SCEC2024 Poster #176: Waveguide or not? Revised ground motion simulations for greater Los Angeles from the M7.8 ShakeOut earthquake scenario

Group B, Poster #176, Ground Motions (GM)

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

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Poster Presentation

2024 SCEC Annual Meeting, Poster #176, SCEC Contribution #13769 VIEW 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 Accele...rations 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.
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