Archives of Seismological Society of America recent issues

[Articles] An Overview of the Damaging and Low Magnitude Mw 4.8 La Paca Earthquake on 29 January 2005: Context, Seismotectonics, and Seismic Risk Implications for Southeast Spain

2007-06-11

This article presents an overview of the La Paca earthquake of magnitude m_bLg 4.7, which occurred on 29 January 2005, with its epicenter located near the town of Avilés in the Murcia region in southeast Spain. Despite its low magnitude, the earthquake caused important damage in two towns of the epicentral area, La Paca and Zarcilla de Ramos. These areas recorded intensities of VI–VII (European Macroseismic Scale, 1998) and sustained estimated economic losses amounting to 10 million ;eu[|20ac|]. Aftershocks continued for more than 2 weeks, producing considerable alarm in the population and mobilizing emergency services from the whole region. The La Paca seismic series is the third registered in the region in the past 8 years, being preceded by the Mula (1999) and southwest Bullas (2002) seismic series. These main events had also low magnitudes (m_bLg 4.8) and caused damage levels similar to the 2005 earthquake. The case is an example of a moderate seismic zone where low-magnitude and frequent earthquakes have important implications on the seismic hazard and risk of the region. Although these are not the largest expected earthquakes, they have yielded important information for improving the knowledge of the seismic activity of the area. With this aim in mind, different topics have been analyzed from a multidisciplinary perspective, including seismicity, local tectonics and surface geology, focal mechanisms, macroseismic effects, and ground motion. Results indicate a local tectonic interpretation, consistent with a strike-slip focal mechanism, the confirmation of a triggering process between the 2002 and 2005 earthquakes, a geotechnical and ground-motion characterization for the damaged sites (supporting local amplification effects and estimated peak ground acceleration values of ~0.1g), and an understanding of damage patterns in relation to local building trends. The results may be used as guidelines for future revisions of the Spanish Building Code (Norma de la Construcción Sismorresistente Española [NCSE-02], 2002). The study results should contribute to risk mitigation in a region where strong-motion records from the maximum expected earthquakes are not available. This approach can be extended to other regions with similar seismic backgrounds and a lack of strong-motion records.

[Articles] The 2004 Mw 4.4 Rotenburg, Northern Germany, Earthquake and Its Possible Relationship with Gas Recovery

2007-06-11

We study the 20 October 2004 M_w 4.4 Rotenburg (Wümme)/Neuenkirchen earthquake, located in a previously aseismic region in the northern German sedimentary basin. We constrain the source parameter by using different techniques. A possible relationship between this event, the regional tectonic setting, and local gas recovery is investigated. Different waveform inversion and modeling approaches constrain the depth of the mainshock between 5 and 7 km. The source mechanism was oblique normal faulting on planes striking roughly north–south. An inversion for kinematic rupture parameters indicates a unilateral rupture propagation toward the north, consistent with the higher macroseismic intensities found toward the north in the region of Hamburg compared with those at a similar distance toward the south in the region of Hannover. Relocations of the mainshock and three of the largest aftershocks indicate that these events occurred within a few kilometers of three major gas fields and at depth close to gas production intervals. Comparison with seismicity triggered in the northern Netherlands by depletion of similar gas reservoirs in a similar tectonic environment suggests that the M_w 4.4 Rotenburg event may be related to gas recovery.

Online material: Focal mechanism and waveform fit.

[Articles] Tsunami Hazard Evaluation of the Eastern Mediterranean: Historical Analysis and Selected Modeling

Salamon, A., Rockwell, T., Ward, S. N., Guidoboni, E., Comastri, A. — 2007-06-11

Seismic sea waves in the eastern Mediterranean have been reported since written history first emerged several thousand years ago. We collected and investigated these ancient and modern reports to understand and model the typical tsunamigenic sources, with the ultimate purpose of characterizing tsunami hazard along the Levant coasts. Surprisingly, only 35% of the tsunami reports could be traced back to primary sources, with the balance remaining questionable. The tsunamis varied in size, from barely noticeable to greatly damaging, and their effects ranged from local to regional. Overall, we list 21 reliably reported tsunamis that occurred since the mid second century b.c. along the Levant coast, along with 57 significant historical earthquakes that originated from the "local" continental Dead Sea Transform (dst) system. An in-depth evaluation shows that 10 tsunamis are clearly associated with on-land dst earthquakes, and therefore, as formerly suggested, they probably originated from offshore, seismogenically induced slumps. Eight tsunamis arrived from the "remote" Hellenic and Cypriot Arcs, one from Italy, and two are left with as yet unrecognized sources. A major conclusion from this work is that onshore earthquakes commonly produce tsunamis along the Levant coastline, and that analogous situations are present elsewhere in the Mediterranean, as well as along the California coast and in another regions with active faults near the coast.

We modeled three typical scenarios, and in light of the Sumatra experience, we examined the more likely severe magnitudes. This of course leads us toward the upper range of expected run-ups. The models show that sooner than five minutes after a strong earthquake produces an offshore slump, which occurs after close to a third of the large dst earthquakes, a 4- to 6-m run-up may flood part of the Syrian, Lebanese, and Israeli coasts. Tsunamis from remote earthquakes, however, arrive later and produce only 1- to 3-m run-ups, but are more regional in extent.

Online material: Tsunami modeling and reports.

[Articles] Unveiling the Sources of the Catastrophic 1456 Multiple Earthquake: Hints to an Unexplored Tectonic Mechanism in Southern Italy

Fracassi, U., Valensise, G. — 2007-06-11

We revisited data related to the 1456 seismic crisis, the largest earthquake to have ever occurred in peninsular Italy, in search of its causative source(s). Data about this earthquake consist solely of historical reports and their intensity assessment.

Because of the age of this multiple earthquake, the scarcity and sparseness of the data, and the unusually large damage area, no previous studies have attempted to attribute the 1456 events to specific faults. Existing analytical methods to identify a likely source from intensity data also proved inappropriate for such a sparse dataset, since historical evidence suggests that the cumulative damage pattern contains at least three widely separated events.

We subdivided the 1456 damage pattern into three independent mesoseismal areas; each of these areas falls onto east–west tectonic trends previously identified and marked by deep (>10 km) right-lateral slip earthquakes. Based on this evidence we propose (1) that the 1456 events were generated by individual segments of regional east–west structures and are evidence of a seismogenic style that involves oblique dextral reactivation of east–west lower crustal faults; (2) that each event may have triggered subsequent but relatively distant events in a cascade fashion, as suggested by historical accounts; hence (3) that the 1456 sequence reveals a fundamental but unexplored mechanism of tectonic deformation and seismic release in southern Italy. This style dominates the region that lies between the northwest–southeast system of large extensional faults straddling the crest of the southern Apennines and the buried outer front of the chain.

Although the quality of the available information concerning the 1456 earthquake is naturally limited, we show that the overlap of the damage distribution, the orientation and characteristics of regional tectonic structures, the seismicity patterns, and the focal mechanisms all concur with our interpretations and would be difficult to justify otherwise.

[Articles] 12,000-Year-Long Record of 10 to 13 Paleoearthquakes on the Yammouneh Fault, Levant Fault System, Lebanon

Daeron, M., Klinger, Y., Tapponnier, P., Elias, A., Jacques, E., Sursock, A. — 2007-06-11

We present results of the first paleoseismic study of the Yammoûneh fault, the main on-land segment of the Levant fault system within the Lebanese restraining bend. A trench was excavated in the Yammoûneh paleolake, where the fault cuts through finely laminated sequences of marls and clays. First-order variations throughout this outstanding stratigraphic record appear to reflect climate change at centennial and millennial scales. The lake beds are offset and deformed in a 2-m- wide zone coinciding with the mapped fault trace. Ten to thirteen events are identified, extending back more than ~12 kyr. Reliable age bounds on seven of these events constrain the mean seismic return time to 1127 ± 135 yr between ~12 ka and ~6.4 ka, implying that this fault slips in infrequent but large (M ~7.5) earthquakes. Our results also provide conclusive evidence that the latest event at this site was the great a.d. 1202 historical earthquake, and suggest that the Yammoûneh fault might have been the source of a less well-known event circa a.d. 350. These findings, combined with previous paleoseismic data from the Zebadani valley, imply that the parallel faults bounding the Beqaa release strain in events with comparable recurrence intervals but significantly different magnitudes. Our results contribute to document the clustering of large events on the Levant fault into centennial episodes, such as that during the eleventh through twelfth centuries, separated by millennial periods of quiescence, and raise the possibility of a M >7 event occurring on the Yammoûneh fault in the coming century. Such a scenario should be taken into account in regional seismic-hazard assessments and planned for accordingly.

