The distribution and frequency of prehistoric surface ruptures on the San Jacinto and San Andreas faults near their junction.

Nate W. Onderdonk, Sally F. McGill, & Thomas K. Rockwell

Under Review September 30, 2017, SCEC Contribution #6238

The San Andreas and San Jacinto faults are the primary plate boundary structures in southern California and present a large earthquake hazard for the region. The northern end of the San Jacinto fault forms a 2 km-wide releasing step with the San Andreas fault, and they are typically regarded as separate faults. In this paper we use the spatial distribution and frequency of ground rupturing earthquakes in the area where these two faults approach each other to investigate how the two faults may interact. We present new paleoseismic data from the Mystic Lake site on the northern San Jacinto fault that spans the past 3700 years and compare the last 2000 years to other nearby paleoseismic sites on the San Jacinto and San Andreas faults. We find that: (1) the current open interval on these two faults is longer than their average recurrence intervals, but that similar intervals have occurred in the past 2000 years; (2) the San Andreas and San Jacinto faults have probably ruptured together multiple times; and (3) a joint rupture of the San Jacinto fault with the Mojave section of the San Andreas fault may be a more likely source of a major earthquake in southern California than a rupture on the San Andreas fault from the Mojave to the southern end of the fault.

Citation
Onderdonk, N. W., McGill, S. F., & Rockwell, T. K. (2017). The distribution and frequency of prehistoric surface ruptures on the San Jacinto and San Andreas faults near their junction.. Geology, (under review).


Related Projects & Working Groups
SoSAFE