We conclude that (1) the peak nonlinear responses due to the modified records and the scaled-real records are similar if the peaks and troughs effects and response spectral shape effects are taken into account adequately in the choice of scaling factors for the real records; and (2) the peak nonlinear responses due to the simulated and modified records are similar. These findings, though obtained based on limited sets of ground-motion records, are in agreement with previous studies and highlight the need for judicious choices in cases involving the scaling of records. The results also suggest that stochastically simulated records may be an appropriate way to capture overall response potential for both linear and nonlinear structural systems, at least over a range of periods from 0.1 to 2 sec.

We analyzed both ground acceleration spectra of the mainshocks and single-station spectral ratios of broadband seismograms in an extended magnitude range (2.7≤M_w≤5.7). Our results show that neither the spectral amplitudes of recorded ground motions nor the spectral ratios can be fit by a high stress-drop source. Instead we find that the observations are consistent with a low stress drop, our best estimates ranging between 6 and 25 bars, in agreement with the relatively long source duration and small coseismic slip.

We interpret the low stress of the 2002 Molise earthquakes in terms of lower energy release mechanisms due to the reutilization of faults reactivated opposite to their original sense of slip.

Estimates of U_seis for the time intervals 0–1999, 1500–1999, and 1964–2006 using historic and instrumental catalogs suggest a considerable seismic slip deficit when compared with the total slip expected from the temporal extrapolation of geodetic measurements. This suggests a recent weakly coupled plate contact and a total slip predominantly accommodated aseismically despite the 365 A.D. event. High seismic activity up to magnitudes of about M_w 6 is found accompanying the aseismic slip at the plate contact.

Investigations of the seismicity distribution hint at a spatial variability of the seismic energy release in the western forearc of the HSZ. Activity in the proximity of the 365 A.D. epicenter is observed to be an order of magnitude larger than toward the northwest between Crete and Peloponnese. This spatiotemporal variability of the seismic energy release and seismic coupling suggests that subregions of the plate contact undergo alternating periods of locking followed by intermediate magnitude events and aseismic sliding. Thus, the behavior of significant portions of the plate contact southwest of Crete may be described as conditionally stable.

The answer to this question is rooted in significant differences between point-source and finite-source stochastic simulation methodologies, specifically as implemented in SMSIM (Boore, 2005) and EXSIM (Motazedian and Atkinson, 2005) to date. Point-source and finite-fault methodologies differ in general in several important ways: (1) the geometry of the source; (2) the definition and application of duration; and (3) the normalization of finite-source subsource summations. Furthermore, the specific implementation of the methods may differ in their details. The purpose of this article is to provide a brief overview of these differences, their origins, and implications. This sets the stage for a more detailed companion article, "Comparing Stochastic Point-Source and Finite-Source Ground-Motion Simulations: SMSIM and EXSIM," in which Boore (2009) provides modifications and improvements in the implementations of both programs that narrow the gap and result in closer agreement. These issues are important because both SMSIM and EXSIM have been widely used in the development of ground-motion prediction equations and in modeling the parameters that control observed ground motions.

In the M_w 5.0 mainshock, mean-horizontal peak ground acceleration exceeded 0.6g at three stations. The strongest peak vector acceleration was 1164 cm/sec², or about 1.19g, at the station MOGL (~0.4 km from the epicenter) at a frequency of about 3 Hz. The mean-horizontal peak velocity exceeded 12 cm/sec at the four nearest stations, and the peak vector velocity at the strongest was 54 cm/sec. The duration of the strongest shaking was about 2 sec at these stations. The observed peak accelerations and peak velocities decreased more rapidly with distance than predicted, to values below the median predictions at most stations in the Reno metropolitan area. The shallow source depth is, to some degree, a likely contributing factor to these unusual observations. The motions in the residential neighborhood near the epicenter are significantly greater than the seismic forces required by the 1985 Uniform Building Code Zone 3 that was in effect when the Mogul neighborhood was developed, but damage to the wood-frame structures in the neighborhood was minimal.

We also performed an inversion of the available macroseismic observations, based on the kinematic function (KF) for the radiation of body waves from a line source. Because the problem is nonlinear and also bimodal for pure dip-slip mechanisms, we used a sharing niching genetic algorithm to perform the inversion. The new epicenter obtained from the KF intensity inversion is consistent with the new instrumental solution, giving a location in the lower crust (25–30 km) and close to the eastern coast of the Oslofjord, near the junction of two major fault zones. The KF inversion was moreover able to constrain the fault-plane mechanism with an almost vertical rupture plane striking north-northeast–south-southwest (206°–212°) with a mixed mechanism (approximately 64° rake angle), the ambiguity of which is resolved by the polarities of the Pn onset observed on the horizontal components of the Uppsala Wiechert seismograph. The magnitude of the event has previously been assessed to M_S 5.4, based on the size of the felt area observations, and a value of M_S~5.4 has now been independently confirmed both through the new intensity inversion and through the instrumental data.

