SCEC Award Number 20111 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Study of fault zone damage and healing in the 2019 Ridgecrest aftershock sequence
Investigator(s)
Name Organization
John Vidale University of Southern California
Other Participants Wei Wang, post-doc, USC
SCEC Priorities 2c, 3a, 3d SCEC Groups FARM, Seismology, SDOT
Report Due Date 03/15/2021 Date Report Submitted 03/11/2021
Project Abstract
 Tectonic faults, as zones of weakness, are central to plate tectonics. By velocity weakening and strain localization, faults generate earthquakes from plate motions. One outstanding question is just how faults weaken as they break, and then restrengthen afterwards. Faults may heal through a variety of deformation processes; closure of cracks by indentation, sealing of cracks by chemical processes, or redistribution of fluids, for example. The 2019 M7.1 Ridgecrest earthquake and its M6.4 foreshock occurred near the town of Ridgecrest on July 5 and were the first such strong events in southern California for 20 years. Numerous individual stations and fault crossing arrays were recorded as a community resource, providing unprecedented measurements in the ensuing weeks. We examined the repeating earthquakes [Trugman et al., 2020] to identify the changes in seismic wave speed. We use the travel time difference of P and S waves using the repeating events to measure the velocity changes of the fault zone and wallrock. Our results resolve the NW-ES-oriented main fault below the linear array B3 has a width of 1.2 km, and a second, perpendicular fault is not as clear but has the width about 0.8 km. We only detect a subtle healing process only within the fault zone, i.e. less than 1% shear-wave velocity increased during the less than one-month period of the array working. This study provides knowledge of the fault zone structure and the earthquake cycle for the 2019 M7.1 Ridgecrest earthquake.
Intellectual Merit The performed results measure the fault zone healing process of the faults for the M7.1 Ridgecrest mainshock and the M6.5 foreshock. Also, the results reveal the fault widths for the main fault and a perpendicular one. The stronger healing processing for S wave in the fault zone rather than for P wave or in the wallrock indicates fluids may be involved in the healing process.
Broader Impacts This research provides important constraints on fault zone structure. The nonlinear behavior of the material within the fault zone is key to understand the earthquake cycling and nucleation. The differences in healing process for P and S wave may indicate the influence of the fluid in fault weakening and the earthquake cycling. These discoveries are relevant to rock mechanics and laboratory experiments.
Exemplary Figure Figure 3 in the report.

Caption: Figure 3. (A) Location of the repeaters (red dot) and two dense arrays. The triangles mark the individual stations of the two dense arrays. The red and green sections highlight the detected fault zones. (B) P wave arrival time delays relative to the earliest event (38582751) along the southern dense array (B3 in A). (C) S wave arrival time delays relative to the earliest event along the southern dense array in Figure 2A. The inferred fault zones are delimited by black dashed lines. (D) illustrates the average S arrival time delay within the fault zone marked in (B) and (C) versus to the occurrence time relative to the M7.1 Ridgecrest earthquake time.