[Articles] Tectonic Processes in the Jan Mayen Fracture Zone Based on Earthquake Occurrence and Bathymetry

2007-06-11

Jan Mayen is an active volcanic island situated along the mid-Atlantic Ridge north of Iceland. It is closely connected with the geodynamic processes associated with the interaction between the Jan Mayen Fracture Zone (jmfz) and the slowly spreading Kolbeinsey and Mohns Ridges. Despite the significant tectonic activity expressed by the frequent occurrence of medium to large earthquakes, detailed correlation between individual events and the causative faults along the jmfz has been lacking. Recently acquired detailed bathymetric data in the vicinity of Jan Mayen has allowed us to document such correlation for the first time. The earthquake of 14 April 2004 (M_w 6), which occurred along the jmfz, was studied in detail and correlated with the bathymetry. Locations of aftershocks within the first 12 hours after the mainshock outline a 10-km-long fault plane. Interactions between various fault systems are demonstrated through locations of later aftershocks, which indicate that supposedly normal fault structures to the north of the ruptured fault, in the Jan Mayen Platform, have been reactivated. Correlation of the waveforms shows that events located on these structures are significantly different from activity at neighboring structures. Coulomb stress modeling gives an explanation to the locations of the aftershocks but cannot reveal any information about their mechanisms.

[Articles] Aftershock Detection Thresholds as a Function of Time: Results from the ANZA Seismic Network following the 31 October 2001 ML 5.1 Anza, California, Earthquake

Kilb, D., Martynov, V. G., Vernon, F. L. — 2007-06-11

We examine aftershock detectability thresholds for events in the initial part of the 31 October 2001, M_L 5.1 sequence in southern California. This sequence occurred directly below the broadband anza seismic network, which recorded continuous waveform data at 13 azimuthally well-distributed stations within the study region (seven had epicentral distances < 20 km). Of the 608 aftershocks (0 < M_L < ~2.8) in the initial 2 hr of this sequence, the first five aftershocks recorded were only identifiable at stations within 30 km after applying a high-pass filter. Using a cluster (radius ≤ 1.1 km) of 200 representative aftershocks, we track the maximum seismogram amplitude versus earthquake magnitude. This relationship helps us quantify the visibility of aftershocks within the mainshock coda and assess our detection capabilities. We estimate that detectable aftershocks within the mainshock coda include (1) those over magnitude ~3 that are within 15 km of the network centroid that occur 12 sec or more into the sequence, and (2) those over magnitude ~2 that are within 30 km of the centroid of the network that occur 60 sec or more into the sequence. We find a lack of large aftershocks in this sequence. The largest aftershock (M_L ~2.8) is substantially smaller than the mainshock (M_L 5.1). We suggest this relatively large-magnitude differential is dictated by a combination of factors that includes complexity of the San Jacinto fault system and the lack of large earthquakes in the region in the past ~20 years.

Online material: Quicktime movies juxtaposing a 3.2 aftershock in the coda of a 5.1 mainshock.

[Articles] Wavelet Transform Methods for Azimuth Estimation in Local Three-Component Seismograms

Galiana-Merino, J. J., Rosa-Herranz, J., Jauregui, P., Molina, S., Giner, J. — 2007-06-11

We have developed a new method for estimating azimuth in local three- component seismograms through wavelet analysis. The proposed process proceeds in three stages. First, the seismogram is filtered through a wavelet packet approach. Second, the first arrival is determined through a P picker similar to a short-term- average long-term-average scheme that is applied on the wavelet domain. Finally, an adaptive-length window around the pick is selected and used for determining the azimuth, using the property of linear polarization of the first arrival.

The proposed method has been applied to three-component short-period seismograms for local earthquakes recorded by the seismic network of Alicante province in southeastern Spain. The locations of these events were previously obtained through the software HYPO71PC (Lee and Valdes, 1989) and the seismograms recorded by four analog stations of vertical component distributed within the province of Alicante. The results obtained by the wavelet-based algorithm have been compared with the azimuth angles obtained through the results from the location software. The comparison indicates that the proposed algorithm can determine the azimuth of the analyzed events to within a mean bias of 4.5°.

[Articles] Regional Travel-Time Uncertainty and Seismic Location Improvement Using a Three-Dimensional a priori Velocity Model

Flanagan, M. P., Myers, S. C., Koper, K. D. — 2007-06-11

We demonstrate our ability to improve regional travel-time prediction and seismic event location accuracy using an a priori 3D velocity model of Western Eurasia and North Africa (WENA1.0). Travel-time residuals are assessed relative to the iasp91 model for approximately 6000 Pg, Pn, and P arrivals, from seismic events having 2 epicenter accuracy between 1 km and 25 km (ground truth 1 [GT1] and GT25, respectively), recorded at 39 stations throughout the model region. Ray paths range in length between 0° and 40° (local, regional, and near teleseismic) providing depth sounding that spans the crust and upper mantle. The dataset also provides representative geographic sampling across Eurasia and North Africa including aseismic areas. The WENA1.0 model markedly improves travel-time predictions for most stations with an average variance reduction of 29% for all ray paths from the GT25 events; when we consider GT5 and better events alone, the variance reduction is 49%. For location tests we use 196 geographically distributed GT5 and better events. In 134 cases (68% of the events), locations are improved, and average mislocation is reduced from 24.9 km to 17.7 km. We develop a travel-time uncertainty model that is used to calculate location coverage ellipses. The coverage ellipses for WENA1.0 are validated to be representative of epicenter error and are smaller than those for iasp91 by 37%. We conclude that a priori models are directly applicable where data coverage limits tomographic and empirical approaches, and the development of the uncertainty model enables merging of a priori and data-driven approaches using Bayesian techniques.

Online material: Correction surfaces and histograms of travel-time residuals for 40 stations.

[Articles] Seismic Velocity Structure and Seismotectonics of the Eastern San Francisco Bay Region, California

Hardebeck, J. L., Michael, A. J., Brocher, T. M. — 2007-06-11

The Hayward Fault System is considered the most likely fault system in the San Francisco Bay Area, California, to produce a major earthquake in the next 30 years. To better understand this fault system, we use microseismicity to study its structure and kinematics. We present a new 3D seismic-velocity model for the eastern San Francisco Bay region, using microseismicity and controlled sources, which reveals a ~10% velocity contrast across the Hayward fault in the upper 10 km, with higher velocity in the Franciscan Complex to the west relative to the Great Valley Sequence to the east. This contrast is imaged more sharply in our localized model than in previous regional-scale models. Thick Cenozoic sedimentary basins, such as the Livermore basin, which may experience particularly strong shaking during an earthquake, are imaged in the model.

The accurate earthquake locations and focal mechanisms obtained by using the 3D model allow us to study fault complexity and its implications for seismic hazard. The relocated hypocenters along the Hayward Fault in general are consistent with a near-vertical or steeply east-dipping fault zone. The southern Hayward fault merges smoothly with the Calaveras fault at depth, suggesting that large earthquakes may rupture across both faults. The use of the 3D velocity model reveals that most earthquakes along the Hayward fault have near-vertical strike-slip focal mechanisms, consistent with the large-scale orientation and sense of slip of the fault, with no evidence for zones of complex fracturing acting as barriers to earthquake rupture.

Online material: Velocity model validation experiments and additional seismicity plots.

