Frequent Joint Earthquake Rupture of the San Andreas and San Jacinto Faults

Alba M. Rodriguez Padilla, Michael E. Oskin, Thomas K. Rockwell, Irina Delusina, & Drake M. Singleton

Under Review February 18, 2021, SCEC Contribution #10924

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 junction1. Measuring this frequency is challenging because dating uncertainties limit correlation of paleoseismic events at different faults, requiring a direct approach to measuring rupture through an earthquake gate. Here we show through documentation and modeling of secondary fault slip 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. Gate-breaching events taper steeply and halt abruptly as they transfer slip between faults. Comparison to independent chronologies shows 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.

Rodriguez Padilla, A. M., Oskin, M. E., Rockwell, T. K., Delusina, I., & Singleton, D. M. (2021). Frequent Joint Earthquake Rupture of the San Andreas and San Jacinto Faults. Geology, (under review).