Evidence for shared earthquakes between San Andreas and San Jacinto
faults

Date:
December 14, 2021
Source:
University of California - Davis
Summary:
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.



FULL STORY ==========================================================================
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 Dec. 7 in
the journal Geology.


========================================================================== Large earthquakes involving multiple faults increase the threat of strong ground shaking. However, each of these faults on their own can generate a large-magnitude (7.5 or above) earthquake, said Alba Rodri'guez Padilla,
a graduate student at UC Davis and first author on the paper.

"Typically, we think earthquakes will remain confined to a single fault, especially for "mature" faults such as the San Andreas and the San
Jacinto, which are well-established, geometrically simple plate boundary faults," Rodri'guez Padilla said. But researchers previously have shown
that it's theoretically possible for an earthquake to transfer from one
fault to another where they come close together at Cajon Pass, north of
Los Angeles, she said.

"However, prior to our study, there was no direct physical evidence
that these joint ruptures, or shared earthquakes, do in fact occur,"
Rodri'guez Padilla said.

Between the south end of the San Andreas Fault and the northern end of
the San Jacinto lies a small fault, the Lytle Creek Ridge Fault. This
fault would slip only when there is an earthquake shared across the two
bigger faults.

The Lytle Creek Ridge Fault does not itself do any work during these
shared earthquakes, just acting as a passive marker, Rodri'guez Padilla
said.

20% to 23% of earthquakes shared To get evidence of potential shared earthquakes, Rodri'guez Padilla and colleagues hand-dug a trench 15
meters long and 1.5-3 meters deep into the Lytle Creek Ridge Fault. They identified signs of three earthquake events in the past 2,000 years,
based on radiocarbon and pollen dating.

That compares to 15 known earthquakes on the San Andreas and 13 on the San Jacinto over the same time. Based on that, the team concluded that 20% to
23% of earthquakes on these major faults are shared with the other fault.

Next, they simulated the historically recorded earthquakes of 1812 and
1857 to see if these could have been multi-fault earthquakes. Based on
the simulations, they discarded the 1857 earthquake and found that the
1812 earthquake was capable of jumping faults.

Additional co-authors on the paper are Professor Michael Oskin and
project scientist Irina Delusina, UC Davis Department of Earth and
Planetary Sciences; and Thomas Rockwell and Drake Singleton, San Diego
State University. Julian Lozos, California State University Northridge,
and Kelian Dascher-Cousineau, UC Santa Cruz, helped dig the trench.

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

========================================================================== Story Source: Materials provided by
University_of_California_-_Davis. Original written by Andy Fell. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Alba M. Rodriguez Padilla, Michael E. Oskin, Thomas K. Rockwell,
Irina
Delusina, Drake M. Singleton. Joint earthquake ruptures of the San
Andreas and San Jacinto faults, California, USA. Geology, 2021;
DOI: 10.1130/G49415.1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211214134949.htm

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