[Articles] A Regional Ground-Motion Excitation/Attenuation Model for the San Francisco Region

Malagnini, L., Mayeda, K., Uhrhammer, R., Akinci, A., Herrmann, R. B. — 2007-06-11

By using small-to-moderate earthquakes located within ~200 km of San Francisco, we characterize the scaling of the ground motions for frequencies ranging between 0.25 and 20 Hz, obtaining results for geometric spreading, Q(f), and site parameters using the methods of Mayeda et al. (2005) and Malagnini et al. (2004). The results of the analysis show that, throughout the Bay Area, the average regional attenuation of the ground motion can be modeled with a bilinear geometric spreading function with a 30-km crossover distance, coupled to an anelastic function exp(–fr/Q(f), where: Q(f) = 180 f^0.42. A body-wave geometric spreading, g(r) = r^–1.0, is used at short hypocentral distances (r < 30 km), whereas g(r) = r^–0.6 fits the attenuation of the spectral amplitudes at hypocentral distances beyond the crossover.

The frequency-dependent site effects at twelve of the Berkeley Digital Seismic Network stations were evaluated in an absolute sense using coda-derived source spectra. Our results show the following. (1) The absolute site response for frequencies ranging between 0.3 Hz and 2.0 Hz correlate with independent estimates of the local magnitude residuals (M_L) for each of the stations. (2) Moment magnitudes (M_w) derived from our path and site-corrected spectra are in excellent agreement with those independently derived using full-waveform modeling as well as coda-derived source spectra. (3) We use our weak-motion-based relationships to predict motions regionwide for the Loma Prieta earthquake, well above the maximum magnitude spanned by our data set, on a completely different set of stations. Results compare well with measurements taken at specific National Earthquake Hazards Reduction Program site classes. (4) An empirical, magnitude-dependent scaling was necessary for the Brune stress parameter to match the large-magnitude spectral accelerations and peak ground velocities with our weak-motion-based model.

Online material: Tables of peak ground acceleration, peak ground velocity, and pseudo-spectral acceleration at 0.3 sec, 1.0 sec, and 3.0 sec.

[Articles] Anomalous Propagation of Long-Period Ground Motions Recorded in Tokyo during the 23 October 2004 Mw 6.6 Niigata-ken Chuetsu, Japan, Earthquake

Furumura, T., Hayakawa, T. — 2007-06-11

Unusually large (>5 cm) and prolonged shaking associated with long- period ground motions at periods of about 7 sec were observed in central Tokyo during the M_w 6.6 Niigata-ken Chuetsu earthquake of 23 October 2004. The long- period ground motions caused significant resonance in high-rise buildings of about 70 floors in height. Thus, it is an urgent matter to understand the development and amplification properties of long-period ground motions in Tokyo associated with large earthquakes.

In this study, we use numerous waveform records from 585 stations in a nationwide accelerometer network (K-NET, KiK-net) and 495 intensity meters in the area around Tokyo. The data reveal that the long-period ground motion is characterized in most part by a surface, Rayleigh wave generated at the northern edge of Kanto basin, and the surface wave is developed as propagating through a thick cover of sediments (>3000–4000 m) that overlies rigid bedrock.

To complement the observational data, we conducted a large-scale computer simulation of seismic-wave propagation by employing the Earth Simulator supercomputer with a detailed source-slip model and a high-resolution 3D sedimentary structural model of central Japan. The results of the computer simulation demonstrate that the anomalously prolonged ground shaking of the long-period signal recorded in the center of Tokyo occurred because of the stagnation of seismic energy resulting from the multipathing and focusing of Rayleigh waves toward the bottom of the Kanto basin from surrounding mountain regions with interaction to the 3D basin structure.

Online material: Animations of long-period ground motions in Tokyo.

[Articles] Sensitivity of Ground-Motion Simulations to Earthquake Source Parameters: A Case Study for Istanbul, Turkey

Sorensen, M. B., Pulido, N., Atakan, K. — 2007-06-11

Following the disastrous earthquakes in Izmit and Düzce along the North Anatolian fault in 1999, the earthquake hazard in the Istanbul area became a great concern. In this study we simulate strong ground motions caused by a scenario earthquake (M 7.5) in the Marmara Sea, and investigate the effect of varying the input parameters on the broadband frequency ground motion. Simulations are based on a multiasperity source model that involves the combined rupture of the North Anatolian fault segments beneath the Marmara Sea. We use a hybrid model combining a deterministic simulation of the low frequencies (0.1–1.0 Hz) with a semistochastic simulation of the high frequencies (1.0–10.0 Hz). Computation at each frequency range is performed separately and the total ground motion is combined in the time domain. Computations are linear and are performed at bedrock level, thereby not taking any effect of local geological conditions into account. We calculate a total of 17 earthquake scenarios corresponding to different source and attenuation parameters to study their effect on the ground motion. The most significant parameters in terms of ground-shaking level are the rise time, rupture velocity, rupture initiation point, and stress drop. The largest variability of strong ground motions is observed in regions adjacent to asperities and is associated with frequencies higher than 5 Hz. For lower frequencies our simulated velocity spectra within the Istanbul area are fairly stable among scenarios. The average standard deviations of all ground-motion measures are less than 35% of the mean.

Online material: Figures of peak ground acceleration and peak ground velocity and their differences to the reference scenario values.

[Articles] Assessment of a Nonlinear Dynamic Rupture Inversion Technique Applied to a Synthetic Earthquake

Corish, S. M., Bradley, C. R., Olsen, K. B. — 2007-06-11

Dynamic rupture inversion is a powerful tool for learning why and how faults fail, but much more work has been done in developing inversion methods than evaluating how well these methods work. This study examines how well a nonlinear rupture inversion method recovers a set of known dynamic rupture parameters on a synthetic fault based on the 2000 western Tottori, Japan earthquake (M_w 6.6). Rupture evolution on the fault is governed by a slip-weakening friction law. A direct-search method known as the neighborhood algorithm (Sambridge, 1999) is used to find optimal values of both the initial stress distribution and the slip-weakening distance on the fault, based on misfit values between known and predicted strong-motion displacement records. The yield stress and frictional sliding stress on the fault are held constant. A statistical assessment of the results shows that, for this test case, the inversion succeeds in locating all parameters to within ±14% of their true values. With the model configuration used in this study, the parameters located in the central rupture area are better resolved than the parameters located at the sides and bottom of the fault. In addition, a positive linear correlation between the mean initial stress and the slip-weakening distance is identified. The investigation confirms that dynamic rupture inversion is useful for determining rupture parameters on the fault, but that intrinsic trade-offs and poor resolution of some parameters limit the amount of information that can be unambiguously inferred from the results. In addition, this study demonstrates that using a statistical approach to assess nonlinear inversion results shows how sensitive the misfit measure is to the various parameters, and allows a level of confidence to be attached to the output parameter values.

[Articles] A Constitutive Criterion for the Fault: Modified Velocity-Weakening Law

Miguel, L. F. F., Riera, J. D. — 2007-06-11

Expanding on a previous article, constitutive laws for solid friction are examined jointly with available experimental results. The models are evaluated by means of numerical dynamic analysis of two sliding blocks simulating adjacent fault sections. Effective constitutive laws are determined as relations between mean values of the relevant variables on a selected area of the sliding surfaces. The material is initially assumed elastic and homogeneous and the influence of nonhomogeneity evaluated next by modeling mass density, Young’s modulus, and friction coefficient as correlated random fields. The effect of fractures in rock close to the fault is also numerically assessed. Finally, the influence of rupture of protrusions (microasperities) between the sliding surfaces is analyzed. The influence of size of the averaging interface area on the parameters of the effective constitutive law is then obtained by means of Monte Carlo simulation. When the rock regions adjacent to the fault are assumed to be linearly elastic and homogeneous or nonhomogeneous no size effect is observed. On the other hand, when the friction coefficient is characterized by a random field, a size effect is detected. Fracture occurrence in the region surrounding the fault does not cause significant alteration of the macroconstitutive law, producing only minor perturbations of the mean law determined without fracture, but it should also introduce high-frequency slave vibrations. Finally, a macroconstitutive law that takes into account the shear rupture of microasperities on the sliding surfaces is suggested. The proposed modified velocity-weakening law, constitutes a more general and flexible constitutive law.

