequate for representing variations in ambient noise levels.

[Short Notes] Equivalence of the Green's Function for a Full-Space to the Direct-Wave Contributions for a Half-Space and a Layered Half-Space

Zhang, H., Chen, X. — 2009-01-21

As a part of a series of studies on the dynamic rupture processes based on a modern boundary integral equation method, we prove that contribution of direct waves to the Green’s function for a half-space and that to the Green’s function in source layer for a layered half-space, which are both expressed in a cylindrical coordinate system, are strictly equivalent to the well-known analytic Green’s function for full-space. Because all of the hypersingularities in boundary integral equations (BIEs) come from the direct waves and contribution of direct waves can be evaluated analytically, the results obtained in this study are very useful in the simplification of the BIEs, especially for the case of layered half-space, in which the treatment on separation of singular parts is otherwise extremely complicated in mathematics and hence impracticable.

[Short Notes] Frequency-Dependent Asymmetry of Seismic Cross-Correlation Functions Associated with Noise Directionality

Frank, S. D., Foster, A. E., Ferris, A. N., Johnson, M. — 2009-01-21

Cross correlation of ambient noise recorded by broadband seismometers has become a popular data analysis technique. Because time-averaged cross-correlation functions provide estimates of the Green’s function between two stations, they provide arrival information for waves propagating in opposite directions along a line connecting the stations. Under certain circumstances, amplitude asymmetry in these functions can be associated with preferred directionality of the noise field surrounding the stations. An interesting example of asymmetric cross-correlation functions comes from the site of the 1992 Pakistan Himalayas passive source broadband seismic experiment (PAKH). Here, the comparison of causal and acausal signal-to-noise ratios in several period bands gives an indication of the noise field directivity. The analysis also reveals frequency-dependent asymmetry of the cross-correlation functions, suggesting multiple sources of the ambient noise. At PAKH, some portion of the 6–10 sec noise field band may be attributed to distant ocean wave activity, as seen in other studies, but other period bands show noise propagation from the opposite direction. Our directivity analysis suggests that the regional tectonics of the Indo–Eurasian collision and/or its associated microseismicity may be contributing strongly to the apparent noise field, either by acting as a dominant scatterer or as a noise source.