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We have been testing the applicability of a new simplified source model called the “pseudo point-source model,” which was aimed to calculate strong ground motions (Nozu 2012). The model is simpler, and involves much less model parameters than the conventional characterized source model, which itself is a simplified expression of an actual earthquake source. In the model, the spatio-temporal distribution of slip within a subevent is not modeled. Instead, the source spectrum associated with the rupture of a subevent is modeled and it is assumed to follow the omega-square model. By multiplying the source spectrum with the path effect and the site amplification factor, the Fourier amplitude at a target site can be obtained. Then, combining it with Fourier phase characteristics of a smaller event, the time history of strong ground motions from the subevent can be calculated. Finally, by summing up contributions from the subevents, strong ground motions from the entire rupture can be obtained. In this study, the model was applied to the main shock of the 2004 southwest-off Kii Peninsula earthquake (Mw7.4), which occurred on September 5, 2004, 23:57 (JST) near the Nankai Trough, within the subducting Philippine Sea plate (e.g., Park and Mori 2005; Bai et al. 2007). Because of its location, strong motion records obtained during the earthquakes are suitable for the validation of strong motion simulation techniques. The pseudo point-source model for the earthquake was developed by appropriately locating point sources on the fault plane, based on the results of the waveform inversion. Then the model was used to simulate strong ground motions in a wide region. It was found that, when the model is used with an appropriate path model, it can accurately simulate strong ground motions at remote stations, even when the stations are located on deep sedimentary basins such as the Osaka, Nobi and Kanto plains.

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Fault slip and surface deformation patterns are essential factors in seismic hazard assessment. However, slip inversions reveal a widely observed shallow slip deficit (SSD) which has not yet been clearly explained. One possible cause of the SSD is distributed plastic deformation in the fault damage zone near the surface. Roten et al. (2017) performed 3D dynamic rupture simulations of the 1992 M7.3 Landers earthquake in a medium governed by Drucker-Prager plasticity. The study showed that while linear simulations tend to underpredict SSD and off-fault deformation (OFD), nonlinear simulations with moderately fractured rock mass can properly reproduce results that are consistent with the 30-60% SSD and around 46% OFD reported in geodetic observations. Analysis of the Roten et al. (2017) results show that discrepancies between linear and nonlinear simulations are only significant in the top hundreds of meters of the surface-rupturing fault. Although inelastic responses in the fault damage zone provide more realistic representations of earthquake physics, it can be computationally expensive or numerically unfeasible (e.g., in adjoint methods) to include nonlinear effects in ground motion simulations. One solution proposed here is to use an equivalent kinematic source (EKS) model that mimics the fault-zone plastic effects. This method generates source-time-functions by modifying the slip rate time histories based on comparisons of linear and non-linear dynamic rupture models, which are then used as input to kinematic simulations. The EKS model is shown to capture the SSD and OFD patterns observed in dynamic simulations with fault-zone plasticity. Further verification of the method is needed before the anticipated use in practical applications such as the Southern California Earthquake Center (SCEC) CyberShake and Broadband platforms.

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Built-up on top of ancient lake deposits, Mexico City experiences some of the largest seismic site effects in the world. The M7.1 intermediate-depth earthquake of September 19, 2017 (S19) collapsed 43 one-to-ten story buildings in the city close to the western edge of the lake-bed sediments, on top of the geotechnically-known transition zone. In this work we explore the physical reasons explaining such a damaging pattern and the long-lasting strong motion records well documented from past events by means of new observations and high performance computational modeling. Besides the extreme amplification of seismic waves, duration of intense ground motion in the lake-bed lasts more than three times those recorded in hard-rock a few kilometers away. Different mechanisms contribute to the long lasting motions, such as the regional dispersion and multiple-scattering of the incoming wavefield all the way from the source. However, recent beamforming observations at hard-rock suggest that duration of the incoming field is significantly shorter than the strong shaking in the lake-bed. We show that despite the highly dissipative shallow deposits, seismic energy can propagate long distances in the deep structure of the valley, promoting also a large elongation of motion. Our 3D simulations reveal that the seismic response of the basin is dominated by surface-waves overtones, and that this mechanism increases the duration of ground motion up to 280% and 500% of the incoming wavefield duration at 0.5 and 0.3 Hz, respectively. Furthermore, our results indicate that the damage pattern of the S19 earthquake is most likely due to the propagation of the fundamental mode in the transition zone of the basin generated as a consequence of a localized basin edge effect. Some conclusions contradicts what has been previously stated from observational and modeling investigations, where the basin itself has been discarded as a preponderant factor promoting long and devastating shaking in Mexico City. Reference: Cruz-Atienza, V. M., J. Tago, J. D. Sanabria-Gómez, E. Chaljub, V. Etienne, J. Virieux and L. Quintanar. Long Duration of Ground Motion in the Paradigmatic Valley of Mexico. Nature – Scientific Reports, 6, 38807; doi:10.1038/srep38807, 2016.

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