3D ground motion simulations for small to moderate magnitude events in the Ridgecrest sequence

Robert W. Graves

Published August 15, 2019, SCEC Contribution #9746, 2019 SCEC Annual Meeting Poster #252

The main goals of this work are to examine the adequacy of existing SCEC 3D Community Velocity Models (CVMs) in simulating near-fault, long period (T > 1 sec) ground motions for small-to-moderate magnitude events in the Ridgecrest area, and to explore possible refinements to the models in order to improve their ability to fit the observed motions. Ultimately, the hope is to develop a well-calibrated and validated 3D velocity model for the Ridgecrest region that can then be used for detailed analyses of the largest events (M6.4 and M7.1) in the 2019 sequence.

The Ridgecrest sequence produced over 100 small-to-moderate magnitude (4<M<6) events that were recorded by the Southern California Seismic Network (SCSN) with about 20 recording sites located within 60 km of the main epicentral region. Testing the velocity models through the use of small-to-moderate magnitude events is attractive because they can typically be treated as point sources at periods longer than about 1 second. Preliminary analysis of the recorded motions indicates that sites very close to the epicentral area (e.g., CLC and CCC) exhibit relatively simple waveforms, suggesting that the subsurface structure near these sites is relatively simple. However, sites to the west of the epicentral area in Ridgecrest (e.g., Q0072) exhibit significant later-arriving, long-period motions for some events, suggesting these arrivals may be basin generated surface waves. In this region, both SCEC CVMs (cvmh and cvmsi) are comprised of smoothly varying velocity structure that is constrained primarily by waveform tomography; i.e., no distinct subsurface geologic features are incorporated in the near-surface of the models. Nonetheless, both models exhibit a low-velocity basin-like structure beneath Ridgecrest that extends to depths of several kms. In the first phase of our work, we will simulate the motions for selected events using both CVMs as is, and then assess the ability of each model to reproduce the observed motions at sites both inside and outside of the Ridgecrest basin. In the second phase, we will explore possible refinements to the velocity models (e.g., sharpening the basin/bedrock interface) that could improve the fit to the observations.

Key Words
cvms, strong ground motion

Citation
Graves, R. W. (2019, 08). 3D ground motion simulations for small to moderate magnitude events in the Ridgecrest sequence. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes