Regional-Scale 3d Ground-Motion Simulations Of M-W 7 Earthquakes On The Hayward Fault, Northern California Resolving Frequencies 0-10 Hz And Including Site-Response Corrections

Large earthquake ground-motion simulations in 3D Earth models provide constraints on site-specific shaking intensities but have suffered from limited frequency resolution and ignored site response in soft soils. We report new regional-scale 3D simulations for moment magnitude 7.0 scenario earthquakes on the Hayward Fault, northern California with SW4. Simulations resolved significantly broader band frequencies (0-10 Hz) than previous studies and represent the highest resolution simulations for any such earthquake to date. Seismic waves were excited by a kinematic rupture following Graves and Pitarka (2016) and obeyed wave propagation in a 3D Earth model with topography from the U.S. Geological Survey (USGS) assuming a minimum shear wavespeed, V-s min, of 500 m/s. We corrected motions for linear and nonlinear site response for the shear wavespeed, V-s, from the USGS 3D model, using a recently developed ground-motion model (GMM) for Fourier amplitude spectra (Bayless and Abrahamson, 2018, 2019a). At soft soil locations subjected to strong shaking, the site-corrected intensities reflect the competing effects of linear amplification by low V-s material, reduction of stiffness during nonlinear deformation, and damping of high frequencies. Sites with near-surface V-s of 500 m/s or greater require no linear site correction but can experience amplitude reduction due to nonlinear response. Averaged over all sites, we obtained reasonable agreement with empirical ergodic median GMMs currently used for seismic hazard and design ground motions (epsilon less than 1), with marked improvement at soft sedimentary sites. At specific locations, the simulated shaking intensities show systematic differences from the GMMs that reveal path and site effects not captured in these ergodic models. Results suggest how next generation regional-scale earthquake simulations can provide higher spatial and frequency resolution while including effects of soft soils that are commonly ignored in scenario earthquake ground-motion simulations.