CyberShake Probabilistic Seismic Hazard Model for Northern California

ABSTRACT

The Statewide California Earthquake Center has developed the CyberShake platform to perform 3D simulation-based probabilistic seismic hazard analysis. Using earthquake event rates, kinematic ruptures, and a 3D seismic velocity model, all constrained by observations, CyberShake utilizes wave propagation simulations and a reciprocity-based approach to generate hundreds of thousands of seismograms per site of interest. These simulation results are used to produce hazard data products including uniform hazard spectra, significant durations, site-specific hazard curves, and regional hazard maps. Previous CyberShake modeling efforts have computed seismic hazard models in southern California and Central California. In this article, we present CyberShake Study 24.8, which implements the CyberShake workflow for 315 sites in the greater San Francisco Bay area, extending physics-based hazard estimates to the entire San Andreas fault system. We construct the velocity model by tiling together three 3D and 1D models and applying a near-surface VS30-based merged taper to the top 700 m. We used the AWP—ODC wave propagation code to perform the ground-motion simulations, and the Graves–Pitarka kinematic rupture generator to create slip time histories for individual events. CyberShake Study 24.8 produces overall slightly reduced hazard compared to the NGA-West2 ground-motion models (GMMs). Specifically, CyberShake produces lower hazard immediately surrounding the San Francisco Bay likely due to use of a minimum VS of 400 m/s, lower hazard in near-fault regions, and higher hazard at long periods near sedimentary basins in the velocity model. We find larger directivity effects for CyberShake in northern California than in southern California, likely due to increased uniformity in fault characteristics and orientation in northern California. Validation results using the 1989 M 6.9 Loma Prieta event show that CyberShake reproduces observed ground motions with reduced bias compared with GMMs and 1D simulations.