Rapid Postearthquake Field Reconnaissance, Paleoseismic Trenching, and GIS-Based Fault Slip Variability Measurements along the Mw 6.4 and Mw 7.1 Ridgecrest Earthquake Sequence, Southern California

Abstract

ABSTRACTUnderstanding fault rupture deformation patterns, especially in complex fault zones, has fundamental implications on seismotectonic studies and hazard mitigation for the built environment. The 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquake ruptures offer an opportunity to quantify deformation patterns and surface displacements from a complex fault rupture. Our field reconnaissance, within 18 hr of the Mw 6.4 event, documented a complex and relatively broad fault deformation zone up to a few kilometers wide in the vicinity of a gas transmission pipeline intersected by the surface rupture south of Highway 178. The subsequent Mw 7.1 event resulted in similarly distributed surface fault deformation within a broad zone that crossed a second gas transmission pipeline located south of Highway 178. On 10 July, fault-normal pipeline assessment trenches were excavated at both locations and provided evidence for late Pleistocene fault rupture on both faults expressed as upward fault truncations and paleoliquefaction features. Subsequent imagery-based rupture-mapping facilitated identification and measurement of offset features that were not identified during field mapping. Fault displacement measurements document a gradual decrease in fault slip on rupture southwestward from the Mw 6.4 epicenter within the study area. However, displacement along the primary Mw 7.1 rupture decreased more drastically southeastward toward a structural complexity at a fault stepover expressed as a horsetail splay. Fault-normal slip distributions showed that majority of the surface deformation was accommodated along discrete fault strands within the primary fault zone across both ruptures. This work expands the catalog of high-fidelity coseismic rupture information used to better understand seismotectonic processes, while contributing to refinement of fault displacement models used in support of critical infrastructure design.