SCEC Award Number 20150 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Refining the timing and mechanics of San Jacinto-San Andreas joint rupture through Cajon Pass
Investigator(s)
Name Organization
Michael Oskin University of California, Davis Thomas Rockwell San Diego State University
Other Participants Alba Mar Rodriguez-Padilla, UC Davis Ph.D. Student and Irina Delusina, UC Davis Project Scientist
SCEC Priorities 2e, 5c, 1d SCEC Groups Geology, SAFS, FARM
Report Due Date 03/15/2021 Date Report Submitted 05/06/2021
Project Abstract
 Large, multi-fault earthquakes increase the threat of strong ground-shaking and reshape event probabilities across a system of faults. Fault junctions act as conditional barriers, or earthquake gates, that stop most earthquakes but permit junction-spanning events when stress conditions are favorable. Constraining the physical conditions that favor multi-fault earthquakes requires information on the frequency of isolated events versus events that activate faults through the junction. Measuring this frequency is challenging because dating uncertainties limit the correlation of paleoseismic events at different faults, requiring a direct approach to measuring rupture through an earthquake gate. We proposed to dig a paleoseismic trench of the Lytle Creek Ridge Fault, a small aseismic low-angle normal fault located within the releasing step-over between the San Andreas and the San Jacinto faults at Cajon Pass. We show that co-rupture of the San Andreas and San Jacinto faults through the Cajon Pass earthquake gate occurred at least three times in the past 2000 years, most recently in the historic 1812 earthquake. We supplement our paleoseismic dataset with quasistatic finite element models of the 1812 and the 1857 events to determine what mechanical conditions favor rupture linkage through Cajon Pass. We found that gate-breaching events taper steeply and halt abruptly as they transfer slip between faults. Comparison to independent chronologies showed that the San Andreas and the San Jacinto faults co-rupture with a frequency of 0.25 to 0.30, making multi-fault events a relatively common occurrence at Cajon Pass.
Intellectual Merit Physical modeling and modern observations show that multiple faults can link up in surprising ways to produce unexpectedly large earthquakes. This is an acute challenge to forecasting seismic hazard because it affects both the frequency and extent of large, damaging, surface-rupturing earthquakes. Up until now, it has remained impossible to concretely show that faults ruptured together in a pre-historic earthquake. In this contribution, we show that the two highest-hazard faults in southern California, the San Andreas and the San Jacinto faults, ruptured together three times in the past 2000 years, most recently in the historical 1812 earthquake, relying on the slip history of an aseismic secondary fault. We estimate the San Andreas and the San Jacinto faults co-rupture with a frequency of 0.25 to 0.30 and a recurrence interval of ~660 years. The relatively common occurrence of gap-bridging events at this location provides crucial information for long-term hazard estimates for the state of California. The approach of combining paleoseismic chronologies with modeling triggered secondary fault slip has never been applied before and has the potential to be used at other junctions of hazardous faults worldwide.
Broader Impacts This project contributes directly to understanding the frequency of multi-fault earthquakes through the densely populated LA Basin, a key component of long-term hazard assessment models for the state of California. This project provided research fellowship support for University of California, Davis graduate student Alba M. Rodriguez, including travel to research conferences and analyses that will contribute to one research publication (submitted).
Exemplary Figure Fig. 3. Top: Comparison of event histories recorded at paleoseismic neighboring sites at the northern San Jacinto and San Andreas faults. The red rectangles outline the area spanned by the probability density functions of events recorded in the LCRF trench. The Wrightwood chronology is from Scharer et al. 9 and the Mystic Lake chronology is from Ordendonk et al. 10. Bottom: Comparison between event rates from the paleoseismic record and UCERF3. The black lines indicate paleoseismic event rates from Wrightwood, Mystic Lake, and the LCRF; Black dots denote trench locations. Colored lines represent UCERF3 event rates for the San Andreas fault (blue), the San Jacinto fault (green), and shared (orange). Shaded areas represent the model range. Faults are divided into segments as defined in UCERF3.
Linked Publications

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