Waveguide or Not? Revised Ground-Motion Simulations for Greater Los Angeles from the <b>M</b> 7.8 ShakeOut Earthquake Scenario

Abstract The ShakeOut scenario of an M 7.8 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, California, 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 structures and SSAF geometry. Spectral accelerations at 3 s 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 Next Generation Attenuation-West2 ground-motion model.