Analysis of Rupture Directivity and Wave Propagation Effects on Simulated Ground Motion for Mw 7 Earthquakes on the Hayward Fault

Abstract As part of the San Andreas fault system, the seismically active Hayward fault is a major contributor to the significant earthquake hazard in the San Francisco Bay Area (SFBA). Accurate ground-motion predictions for large earthquakes on this fault are essential, but challenges arise due to uncertainties in exactly how the fault will rupture in future earthquakes and how wave propagation effects caused by the highly heterogeneous subsurface geological structure will influence site-dependent ground motions. This study investigates the combined influence of rupture kinematics and subsurface structure on ground motion, using simulated broadband seismograms for 50 rupture realizations of an Mw 7 Hayward fault earthquake. Ground motions are generated within the physics-based Earthquake Simulation fault-to-structure exascale computing framework and the Graves–Pitarka kinematic fault rupture generator. In addition to direct comparisons with empirical ground-motion model (GMM) estimates, ensemble-averaged ground-motion intensities are used to isolate and assess source and wave path effects. We find that the local structure causes complex wave propagation and spatial variability not fully represented by the GMMs. Rupture directivity produces nearly symmetric amplification and deamplification along the fault, whereas large-slip patches yield localized influences. Among these factors, the wave propagation effects account for the largest deviation from the GMM predictions. Moreover, they contribute differently across frequencies: directivity effects peak at intermediate frequencies (∼1.5 s), but basin amplification and large-slip patch effects are most prominent at lower frequencies. This study contributes to a broader effort toward understanding key factors controlling ground-motion variability in the SFBA and provides detailed constraints for the development of region-specific, nonergodic GMMs.