Evidence of joint earthquakes between the San Andreas and San Jacinto faults

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The San Andreas and San Jacinto faults have ruptured simultaneously at least three times in the past 2,000 years, most recently in 1812, according to a new study by geologists at the University of California, Davis and San Diego State University. The work was published December 7 in the journal Geology.

Large earthquakes involving multiple faults increase the risk of strong ground shaking. However, each of these faults on its own can lead to a major earthquake (7.5 or higher), said Alba Rodriguez Padilla, a graduate student at the University of California, Davis and first author on the paper.

“Usually, we think earthquakes will remain confined to a single fault, especially for ‘mature’ faults such as San Andreas and San Jacinto, which are well-established faults with simple geometric plate boundaries,” Rodriguez Padilla said. But the researchers previously showed that it was theoretically possible for the earthquake to travel from one fault to another as they came close to each other in Cajon Pass, north of Los Angeles, she said.

“However, prior to our study, there was no direct physical evidence that these joint ruptures, or joint earthquakes, actually occurred,” Rodriguez Padilla said.

Between the southern end of the San Andreas Fault and the northern end of the San Jacinto lies a small fault, the Little Creek Ridge. This fault will only slip when there is a common earthquake across two large faults.

Rodriguez Padilla said that the Little Creek Ridge fault does not itself do any work during these common earthquakes, but only acts as a negative sign.

20% to 23% of earthquakes are common

To obtain evidence of possible joint earthquakes, Rodriguez Padilla and colleagues dug a trench 15 meters long and 1.5 to 3 meters deep in the Little Creek Ridge fault. They have identified signs of three earthquake events in the past 2,000 years, based on radiocarbon and pollen.

The study involved digging a trench 15 meters long and up to 3 meters deep in the San Gabriel Mountains. (Alba Rodriguez Padilla/UC Davis)

This compares to 15 known earthquakes in San Andreas and 13 earthquakes in San Jacinto at the same time. Based on this, the team concluded that 20% to 23% of the earthquakes on these major faults are in common with the other fault.

Next, they simulated historically recorded earthquakes in 1812 and 1857 to see if these were multi-fault earthquakes. Based on simulations, they scrapped the 1857 earthquake and found that the 1812 earthquake was able to jump off the faults.

Other co-authors of the paper are Professor Michael Oskin and project scientist Irina Delosina, Department of Earth and Planetary Sciences at the University of California, Davis; and Thomas Rockwell and Drake Singleton of San Diego State University. Julian Lozos, California State University Northridge, and Killian Dasher-Cosino, UC Santa Cruz, helped dig the trench.

The work was supported by the Southern California Seismological Center, which is funded by the US Geological Survey and the National Science Foundation.

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