[Articles] Wave Gradiometry in the Time Domain

Langston, C. A. — 2007-06-11

A time-domain approach for solving for the change in geometrical spreading and horizontal wave slowness in wave gradiometry is presented based on the use of the analytic signal. The horizontal displacement gradient of a wave is linearly related to the displacement and its time derivative. The coefficients of this relationship give the change of geometrical spreading, the change in radiation pattern, and horizontal slowness. The new time-domain technique incorporates estimates of the instantaneous amplitude and frequency of the three time series to solve uniquely for the wave-field coefficients. The analysis is simpler and more suited to fast array processing of displacement gradient data sets compared with a spectral ratio method.

[Articles] Damping Correction Factors for Horizontal Ground-Motion Response Spectra

Cameron, W. I., Green, R. A. — 2007-06-11

Damping correction factors (dcfs) are used to adjust response spectral values corresponding to damping 5% of critical to other damping levels. Trends in the analytical response of viscously damped, linear-elastic sdof oscillators subjected to finite-duration, sinusoidal base excitations show that dcfs depend on both the frequency and duration of the ground motion, where the latter becomes significantly less influential as damping increases. These analytical trends, in conjunction with correlations relating duration and frequency content to earthquake magnitude, site- to-source distance, site classification, and tectonic setting, are used to explain/study observations in dcfs computed from a large ground-motion database for the central- eastern and western United States. For ≥ 2%, the dcfs proposed by the authors depend on earthquake magnitude, site classification, and tectonic setting, all of which significantly influence the frequency content of ground motions. For = 1%, the dcfs proposed by the authors additionally depend on site-to-source distance, which significantly influences the duration of ground motion. In comparison with the dcfs proposed by the authors, commonly used and recently proposed dcfs were shown to be both too low and too high, depending on the relation, period range, damping ratio, earthquake magnitude, site classification, and tectonic setting. Additionally, the dcf relations proposed in the literature for ground motions exhibiting near-fault effects should not be used for < 5%, and will likely be significantly too high for periods close to that of the near-fault velocity pulse(s) for ≥ 5%.

[Articles] Liquefaction, Ground Oscillation, and Soil Deformation at the Wildlife Array, California

Holzer, T. L., Youd, T. L. — 2007-06-11

Excess pore-water pressure and liquefaction at the Wildlife Liquefaction Array in 1987 were caused by deformation associated with both high-frequency strong ground motion and 5.5-second-period Love waves. The Love waves produced large (~1.5%) cyclic shear strains well after the stronger high-frequency ground motion abated. These cyclic strains generated approximately from 13 to 35% of the excess pore-water pressure in the liquefied layer and caused excess pore-water pressures ultimately to reach effective overburden stress. The deformation associated with the Love waves explains the "postearthquake" increase of pore-water pressure that was recorded at the array. This explanation suggests that conventional methods for predicting liquefaction based on peak ground acceleration are incomplete and may need to consider cyclic strains associated with long-period surface waves. A postearthquake survey of an inclinometer casing indicated permanent shear strain associated with lateral spreading primarily occurred in the upper part of the liquefied layer. Comparison of cone penetration test soundings conducted after the earthquake with pre-earthquake soundings suggests sleeve friction increased. Natural lateral variability of the liquefied layer obscured changes in tip resistance despite a ~1% reduction in volume. The large oscillatory motion associated with surface waves explains ground oscillation that has been reported at some liquefaction sites during earthquakes.

[Articles] Corner Frequency Scaling of Regional Seismic Phases for Underground Nuclear Explosions at the Nevada Test Site

Fisk, M. D. — 2007-06-11

Model fits are used to estimate source spectral corner frequencies of regional seismic phases Pn, Pg, and Lg from underground nuclear explosions (unes) at the Nevada Test Site (nts), based on recordings by Lawrence Livermore National Laboratory (llnl) nts Network stations. The spectra are corrected for instrument response, distance, and station effects, and then network averaged. Explicit information regarding geophysical working-point properties, depth of burial, and some announced yields for nts explosions (Springer et al., 2002) is utilized for the spectral modeling. Scaling of Pn, Pg, and Lg corner frequencies with source yield is examined for sets of explosions at Yucca Flat, Pahute Mesa, and Rainier Mesa with similar material properties (medium type, density, velocities, and gas porosity). The unes are limited to those in media with gas porosity of 10% or less so that a source spectral model with a rolloff of f^–2 is applicable. Model comparisons are also examined for regional P/Lg discriminants. A key result is that Lg corner frequencies for nts unes exhibit similar scaling with source size as for P waves, but shifted lower, analogous to observations by Fisk (2006) for the Lop Nor, Semipalatinsk, and Novaya Zemlya test sites. This appears to corroborate an important effect, that is, that major contributions to S-wave generation by explosions occur near the source with a similar length scale, comparable to the elastic radius, as for P waves from explosions. Although the explicit physical mechanism is not yet understood, the implications are important regarding where and perhaps how S waves are predominantly generated by explosions. A related, key result is that the increasing separation of P/Lg at higher frequencies between nts explosions and earthquakes has a consistent model-based explanation, as at other nuclear test sites, in terms of the difference between explosion P and S corner frequencies.

[Articles] Identification of Delay-Fired Mining Explosions Using Seismic Arrays: Application to the PDAR Array in Wyoming, USA

Arrowsmith, S. J., Hedlin, M. A. H., Arrowsmith, M. D., Stump, B. W. — 2007-06-11

We extend a time-frequency discrimination algorithm, developed in an earlier article (Arrowsmith et al., 2006), for application to seismic-array data. Spectrograms evaluated at each component of an array are stacked and then converted into binary form for computation of discriminants. Because noise can bias the discriminants, we develop a procedure for removing the effect of noise on the discriminants. The binary spectrograms are randomized where the spectral amplitude of the signal is similar to the mean spectral amplitude of the pre-event noise at that frequency. The formulism of Arrowsmith et al. (2006) is further extended by modifying the objective function used to optimize the values of input parameters and by removing high-frequency and low-frequency spectral content. We apply the method to a dataset of regional recordings of earthquakes and delay-fired mine blasts recorded at the Pinedale seismic array in Wyoming. Our results show that the utilization of array data improves the success rate for source identification. Furthermore, we find that incorporating the noise-correction procedure increases the separation between earthquakes and cast overburden blasts (the largest type of delay-fired mine blasts). In total, the algorithm successfully identifies 97.4% of the events (74 of a total of 76 events, which comprise earthquakes and cast overburden blasts).

[Articles] Imaging the Three-Dimensional Crust of the Korean Peninsula by Joint Inversion of Surface-Wave Dispersion and Teleseismic Receiver Functions

Yoo, H. J., Herrmann, R. B., Cho, K. H., Lee, K. — 2007-06-11

A detailed study of the 3D variation of shear-wave velocities in the southern part of the Korean Peninsula is made by combining high-frequency surface- wave tomography results of Cho et al. (2006b) with teleseismic P-wave receiver functions at 80 locations on the peninsula. Receiver functions were derived from high-gain acceleration, short-period, and broadband digital data streams of the Korea Meteorological Administration (kma) and Korean Institute for Geosciences and Mineral Resources (kigam) networks. Vertical cross sections trace the lateral variation in the depth to the Moho, the variation of low velocities near the surface, and the variable thickness of the transition from surface velocities to midcrustal velocities. The derived crustal structure provides new insights on the evolution of the Korean crust.

