Waveguide or not? Revised ground motion simulations for greater Los Angeles from the M7.8 ShakeOut earthquake scenario

Te-Yang Yeh, & Kim B. Olsen

Published September 8, 2024, SCEC Contribution #13769, 2024 SCEC Annual Meeting Poster #176 (PDF)

The ShakeOut scenario of a M7.8 northwestward rupture on the southern San Andreas fault (SSAF) (Jones et al., 2008) predicted significant long-period ground motion amplification in the greater Los Angeles, CA, area, caused by a waveguide from interconnected sedimentary basins. However, the early ShakeOut ground motion simulations omitted important model features with immature versions of the velocity structure and fault geometry. Here, we present 0-1 Hz 3D numerical wave propagation simulations for the ShakeOut scenario including surface topography, as well as updated high-resolution velocity structure in the SCEC Community Velocity Model CVM-S4.26M01 and SSAF fault geometry. Spectral Accelerations at 3s are increased by the local high-resolution basin models (25-45%) but decreased from complexity in velocity and density updates outside the basins (65-100%) and inclusion of surface topography (~30%). The updated model reduces the simulated long period ground motions in the waveguide by 60-70%, bringing the predictions significantly closer to the values by a leading NGA-West2 ground motion model. The final velocity model is validated against long-period (0-1 Hz) strong motion recordings from a series of M4.1-M4.4 events in Southern California. Of particular interest, the synthetics from a M4.3 event in Bombay Beach, CA, located near the rupture nucleation for the ShakeOut scenario and channeling waves along the SSAF corridor, show a much improved fit to recordings along the waveguide and in the Los Angeles basin using the updated model, as compared to those from the 2008 model.

Key Words
ShakeOut, wave propagation, waveguide effect, topographic effects, CVM

Citation
Yeh, T., & Olsen, K. B. (2024, 09). Waveguide or not? Revised ground motion simulations for greater Los Angeles from the M7.8 ShakeOut earthquake scenario. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)