SCEC Award Number 20103 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Trenching the causative faults of the 2019 Ridgecrest sequence
Name Organization
Rich Koehler University of Nevada, Reno Ian Pierce University of Nevada, Reno Alana Williams Arizona State University Ramon Arrowsmith Arizona State University
Other Participants
SCEC Priorities 5b, 3e, 1a SCEC Groups Geology, CXM, SAFS
Report Due Date 03/15/2021 Date Report Submitted 03/15/2021
Project Abstract
The 2019 Ridgecrest earthquake sequence consisted of a M6.4 rupture along the Salt Wells Valley left-lateral strike-slip fault on July 4th, 2019, followed by a M7.1 rupture of the nearly perpendicular Paxton Ranch right-lateral strike-slip fault on July 5th, 2019. Here we chose three sites, two along the Paxton Ranch fault and one along the Salt Wells Valley fault to investigate the paleoseismic rupture history of each of these faults. We excavated five trenches among the three sites. Each trench exposed evidence of past earthquake ruptures, although the timing remains uncertain pending dating results. The clearest event chronology was exposed in the trench at the basin site along the southern part of the Paxton Ranch fault, and showed clear evidence for at least two prior latest Pleistocene/Holocene surface rupturing events, with vertical displacements similar to the 2019 event. Assuming that the ~120 cm dextral displacement from the most recent event at this site was similar for each of these three total events and an estimate age of the oldest deposits based on the age of pluvial Searles Lake (~12 ka), we estimate a local slip rate of 0.3 mm/yr for the southernmost part of the fault. Due to the lower depositional rates at the other sites, unraveling a specific event chronology in the other trenches has proven challenging, however, future dating results will provide broad time constraints on prior deformation.
Intellectual Merit This study contributes incremental advances to our understanding of the number of pre-2019 earthquakes along the causative faults of the Ridgecrest earthquake sequence and represents the first paleoseismic trenching results along these faults. The results also are used to infer a latest Pleistocene slip rate which places the Ridgecrest faults in the context of the greater southern Walker Lane shear zone.
Broader Impacts The project has fostered mentoring of graduate students and post docs who played major roles in conducting the field work and developing reports.
Exemplary Figure Figure 8. Oblique aerial photo of the M7.1 Basin trench site prior to excavation. Upstream facing scarp indicated by red arrows.
Figure 9. Basin trench log. Fault terminations in units 2 and 4 provide evidence of 2-3 paleoearthquakes prior to the 2019 rupture. Units 3-8 are lacustrine deposits likely from the last lake cycle ~12-17 ka. OSL samples are indicated by yellow circles.