[Articles] Spatial Distribution of Coda Q in South Korea

Yun, S., Lee, W. S., Lee, K., Noh, M. H. — 2007-06-11

Analyzing spectral attenuation of coda waves, we first mapped coda Q- values in the crust over the whole of South Korea. During the period from 1995 to 2004, we selected 574 north–south-component seismograms with epicentral distances less than 100 km from 328 earthquakes with magnitudes between 1.4 and 5.2. We estimated coda Q-values using the single isotropic scattering model at center frequencies of 1.5, 3, 6, 9, 12, 15, and 18 Hz, and found significant spatial variation over all frequency ranges and strong frequency dependence of coda Q in the region. The estimated coda Q-value at 1 Hz (Q₀) and the -value ranges are 80–300 and 0.4–1.1, respectively. The values have strong correlation with the regional geology in the Korean peninsula. Low Q₀-values are mainly obtained in the regions comprising sedimentary strata in southeastern South Korea, whereas granite regions in the northern part of South Korea show high Q₀. The Q₀-values in the study area agree well with those of the eastern China and Kyushu, western Japan. Furthermore, our -values are also in good agreement with those of Japan.

Online material: Locations of seismometers, list of event-station pairs, and linear regression of Q_C measurements.

[Short Notes] Holocene Slip Rate for the Western Segment of the Castle Mountain Fault, Alaska

Willis, J. B., Haeussler, P. J., Bruhn, R. L., Willis, G. C. — 2007-06-11

The western segment of the Castle Mountain fault poses a significant seismic hazard to the most populated region of south-central Alaska. We identify a previously unrecognized margin of a postglacial outwash channel that is offset right laterally 36 ± 4 m across the western segment of the Castle Mountain fault. This offset occurred after glaciers withdrew from the lowland 11,300–15,380 cal yr b.p. and after outwash channel margins were cut and stabilized 11,210–13,470 cal yr b.p. Using these ages and the measured separation, we obtain a maximum slip rate of 3.0 ± 0.6 mm yr^–1 and a minimum slip rate of 2.8 ± 0.7 mm yr^–1. These are the first lateral slip rates for the Castle Mountain fault established by a field measurement. Based on timing of the most recent earthquake, 670 ± 60 yr b.p., the Castle Mountain fault could have accumulated an average single-event slip of about 1.9 m (extremes range from 1.3 to 2.6 m). The fault consists of two segments; a surface-rupturing earthquake likely will be limited to the 62-km-long western segment. Area-magnitude regression calculations suggest that such an earthquake on the western Castle Mountain fault would have a moment magnitude of 6.9 to 7.3.

[Short Notes] Wave Propagation in Irregularly Layered Elastic Models: A Boundary Element Approach with a Global Reflection/Transmission Matrix Propagator

Ge, Z., Chen, X. — 2007-06-11

A direct boundary element method that uses the full-space Green’s function is proposed for calculating elastic wave propagation in two-dimensional irregularly stratified models. The global matrix equation becomes larger as the number of layers increases. These equations are usually solved by improved block Gaussian elimination, conjugate gradient algorithms, or other approaches based on different approximations. In this article, we adopt the global generalized reflection/transmission matrix method (Chen, 1990, 1995, 1996) to solve this problem. This method can prevent excessive requirement of both computer memory and CPU time. The method is validated by comparing its results with those obtained using the finite- difference method.

[Errata] Earthquake Ground-Motion Prediction Equations for Eastern North America

Atkinson, G. M., Boore, D. M. — 2007-06-11

[Articles] Recovering Seismic Displacements through Combined Use of 1-Hz GPS and Strong-Motion Accelerometers

Emore, G. L., Haase, J. S., Choi, K., Larson, K. M., Yamagiwa, A. — 2007-04-08

Retrieving displacement from seismic acceleration records is often difficult because unknown small baseline offsets in the acceleration time series will contaminate the doubly integrated record with large quadratic errors. One-hertz Global Positioning System (gps) position estimates and collocated seismic data are available from the 2003 M_w 8 Tokachi-Oki (Hokkaido) earthquake. After a process of correcting for possible misorientation of the seismic sensors, an inversion method is used to simultaneously solve for ground displacement with both data sets as input constraints. This inversion method takes into account the presence of unknown offsets in the acceleration record, and the relatively large uncertainties in the estimated 1-Hz gps positions.

In this study, 117 channels of seismic data were analyzed. Only 5% of the time does the static displacement retrieved from traditional baseline correction processing without gps information agree with the absolute displacement measured with 1-Hz gps to within the errors of the gps data. In solving simultaneously for constrained displacements that agree with both the seismic and gps data sets, an optimal solution was found that included only one- or two-step functions in the acceleration records. Potential explanations for the offsets are analyzed in terms of tilt of the sensor or electronic noise. For nine stations, clear misorientations of the seismic sensors of more than 20 deg from the reported orientation were found. For this size event, the 30-sec sampled gps solutions were also a sufficient constraint for establishing the offset errors and recovering reliable displacements. The results significantly extend the frequency band over which accelerometer data are reliable for source inversion studies.

Online material: Plots of constrained seismograms with metadata.

[Articles] Estimating Local Vp/Vs Ratios within Similar Earthquake Clusters

Lin, G., Shearer, P. — 2007-04-08

We develop and test a method to estimate local V_p/V_s ratios for compact similar earthquake clusters using the precise P and S differential times obtained using waveform cross-correlation. We demonstrate how our technique works using synthetic data and evaluate likely errors arising from near-source takeoff angle differences between P and S waves. We use a robust misfit function method to compute V_p/V_s ratios for both synthetic data sets and several similar event clusters in southern California, and use a bootstrap resampling approach to estimate standard errors for real data. Our technique has higher resolution for near-source V_p/V_s ratios than typical tomographic inversion methods and provides constraints on near-fault rock properties.

[Articles] Comparison of Location Procedures: The Kara Sea Event of 16 August 1997

Schweitzer, J., Kennett, B. L. N. — 2007-04-08

Various location procedures and velocity models are compared for the M_L 3.3 Kara Sea event of 16 August 1997. This event has been the subject of considerable discussion because of its geographical position and the difficulty in obtaining a reliable focal-depth estimate. A comprehensive data set was extracted by (re-)reading the records from all available stations. These readings have then been used in a sequence of location experiments to examine the effect of using different velocity models to describe the travel times of the phases, and also to compare the use of a fully nonlinear scheme (shakeNA; Sambridge and Kennett, 2001) and a linearized location algorithm (hyposat; Schweitzer, 2001, 2002). A standard least-squares misfit criterion has been used for direct comparisons between the two methods.

The results confirm both the importance of S-wave information in assessing the depth of regional events, and the need to apply a reliable velocity model to place the strongest constraints on the location of the event. Even with only a limited data set, but an adequate velocity model, it is possible to find the position of the Kara Sea event close to the most probable locations; however, there is then no depth resolution. Reported error ellipses from standard data centers tend to have relatively small error ellipses. With the commonly made assumption that the reading errors and the a posteriori residuals have an unbiased normal distribution, such inversion results may indicate an unreasonably high resolution and accuracy of the solution.

The epicenter estimates for the whole data set using the range of different techniques agree quite well, with some overlap of the estimated confidence regions. The observed seismic source was most likely an earthquake in the middle or lower crust at about 10–30 km depth.

[Articles] Wave Gradiometry in Two Dimensions

Langston, C. A. — 2007-04-08

The spatial displacement gradient of a seismic wave is related to displacement and velocity through two spatial coefficients for any one dimension. One coefficient gives the relative change of wave geometrical spreading with distance and the other gives the horizontal slowness and its change with distance. The essential feature of spatial gradient analysis is a time-domain relation between three seismograms that yields information on the amplitude and phase behavior of a seismic wave. Filter theory is used to find these coefficients for data from 2D areal arrays of seismometers, termed gradiometers. A finite-difference star is used to compute the displacement gradient for irregularly shaped gradiometers, and a relation for the frequency-dependent error in the displacement gradient is obtained and applied to ensure accurate estimates. This kind of array analysis is useful for gradiometers at any distance from a source and yields a variety of time-domain and frequency-domain views of wave-amplitude changes and horizontal phase velocity estimates across the gradiometer. For example, time-dependent horizontal slowness and wave-azimuth plots are natural results of the analysis. These time-domain maps may be used in conjunction with time–distance and horizontal slowness–distance models to locate seismic sources or may be used directly to study earth structure. These methods are demonstrated by using data from a small-aperture (~40 m) seismic gradiometer.