The field applicability of the c^L estimation methods based on the proposed formulas—which we name the SPAC+L, SPAC-L, and CCA-L (where CCA stands for centerless circular array) methods—is demonstrated by analysis results for two test sites. Among the three techniques proposed, the SPAC+L method distinguished itself as the best-performing. The valid wavelength ranges of the three methods had lower limits in the area of 2–5 times the array radius r and upper limits in the area of 10–25r.

Similar circular-array microtremor techniques that involve c^L alone were proposed in recent years, but the methods we present here are either logistically less expensive or mathematically simpler than any of them.

Focal mechanisms of fluid-induced earthquakes are characterized by a non-double-couple part. However, we show and discuss that the double-couple moment tensor approximation is valid as a source model.

Then, we propose a nonlinear inversion method of the direct P-, SV- and SH-wave amplitudes, based on a simulated annealing algorithm to determine double-couple focal mechanisms. Simultaneously, we determine the associated uncertainty. We take into account three sources of uncertainty related to the convergence process of the inversion to the amplitude picking uncertainty caused by the noise level and to the uncertainty of the event location. First, we test our method on synthetic data. Second, we apply the method on four events induced in the Soultz-sous-Forêts geothermal field whose focal mechanisms were already determined by Charlety et al. (2007). We obtain focal mechanisms with uncertainties containing the solutions previously determined.

Finally, we evaluate the required minimum number of sensors and their geometrical configuration to obtain a focal mechanism. The direction of the nodal planes and the type of mechanism are retrieved for data sets as small as three 3-component stations. The tests also reveal that the reliability of the fault plane solution depends on the configuration the stations used. It also seems that the coverage of the focal sphere by the stations, that is, the opening angle of the network and the coverage of several quadrants, has an influence on the reliability of the fault plane solution retrieval. The use of only one 3-component sensor allows retrieval of the type of focal mechanism in most of the cases studied.

An unexpected result of the study is the finding that ground motions for events of M<5.5 in California attenuate more rapidly with distance than predicted by the recent Pacific Earthquake Engineering Research Center–Next Generation Attenuation (PEER–NGA) ground-motion prediction equations for shallow crustal earthquakes in active tectonic regions (Abrahamson and Silva, 2008; Boore and Atkinson, 2008; Campbell and Bozorgnia, 2008; Chiou and Youngs, 2008). It appears that the limited M<5.5 data used in the PEER-NGA study are not representative of ground motions from M<5.5 events in California in general. (At larger magnitudes, however, the California ShakeMap observations converge with expectations based on the PEER-NGA equations.) By contrast, the California ground motions for M<5.5 at distances less than 100 km appear consistent with those that would be predicted by the ground-motion model of Atkinson and Boore (2006) for eastern North America, if an adjustment is made for a factor of 2 difference in stress drop between eastern and western North America. This attests to the need to include a broad range of magnitudes and distances in the development of comprehensive ground-motion prediction models.

We test our ability to predict strong ground motion by using our attenuation and source-scaling relations derived from the small-magnitude recordings to stochastically simulate peak ground acceleration, peak ground velocity, and 5% damped response spectral ordinates over a range of magnitudes and distances. The residuals of this simulation are found to be largely independent of distance and magnitude. We compare our attenuation relations against other relations derived for Japan. The residuals of these relations are analyzed and compared against those obtained from the model found in this study. We find that, in this study, the prediction of strong ground motions is possible using small-magnitude data and that the validity of the prediction extends across all magnitudes available for comparison (2.0≤M_JMA≤7.8). On the other hand, by using an alternative published predictive relation for Japan, derived using large-magnitude events, peak ground acceleration is significantly overestimated for small-magnitude earthquakes.

The residual variance has been decomposed into interevent, interstation, and record-to-record components by applying a random effect regression scheme. The interevent and interstation error distributions have been analyzed as function of periods to detect sites and events for which predicted values significantly deviate from observations. For periods up to 0.35 sec, the interstation is the dominant component of variance, indicating that an improvement in the site classification could lead to a refinement of the GMPEs. For longer periods, the three components of variance provide similar contributions, indicating that a reduction of the uncertainty can be achieved by reducing the epistemic uncertainty affecting the physical model. The interevent error highlights the peculiarity of few earthquakes, suggesting that the evaluation of regional GMPEs can be important when specific scenario studies should be carried out. The interstation variability allows us to detect stations with peculiar site response and to assess the goodness of the considered site classification scheme.

Response along the random interface is computed for different frequencies and the source configurations. The interface motion is found to strongly depend upon the incident system. Two incident system configurations are identified that produce the interface motion that is considerably amplified or reduced when compared to the corresponding displacements due to a single unit primary source. The two systems share the same geometry and the source amplitudes but have different source amplitude phases.