Paleoseismology of the Southern Clark Strand of the San Jacinto Fault Zone, Southern California

Michael T. Buga, & Thomas K. Rockwell

In Preparation April 2012, SCEC Contribution #1635

We present results from a new paleoseismic trench site on the Clark strand of the San Jacinto Fault Zone in Clark Lake, western Salton Trough, southern California. The trench results suggest that most late Holocene slip along the Clark fault at Clark Lake is localized within a narrow zone, as there are no significant faults in trenches excavated across two other lineaments where faults have been mapped. We have identified up to ten past surface ruptures that have occurred in the last 2,600 years. Using interseismic periods, we calculate an average recurrence interval (RI) of 227 ± 114 years. This corresponds well to the record at Hog Lake, located about 50 km to the northwest, where the RI for major ruptures is estimated at 235 ± 116 years. The two most recent events at Clark Lake are likely the November 22, 1800 and ca 1550 earthquakes, and correlate to events 2 and 3 at Hog Lake, based on mapping of small geomorphic offsets along the Clark fault. The next three events, E3 to E5, correlate to a cluster of events, E-4 to E-8, at Hog Lake. Two events at Hog Lake are not seen at Clark Lake, E-5 and E-7, suggesting that these ruptures in the Hog Lake cluster correspond to earthquakes that ruptured only the northern part of the zone, similar to what occurred in the 1918 M6.9 earthquake. These observations suggest that the entire Clark fault fails together in some large earthquakes (Mw7.3) whereas the northern Clark fault may fail more frequently in M6.5-6.9 earthquakes, as occurred in 1899 and 1918.

Buga, M. T., & Rockwell, T. K. (2012). Paleoseismology of the Southern Clark Strand of the San Jacinto Fault Zone, Southern California. Bulletin of the Seismological Society of America, (in preparation).