[Articles] A Seismoacoustic Analysis of the Gas-Pipeline Explosion near Ghislenghien in Belgium

Evers, L. G., Ceranna, L., Haak, H. W., Le Pichon, A., Whitaker, R. W. — 2007-04-08

A high-pressure gas pipeline exploded near Ghislenghien in Belgium, on 30 July 2004. Seismic energy of this event was recorded at three nearby seismometers. The origin time of the explosion was determined by analyzing the Rayleigh waves and found to be 06h55m27±2sec coordinated universal time (utc). Ground truth of the explosion was invoked to assess the accuracy of the location derived from infrasound data recorded over Western Europe. Different infrasonic phases were identified by using array-processing techniques and raytracing through atmospheric models. The analysis showed that the derived location was situated 13 km from the true location. The total area of the uncertainty ellipse, or area to be searched if the source was of unknown origin, was 690 km². After the origin time and location, the yield was calculated by comparing infrasonic stratospheric amplitudes with those from a high-explosives dataset and was estimated at 40.9 tons high- explosives equivalent. In summary, this study illustrates the capability of seismic and infrasound data in forensic investigations. Furthermore, the potential of infrasound as a monitoring or verification technique is addressed.

[Articles] Correlation of Lg Amplitude Ratios from Peaceful Nuclear Explosions to Crustal Structure in Northern Eurasia

Li, H., Morozov, I. B., Smithson, S. B. — 2007-04-08

Because the Lg phase is strongly affected by the variations of crustal structure in some areas, understanding these factors is important for nuclear test monitoring. This study examines the effects of regional tectonic structure on Lg propagation by using the Peaceful Nuclear Explosion (pne) profiles in Russia. The logarithms of Lg/Sn and Lg/Pcoda pne-amplitude ratios within the 0.5- to 3-Hz frequency band are measured and correlated with the regional crustal structures. Both ratios are found to decrease within the areas with thick, low-velocity sedimentary cover and across tectonic boundaries with abrupt variations of crustal thickness. For the offset derivatives of both the logarithmic ratios, a linear relationship to the slopes of the Moho and the vertical travel times within the sedimentary cover is determined. A strong negative slope is observed along both crustal thinning and crustal thickening segments, and a positive slope follows the negative slope across a crustal thickening segment. Therefore, crustal thinning appears to affect the Lg propagation stronger than crustal thickening, which could be due to focusing Lg energy within the crustal wave guide. The log-amplitude regression provided in this article could be useful for predicting Lg behavior in areas where amplitude measurements are not available.

[Articles] Detailed Analysis of Wave Propagation beneath the Campi Flegrei Caldera, Italy

Nisii, V., Saccorotti, G., Nielsen, S. — 2007-04-08

We investigate the complex propagation of seismic waves beneath the Campi Flegrei caldera, Italy, using multichannel recordings of artificial explosions. The sources consisted of air gun explosions shot in the Gulf of Pozzuoli at offsets ranging between 3 and 7 km. A multichannel recording device was deployed in the Solfatara crater and consisted of ten vertical-component and two three-component short-period seismometers with a maximum aperture of about 150 m. The zero-lag correlation (zlc) technique was adopted to estimate horizontal slowness and backazimuth of coherent waves crossing the array. For sources located in the northern sector of the Gulf, with maximum offset 5 km, ray parameters and backazimuths are in agreement with those predicted for the 1D velocity model used for routine locations. For sources at offsets larger than ~5 km, the zlc curves depict prominent maxima associated with a secondary phase propagating with a lower velocity than the first-arrival P wave. Using finite-difference synthetic seismograms generated for a 2D realistic velocity model, we explain these late arrivals in terms of a lateral velocity variation located at depths of about 1 km. Such discontinuity would correspond to a positive V_p anomaly imaged by a recent 3D tomographic study, and interpreted as the submerged southern rim of Campi Flegrei caldera collapsed during the explosive eruption of 12 ky b.p. The small spacing among adjacent shot points allowed simultaneous wave-field decomposition at the source and receiver arrays. Using a modified version of the double-beam method, we retrieve the independent variation of horizontal slowness at both the source and receiver regions. For both cases, we found azimuthal deviations as large as 50° with respect to the great circle path. At the source region, these discrepancies may be interpreted in terms of ray bending at the interface of the aforementioned positive anomaly. At the receiver array, the observed anomalies may be attributed to either velocity variations marking the Solfatara crater rim, or to a near-receiver, low-velocity body whose position would coincide with negative gravimetric anomalies and a high V_p/V_s ratio region inferred by independent geophysical and seismological studies.

[Articles] The October 2005 Georgian Bay, Canada, Earthquake Sequence: Mafic Dykes and Their Role in the Mechanical Heterogeneity of Precambrian Crust

Dineva, S., Eaton, D., Ma, S., Mereu, R. — 2007-04-08

On 20 October 2005 at 21:16 UTC, a moderate earthquake (m_N 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thornbury, Ontario (44.67° N, 80.46° W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low- and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were installed in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral parameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M₀ 4.5 x 10¹⁴ N m and corner frequency of 3.7 Hz, based on a spectral fit using Brune’s source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reverse- sense of slip and northwest-striking nodal planes. The reverse mechanism and midcrustal focal depths (10–12 km) are characteristic, in general, of more abundant seismicity located ~200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2–6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie ~200 km to the south. We attribute this north–south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precambrian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.

[Articles] Surface Ruptures Associated with the 1937 M 7.5 Tuosuo Lake and the 1963 M 7.0 Alake Lake Earthquakes and the Paleoseismicity along the Tuosuo Lake Segment of the Kunlun Fault, Northern Tibet

Guo, J., Lin, A., Sun, G., Zheng, J. — 2007-04-08

Examination of aerial photos taken in the late 1960s and field mapping allowed us to re-evaluate the surface ruptures associated with the 1937 M 7.5 Tuosuo Lake and 1963 M 7.0 Alake Lake earthquakes along the Tuosuo Lake and Alake Lake segments, central part of the Kunlun fault. Individual ruptures can be distinguished by their freshness of surface expression, slip distributions, and surficial geometry; they are separated by the geometrical bend that probably acts as an effective barrier to rupture propagation of these earthquakes. The rupture (~40 km long with 1–2 m left-lateral slip) of the 1963 Alake Lake earthquake occurred on the west of the bend, whereas that of the 1937 Tuosuo Lake earthquake (about 150 km long with 4.1 m mean left-lateral offset) occurred on the east of the bend. The change of dip- slip sense reflects the characteristics of jogs and bends along the segment boundaries. Systematically and progressively displaced terrace risers and gullies indicate repeated activity of the fault during the late Quaternary. Five trenches excavated across the surface ruptures along the Tuosuo Lake segment reveal distinct indicators of paleoseismic events. At least nine earthquakes are constrained in the Holocene deposits, and a 630 ± 130 yr, upper-bound recurrence interval of large earthquakes in the past 2000 years is determined.

[Articles] Relationships between Felt Intensity and Instrumental Ground Motion in the Central United States and California

Atkinson, G. M., Kaka, S. I. — 2007-04-08

In this study, we develop empirical relationships between instrumental ground-motion parameters and observed Modified Mercalli Intensity (mmi) by using data from felt moderate earthquakes in the central United States (cus) that were also recorded on broadband seismographic networks and strong-motion recorders in the cus region. The data are calibrated and supplemented at higher intensities based on observations in California. mmi for ShakeMap applications in the cus region, and in California, can be predicted from recorded peak ground velocity (pgv), in cm/sec, with a standard deviation of 0.8 mmi units, using the following equation:

MMI = 4.37 + 1.32(log PGV) log PGV ≤ 0.48

MMI = 3.54 + 3.03(log PGV) log PGV ≥ 0.48

There are weak-magnitude and distance-dependent trends in the residuals for this relationship. These trends, if not removed, may lead to apparent regional dependencies in mmi versus ground-motion amplitude relationships. Refined relationships that include magnitude and distance as predictive variables that are applicable throughout North America are defined.

