Shallow Fault Zone Structure Affects Rupture Dynamics and Ground Motions of the 2019 Ridgecrest Sequence to Regional Distances

Abstract

Seismic faults are surrounded by damaged rocks with reduced rigidity and enhanced attenuation. These damaged fault zone structures can amplify seismic waves and affect earthquake dynamics, yet they are typically omitted in physics-based regional ground motion simulations. We report on the significant effects of a shallow, flower-shaped fault zone in foreshock-mainshock 3D dynamic rupture models of the 2019 Ridgecrest earthquake sequence. We find that the fault zone structure both amplifies and reduces ground motions not only locally but at distances exceeding 100 km. This impact on ground motions is frequency- and magnitude-dependent, particularly affecting higher frequency ground motions from the foreshock because its corner frequency is closer to the fault zone's fundamental eigenfrequency. Within the fault zone, the shallow transition to a velocity-strengthening frictional regime leads to a depth-dependent peak slip rate increase of up to 70% and confines fault zone-induced supershear transitions mostly to the fault zone's velocity-weakening roots. However, the interplay of fault zone waves, free surface reflections, and rupture directivity can generate localized supershear rupture, even in narrow velocity-strengthening regions, which are typically thought to inhibit supershear rupture. This study demonstrates that shallow fault zone structures may significantly affect intermediate- and far-field ground motions and cause localized supershear rupture penetrating into velocity-strengthening regions, with important implications for seismic hazard assessment.