TRIGGERED SLIP IN SOUTHERN CALIFORNIA AS A RESULT OF THE APRIL 5, 2010 EL MAYOR-CUCAPAH EARTHQUAKE

The magnitude and aftershock distribution from the M_w 7.2 El Mayor-Cucapah earthquake prompted our search for the expected triggered slip on several faults over a broad area extending roughly 100 km north of the U.S.-Mexico border. We mapped right-lateral triggered slip on the San Andreas, Imperial, Coyote Creek, and Superstition Hills faults, all previously known to experience triggered slip during significant regional earthquakes. Triggered slip varied from sub-millimeter to locally in excess of 20 mm. Triggered slip (dextral) was also mapped, for the first time, on the Brawley and Wienert faults (~10 mm and <5 mm slip maxima, respectively), as well as normal displacement on the Kalin Fault within the Brawley Seismic Zone. Additionally we mapped slip on two strands of the Laguna Salada fault that extended across the border northward for at least 5.5 km; dextral slip on these faults was commonly in the 10-20 mm range but locally as great as 35 mm and vertical separation was up to 30 mm.

Notable in this event was triggered slip on northeast-trending faults within the Yuha Desert area. Minor (sub-millimeter) slip was observed along a short section of the Yuha Wells Fault, but more significant was up to 45 mm left-lateral slip on a previously unknown northeast-trending fault about 10 km southeast of the Yuha Wells Fault.

A brief search found no triggered slip on some other well-known faults in the region, including the Elsinore, Superstition Mountain, and Elmore Ranch faults. Likewise, we saw no new slip on the Rico Fault or on short faults northwest of Plaster City.