[Articles] Empirical Prediction Equations for Peak Ground Velocity Derived from Strong-Motion Records from Europe and the Middle East

Akkar, S., Bommer, J. J. — 2007-04-08

Peak ground velocity (pgv) has many applications in earthquake engineering, but there are relatively few prediction equations for this parameter in comparison with the large numbers of equations for estimating peak ground acceleration and response spectral ordinates. This lack of empirical equations for pgv has led to widespread use of the practice of scaling peak velocity from the 5%-damped response spectral ordinate at 1 sec, which is a poor substitute for direct prediction of the parameter. Responding to the need to provide equations for the prediction of pgv, this article derives new equations using the strong-motion database for the seismically active areas of Europe and the Middle East, following a new processing of all of the records. A total of 532 strong-motion accelerograms recorded at distances of up to 100 km from 131 earthquakes with moment magnitudes ranging from M 5 to 7.6 are used to derive equations for both the larger and the geometric mean of the horizontal components. The predictions are found to be broadly consistent with those from previous European equations, and also with preliminary results from the Next Generation of Attenuation (nga) project, suggesting that systematic differences in ground motions from active crustal regions, if any, are sufficiently small not to prevent the combined use of strong-motion data from southern Europe, western North America, and other tectonically active areas of shallow crustal seismicity.

[Articles] Ground-Motion Characterization of Low-to-Moderate Seismicity Zones and Implications for Seismic Design: Lessons from Recent Mw ~4.8 Damaging Earthquakes in Southeast Spain

Gaspar-Escribano, J. M., Benito, B. — 2007-04-08

We address the ground-motion characterization of three damaging earthquakes that occurred in the low-to-moderate seismicity region of Murcia (southeast Spain) and compare our results with current earthquake-resistant provisions to establish whether those provisions are consistent with the available data or need to be revised.

The analyzed series are the 1999 Mula (m_bLg 4.8, I_EMS = VI), the 2002 southwest Bullas (m_bLg 4.8, I_EMS = VI), and the 2005 La Paca (m_bLg 4.7, I_EMS = VII). Recorded peak ground accelerations are low (pga < 0.025 g), partly because of the low magnitude of the events and the relatively large epicentral distance of the stations (R_ep >20 km). We find that local factors control the distribution of ground motions, possibly in combination with propagation effects. Characteristic spectral shapes, representative of ground motions at each station, are observed. Several ground-motion predictive models and a simulation method are used for estimating accelerations and response spectra in the most damaged towns, where no records exist. The different methods consistently predict similar response spectra for the epicentral areas.

Recorded and predicted normalized spectral shapes exceed the design spectral shape of the Spanish Building Code ncse-02 for all soil categories for intermediate– high frequencies (above about 3 Hz). Moreover, the ncse-02 absolute response spectra are possibly exceeded by the corresponding predicted spectra for the epicentral areas in the same frequency range. Predominant frequencies of common soils and critical periods of most conventional buildings in the Region of Murcia also lie in this frequency range. These factors could explain, in part, the observed damage.

The results raise questions about the definition of the ncse-02 design spectra, and specifically about the design spectral shape used. The approach followed in this work may be used to calibrate other national seismic codes, especially of regions with similar characteristics: moderate seismicity and limited availability of ground-motion data.

[Articles] Seismic-Wave Attenuation and Source Excitation in La Paz-Los Cabos, Baja California Sur, Mexico

Ortega, R., Gonzalez, M. — 2007-04-08

We present results from a regional study of seismic-wave attenuation and source excitation from small-magnitude earthquakes recorded at distances from 6 to 180 km in the La Paz–Los Cabos region, at the south end of the Baja California Peninsula. Data were recorded using 32 strong-motion seismic stations from the La Paz network (lap). A least-squares regression separating the excitation, site, and propagation effects was carried out. We performed the analyses in the time and frequency domains, and we compared these results with results from a coda- normalization method. The propagation term was parameterized to represent a geometrical spreading function and a frequency-dependent Q(f) at a reference distance of 40 km. We estimated the regional attenuation by measuring the maximum amplitude of the S or Lg waves as a function of frequency, defining a continuous piecewise propagation term, D(r, f), after separating the excitation and site terms. Our results show that the attenuation is lower than that of central or northern Mexico. Recorded data were of remarkably good quality despite the fact that the strong- motion network recorded only small earthquakes. Our best model is that of a quality factor of Q(f) = 380f^0.10 with a complex geometrical spreading function.

[Articles] Local Magnitude Determinations for Intermountain Seismic Belt Earthquakes from Broadband Digital Data

Pechmann, J. C., Nava, S. J., Terra, F. M., Bernier, J. C. — 2007-04-08

The University of Utah Seismograph Stations (uuss) earthquake catalogs for the Utah and Yellowstone National Park regions contain two types of size measurements: local magnitude (M_L) and coda magnitude (M_C), which is calibrated against M_L. From 1962 through 1993, uuss calculated M_L values for southern and central Intermountain Seismic Belt earthquakes using maximum peak-to-peak (p-p) amplitudes on paper records from one to five Wood–Anderson (W-A) seismographs in Utah. For M_L determinations of earthquakes since 1994, uuss has utilized synthetic W-A seismograms from U.S. National Seismic Network and uuss broadband digital telemetry stations in the region, which numbered 23 by the end of our study period on 30 June 2002. This change has greatly increased the percentage of earthquakes for which M_L can be determined. It is now possible to determine M_L for all M ≥3 earthquakes in the Utah and Yellowstone regions and earthquakes as small as M <1 in some areas.

To maintain continuity in the magnitudes in the uuss earthquake catalogs, we determined empirical M_L station corrections that minimize differences between M_Ls calculated from paper and synthetic W-A records. Application of these station corrections, in combination with distance corrections from Richter (1958) which have been in use at uuss since 1962, produces M_L values that do not show any significant distance dependence. M_L determinations for the Utah and Yellowstone regions for 1981–2002 using our station corrections and Richter’s distance corrections have provided a reliable data set for recalibrating the M_C scales for these regions. Our revised M_L values are consistent with available moment magnitude determinations for Intermountain Seismic Belt earthquakes.

To facilitate automatic M_L measurements, we analyzed the distribution of the times of maximum p-p amplitudes in synthetic W-A records. A 30-sec time window for maximum amplitudes, beginning 5 sec before the predicted Sg time, encompasses 95% of the maximum p-p amplitudes. In our judgment, this time window represents a good compromise between maximizing the chances of capturing the maximum amplitude and minimizing the risk of including other seismic events.

[Articles] Estimation of the Wave Fields in the Three Geotechnical Zones of Tokyo

Iida, M. — 2007-04-08

We estimate the wave fields in the three geotechnical zones (the hill, alluvial, and reclaimed zones) of Tokyo by analyzing the three components of surface and downhole strong-motion accelerograms recorded at six borehole stations. Although the wave field of the reclaimed zone was already estimated in a recent study, it is partly re-estimated by an improved technique. The target-period range is mainly between 0.1 and 2.0 sec. The wave types of observed ground motions are identified by calculating the theoretical vertical amplitude ratios of body waves and surface waves and the cross-correlation functions between the surface and downhole recordings. In the calculation of the theoretical amplitude ratios, the damping for wave propagation and the damping for the eigenfunctions of surface waves are separately considered. Further, the horizontal components of the surface and downhole recordings are separated into S-wave and surface-wave accelerograms. The main conclusions are summarized as follows: (1) the ground motions observed in the alluvial and reclaimed zones are mostly composed of S waves and Love waves; (2) the Love- wave amplitude ratios are larger than the S-wave ratios in a soft surficial deposit around the theoretical predominant period. The large Love-wave amplitude ratios are excited by the surficial deposit and a deep sedimentary basin; and (3) quality factors near 10 are obtained for the soft deposit around the predominant period.

