Paleoseismic history of the causative faults of the 2019 Ridgecrest, California earthquake sequence

Ian Pierce, Alana Williams, Rich Koehler, Ramon Arrowsmith, & Kathleen Rodrigues

In Preparation May 3, 2025, SCEC Contribution #14195

The 2019 Ridgecrest, California, earthquake sequence featured Mw 6.4 and 7.1 ruptures on along two nearly orthogonal faults: the northeast-striking Salt Wells Valley fault (left-lateral) and northwest-striking Paxton Ranch fault (right-lateral). Such multi-fault or conjugate ruptures pose challenges for seismic hazard assessment. They by increasing increase both the seismic moment release and the complexity of surface deformation. And, our knowledge of the repetition of similar complex ruptures is limited. To examine whether these faults have ruptured together during prior seismic cycles, we excavated five paleoseismic trenches across the two fault strands. On the Salt Wells Valley fault, one trench exposure preserved evidence for the 2019 earthquake only, while a second trench preserved evidence of at least one earlier rupture that when combined with the results of a nearby prior study is limited to ~22-25 ka . In contrast, the Paxton Ranch fault records two definitive, and as many as five, surface-rupturing paleoearthquakes since the latest Pleistocene in addition to the 2019 rupture: one between ~8–2 ka, a second between ~13–12 ka, a possible third ~18 ka, and two possible additional older events. Our data thus indicate that these faults have not commonly co-ruptured in the late Pleistocene/Holocene.

Preliminary slip-rate estimates derived from these paleoearthquake records suggest rates on the Paxton Ranch fault in the range of ~0.2–1.2 mm/yr, whereas the Salt Wells Valley fault appears to have an order of magnitude lower rate of ~0.01-0.07 mm/yr. Although the 2019 Ridgecrest sequence demonstrated the region’s potential for complex multi-fault earthquake behavior, the paleoseismic record hints that such large, multi-fault ruptures do not always involve the same faults. These findings underscore the importance of continued paleoseismic and geodetic investigations in the Walker Lane/Eastern California Shear Zone to better characterize fault interactions and refine regional seismic hazard assessments.

Citation
Pierce, I., Williams, A., Koehler, R., Arrowsmith, R., & Rodrigues, K. (2025). Paleoseismic history of the causative faults of the 2019 Ridgecrest, California earthquake sequence. Seismica, (in preparation).

Related Projects & Working Groups
Earthquake geology