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3D Modeling of Ground Motions for Events in the 2019 Ridgecrest Sequence

In addition to the M6.4 and M7.1 events, the Ridgecrest sequence produced several moderate magnitude (~M5) earthquakes that were also well-recorded by the roughly 300 stations of the Southern California Seismic Network (SCSN). Using these data, we examine the adequacy of existing Southern California Earthquake Center (SCEC) 3D Community Velocity Models (CVMs) to simulate long period (T > 1 sec) ground motions across southern California. Preliminary analysis of the motions from moderate magnitude events indicates that sites in the epicentral area have very simple waveforms, indicating that these events can be treated as point sources at the longer periods and that the subsurface structure near these sites is relatively simple. However, sites to the west of the epicentral area in Ridgecrest exhibit significant later-arriving, long-period motions, suggesting these arrivals may be basin-generated surface waves. Further to the south in the Los Angeles region, the recorded motions exhibit significant amplification (factors of 3-4) and long shaking durations for sites within the deep basins as compared to sites located outside of the basins. In Ridgecrest and across the Mojave Desert, both SCEC CVMs (cvmh and cvmsi) are comprised of smoothly varying velocity structure that is constrained primarily by waveform tomography. To the south, this structure transitions into several deep low-velocity basins in the Los Angeles region, although the details of these structures differ somewhat between the two CVMs. In the first phase of our work, we simulate the long-period motions for the moderate magnitude events using both CVMs and assess the ability of each model to reproduce the observed motions at sites across the southern California region. In the second phase, we extend our analysis to the M6.4 and M7.1 events in order to explore our ability to capture both complex finite-faulting effects along with 3D wave propagation effects.

Presenting Author: Robert Graves

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