[Articles] Site Effects in a Volcanic Environment: A Comparison between HVSR and Array Techniques at Colima, Mexico

Chavez-Garcia, F. J., Dominguez, T., Rodriguez, M., Perez, F. — 2007-04-08

Colima city is the capital of the Mexican federal state of the same name. It is located close to the Pacific coast and is subjected to a large seismic risk. We present a microzonation study in this city, based on microtremors using single-station and array measurements. We applied horizontal-to-vertical spectral ratios (hvsr) analysis to single-station measurements at 310 sites within the city, concentrating measurements in zones that were damaged by the January 2003 (M 7.4) earthquake. The results show that a seismic zonation based exclusively on single-station microtremor measurements is not a reliable alternative when the local geology is complex and site effects are not the result of a single-impedance contrast. For this reason, we applied two independent analysis techniques to array measurements of microtremors: the spatial autocorrelation (spac) method and the refraction microtremor (ReMi) method. We used linear arrays to record 25-sec microtremor windows at eight sites within the city, which were analyzed with those two techniques. The result of both techniques of analysis is a phase-velocity dispersion curve, which can be inverted to obtain a shallow S-wave velocity profile. Two of the sites were the location of shallow (50 m) boreholes, where P- and S-wave velocity profiles were measured using a P-S suspension log. The phase-velocity dispersion curves obtained from the ReMi and spac analyses of the microtremor records showed very good agreement. The velocity profiles inverted from the phase-velocity dispersion curves showed good agreement with the suspension logging measurements at one of the two sites where they were available and poor agreement at the other site. The transfer functions computed from the inverted soil profiles are in good agreement with previous estimates of local amplification from spectral ratios analysis of earthquake records. Our results are compatible with previous indications of site effects and explain the failure of single-station microtremor measurements when the concept of dominant frequency loses its meaning. Finally, we propose an estimate of local site amplification at the city of Colima, which will be useful for future predictions of ground motion at this city.

[Articles] Site-Amplification Effects Based on Teleseismic Wave Analysis: The Case of the Pellice Valley, Piedmont, Italy

Ferretti, G., Massa, M., Isella, L., Eva, C. — 2007-04-08

The investigation of local amplification phenomena by seismic signal analysis is a fundamental step in carefully defining the seismic response of an area. In this study we investigate the use of teleseismic recordings in assessing seismic- wave amplification in the Pellice Valley (northwestern Alps, Italy). Assuming that teleseismic P waves are sensitive to the deep structure of a basin, we deal with the computation of horizontal-to-vertical spectral ratios (hvsrs) and with the estimate of teleseismic P-wave arrival time delays and P-wave amplifications with respect to a reference site. The reliability of the hvsr results obtained by considering teleseismic signals is confirmed by the agreement with the results coming from both the hvsr of noise and hvsr of S wave of local events methods. Strong correlation between the P-wave arrival time delays and the relative P-wave amplifications with respect to thickness of the low-velocity layers and the geometry of the bedrock is found.

[Articles] Determination of Shallow Shear-Wave Velocity at Mississippi Embayment Sites Using Vertical Seismic Profiling Data

Ge, J., Pujol, J., Pezeshk, S., Stovall, S. — 2007-04-08

We used vertical seismic profiling (vsp) data collected in shallow boreholes (about 40–60 m deep) to determine the shear-wave velocity at four sites in the Mississippi embayment in southwestern Tennessee. The source was an air-powered hammer that produces repeatable SH waves, which were recorded by source monitor geophones deployed on the surface very close to the source. Three approaches were used to determine interval velocities: an approximate zero-offset method, a layer- stripping method, and a waveform-matching method. The first two methods use arrival-time picks, whereas the latter is based on the fit of synthetic vsp data to the first half-cycle (approximately) of each trace. The advantage of this method over the other two is that it uses a segment of the data, rather than a single data point. Therefore, the velocities determined using the waveform-matching method are better constrained and are not affected by picking errors, which may translate into significant spurious velocity variations. The source wavelets recorded by one of the monitor geophones and the velocity model computed with the layer-stripping method were used to generate synthetic vertical seismic profiling data for comparison with the actual data. Then the model velocities were modified interactively, one layer at a time, until a satisfactory match was achieved. This required including attenuation in the computation of the synthetic data. The four sites investigated in this study can be divided into two groups: low-velocity sites (Shelby Farms and Covington) and high-velocity sites (Brownsville and Jackson). These last two sites are at larger distances from the embayment axis than the other two, which means that the difference in velocities probably corresponds to the presence of different geologic units. Good agreements between the lithology in the boreholes and the velocity profiles were obtained for all the four sites.

[Articles] Frequency-Dependent Site Amplifications with f >= 0.01 Hz Evaluated from Velocity and Density Models in Central Taiwan

Huang, M.-W., Wang, J.-H., Ma, K.-F., Wang, C.-Y., Hung, J.-H., Wen, K.-L. — 2007-04-08

The frequency-dependent site amplifications at 87 free-field strong- motion station sites in central Taiwan are evaluated from the velocity and density structures constructed from borehole data measured at shallow depths and the velocity models inferred from earthquake data at great depths. Results based on the quarter- wavelength approximation method proposed by Boore and Joyner (1997) show that the site amplifications increase with frequency and are larger in the Western Plain with thick Holocene alluvium than in the Western Foothill with Pleistocene and Miocene formations. Considering wave attenuation, site amplification first increases and then decreases with increasing frequency. The turning frequency, f_t, associated with the maximum amplification varies site by site.

[Short Notes] Quantifying the Media Bias in Intensity Surveys: Lessons from the 2001 Bhuj, India, Earthquake

Hough, S. E., Pande, P. — 2007-04-08

Many seismologists have looked at the 26 January 2001 Bhuj earthquake as a key modern calibration event that could be used to improve estimates of magnitudes of large historic mainshocks in stable continental regions. Since no instrumental data are available for important historic events such as the 1819 Allah Bund, India, and the 1811–1812 New Madrid, central U.S. mainshocks, calibration hinges on comparisons of the macroseismic effects of these earthquakes with those of comparable modern earthquakes for which a reliable, instrumentally determined moment magnitude is available. However, although such a comparison is conceptually straightforward, in practice it is complicated by potentially significant inconsistencies in methods used to quantify macroseismic effects in different regions and/ or times. For the Bhuj earthquake, extensive intensity data sets have been compiled and published from both media accounts and detailed direct surveys. Comparing the two provides a quantification of the previously suspected media bias, whereby earthquake effects can be exaggerated in media accounts. This bias is a strong function of intensity level, with substantial bias at the highest shaking levels and significantly less bias at low intensities. Because only sparse documentary data are in general available for older historic earthquakes, the results of this study suggest that their inferred intensity distributions might be similarly biased. We further use the survey- based intensity values to develop a new relationship between intensities and ground motions.

[Short Notes] Earthquake Environmental Effects in Colombia during the Past 35 Years: INQUA Scale Project

Lalinde P., C. P., Sanchez A., J. J. — 2007-04-08

In this work we present the results of applying the inqua intensity scale to the analysis of the environmental effects of 12 moderate-to-large earthquakes with moment magnitudes (M_w) in the range 5.6–8.1 that shook Colombia between 1970 and 2004. These earthquakes were selected following consideration of their depths, magnitudes, and relevance of effects during shaking. After analysis of shaking effects, we found that earthquakes in Colombia could be assigned inqua intensities between 6 and 11. For a subgroup of seven earthquakes, we found a positive correlation between inqua intensities and M_w defined by a best-fit linear trend with a correlation coefficient of 0.7 and a positive correlation between inqua intensities and Modified Mercalli intensities with a correlation coefficient of 0.6. The inqua scale allowed us to compare local earthquakes with other earthquakes worldwide and to analyze each earthquake to better understand the ground response in Colombia to the shaking during shallow earthquakes. We conclude that, in general, the inqua scale provides a good estimate of intensity that may complement other measures of intensity.