The Effect of Fault Geometry and Minimum Shear Wavespeed on 3D Ground‐Motion Simulations for an $Mw$ 6.5 Hayward Fault Scenario Earthquake, San Francisco Bay Area, Northern California Available to Purchase

We investigated the effects of fault geometry and assumed minimum shear wavespeed (⁠$VSmin$⁠) on 3D ground‐motion simulations (0–2.5 Hz) in general, using a moment magnitude (⁠$Mw$⁠) 6.5 earthquake on the Hayward fault (HF). Simulations of large earthquakes on the northeast‐dipping HF using the U.S. Geological Survey (USGS) 3D seismic model have shown intensity asymmetry with stronger shaking for the Great Valley Sequence east of the HF (hanging wall) relative to the Franciscan Complex to the west (footwall). We performed simulations with three fault geometries in both plane‐layered (1D) and 3D models. Results show that the nonvertical fault geometries result in larger motions on the hanging wall relative to the vertical fault for the same Earth model with up to 50% amplifications in single‐component peak ground velocity (PGV) within 10 km of the rupture. Near‐fault motions on the footwall are reduced for the nonvertical faults, but less than they are increased on the hanging wall. Simulations assuming $VSmin$ values of 500 and $250m/s$ reveal that PGVs are on average 25% higher west of the HF when using the lower $VSmin$⁠, with some locations amplified by a factor of 3. Increasing frequency content from 2.5 to 5 Hz increases PGV values. Spectral ratios of these two $VSmin$ cases show average amplifications of 2–4 (0.5–1.5 Hz) for the lower $VSmin$ west of the fault. Large differences (up to $2×$⁠) in PGV across the HF from previous studies persist even for the case with a vertical fault or $VSmin$ of $250m/s$⁠. We conclude that assuming a $VSmin$ of $500m/s$ underestimates intensities west of the HF for frequencies above 0.5 Hz, and that low upper crustal (depth $<10km$⁠) shear wavespeeds defined in the 3D model contribute most to higher intensities east of the HF.