Regional-Scale 3D Ground-Motion Simulations of Mw 7 Earthquakes on the Hayward Fault, Northern California Resolving Frequencies 0–10 Hz and Including Site-Response Corrections

Abstract

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, VSmin, of 500 m/s. We corrected motions for linear and nonlinear site response for the shear wavespeed, VS, 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 VS material, reduction of stiffness during nonlinear deformation, and damping of high frequencies. Sites with near-surface VS 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.

Site response of sedimentary basins and other geomorphic provinces in southern California

“…For long-period ground motions, such models predict relative de-amplification (less than that provided by the V S30 -scaling function) for shallower depths, and relative amplification for larger depths. The efficacy of depth as an independent variable correlated to site response in basins has also been demonstrated by simulations (Wald and Olsen, 2000;Day et al, 2008;Rodgers et al, 2020), despite the fact that some mechanisms of site response in two-or three-dimensional basin structures (e.g. focusing, basin edge-generated surface waves; Graves, 1993;Kawase, 1996;Graves et al, 1998;Pitarka et al, 1998;Stephenson et al, 2000;Baher and Davis, 2003) are related to more complex geometric attributes such as the shape of the sediment-rock interface at the base of the basin.…”

EQSIM—A multidisciplinary framework for fault-to-structure earthquake simulations on exascale computers part I: Computational models and workflow

“…As leadership computer platforms continue to advance and the ability to compute earthquake phenomenon continues to rapidly increase, it is essential to critically assess the realism of the simulations in multiple dimensions. Previous work has evaluated the realism of ground motion simulations to 5 Hz by comparison with existing GMPEs (Rodgers et al, 2019b; Rodgers et al, 2018a), and recent work illustrates SFBA regional simulations to 10 Hz (Rodgers et al, 2020). A companion paper provides the first assessments of the infrastructure response with fault-to-structure simulations utilizing the EQSIM framework (McCallen et al, in press).…”

A procedure for 3D simulation of seismic wave propagation considering source‐path‐site effects: Theory, verification and application

“…Even so, the computational costs and the limited number of scenarios n 3D simulations of high-frequency motions (e.g., for just one scenario, the computational requirement at 4 Hz is 16 times that at 2 Hz) are not ideal. 30 In view of this, it is advisable to introduce the concept of Bielak's domain reduction, 31,32 which involves the following steps: first, develop a semi-analytical method based on the frequency-wavenumber (FK) domain to simulate seismic wave propagation in a semi-infinite space domain (up to tens or even hundreds of kilometers in depth) due to fault movements; then, numerically calculate the seismic wavefields in the region of interest (with a scale considerably smaller than a semi-infinite space domain) using the SEM, forming a coupled analytical-numerical approach. Finally, the resolved frequency can be significantly increased with limited resources, and further combined with the natural parallelism of SEM, it is expected that broadband ground-motion simulations of 3D localized regions on a large city scale can be achieved.…”

“…For large‐scale scattering problems in elastodynamics involving the two key issues of multiscale seismic wave propagation (wavelengths from a few meters to hundreds of meters) and the overall evaluation of ground motion (from the source to site), supercomputing is generally necessary if fully numerical methods, especially domain‐type methods, are used. Even so, the computational costs and the limited number of scenarios n 3D simulations of high‐frequency motions (e.g., for just one scenario, the computational requirement at 4 Hz is 16 times that at 2 Hz) are not ideal 30 . In view of this, it is advisable to introduce the concept of Bielak's domain reduction, 31,32 which involves the following steps: first, develop a semi‐analytical method based on the frequency‐wavenumber (FK) domain to simulate seismic wave propagation in a semi‐infinite space domain (up to tens or even hundreds of kilometers in depth) due to fault movements; then, numerically calculate the seismic wavefields in the region of interest (with a scale considerably smaller than a semi‐infinite space domain) using the SEM, forming a coupled analytical‐numerical approach.…”