Investigating Near-Fault Ground Motions Using Data Recorded by Dense Arrays Around the 2019 Mw 7.1 Ridgecrest, California, Earthquake Rupture

ABSTRACT Following the 2019 Mw 7.1 Ridgecrest, California, earthquake, 15 dense 1D and 2D arrays (461 sites) were deployed around the main ruptures, including four 1D arrays across the surface ruptures of the mainshock. The dense arrays captured numerous aftershocks and provided an unprecedented dataset for studying near-fault ground motions. We combine the near-fault ground motions measured from the dense arrays with a regional ground-motion dataset and develop a near-fault partially nonergodic ground-motion model (GMM). The partially nonergodic GMM results are used to investigate several aspects of the near-fault ground motions that are related to source and fault zone properties. We find that shallow events tend to produce weaker short-period energy due to smaller stress drop or stronger attenuation but stronger long-period energy from surface-wave excitation. Significant ground-motion variations across the fault zone are observed, which include amplifications within the damage zones due to both reduced seismic velocities and trapped waves at certain periods. We also observe double-couple radiation pattern effects only in long-period ground motions, agreeing with a period-dependent earthquake rupture process. Between 3 and 15 days following the mainshock, the ground motions of aftershocks decrease with time, likely reflecting decreasing stress drops. No temporal evolution of site response and wave propagation effects within the fault zone are observed between 10 and 35 days following the mainshock. Our study demonstrates the capability of dense array data to improve our understanding of earthquake processes, fault zone properties, and near-fault seismic hazard.