SURFACE RUPTURES FROM THE 2019 RIDGECREST EARTHQUAKE SEQUENCE AND OBSERVATIONS APPLICABLE TO FAULT DISPLACEMENT HAZARD ANALYSIS

Fault displacement hazard assessment (FDHA) parameters such as the spatial distribution of surface ruptures, width of deformation zones, and magnitude of displacements typically rely on empirical observations from past surface-rupturing earthquakes. Observations from the July 2019 Ridgecrest earthquakes provides one of the most well-documented surface rupture data sets in a strike-slip faulting environment to inform FDHA. The earthquake sequence produced surface rupture on two fault zones: A M_W 6.4 earthquake ruptured a ~17 km-long section of the NE-striking Salt Wells Valley Fault Zone and the M_W 7.1 earthquake produced a ~50 km-long zone of surface ruptures on the NW-striking Paxton Ranch Fault Zone (PRFZ). Distributed faulting with moderate displacements occurred on subparallel faults surrounding the principal fault traces and fracturing on orthogonal faults was prevalent. Cross faulting was also observed at both ends of the M_W7.1. Using InSAR-derived phase-gradient maps, field teams were able to verify ruptures, often expressed as zones of minor cracking, in a zone up to 15 km wide away from the principal trace of the PRFZ. Geologists measured >750 offsets in the field, and we compare a subset of these with optical-pixel tracking results to examine the data for details of inferred slip gradients as well as potential biases in older surface rupture datasets that rely on field-based measurements. At multiple locations, we surveyed fault-normal transects across the fault zone using a high-precision GPS unit to measure offset cultural and natural features, enabling estimates of the width of deformation and insights into how the underlying geology affects the localization of displacement at the surface. Finally, the complexity of the surface rupture, which occurred on a fault system with low cumulative slip (<5 km), is typical of other recent surface ruptures in the Eastern California Shear Zone. In contrast, recent large surface-rupturing earthquakes on plate boundary faults with large (>10s of kms) cumulative offset, such as the 2002 Denali Fault earthquake, have comparatively simple surface rupture patterns. Cumulative offset may be a first-order control on rupture complexity and empirical data sets developed should consider this as a potential parameter to forecast the distribution of surface faulting.