Erik M. Gordon, Thomas K. Rockwell, Gary H. Girty
Department of Geological Sciences, San Diego State University

The Agua Tibia-Earthquake Valley Fault Zone: reorganization of tectonic slip between the northern Elsinore and southern San Jacinto fault zones

The Agua Tibia-Earthquake Valley (ATEV) fault system parallels the Julian section of the Elsinore fault, extending from the Palomar Mtn area southeast to Whale Peak in the Anza Borrego State Park area. Geomorphological observations were conducted using Google Earth imagery and field checked by foot to determine which fault elements have sustained late Quaternary and Holocene activity, based on scarps and offsets of Quaternary deposits. Clusters of smaller northeast-striking faults were also identified and examined that lie between the ATEV fault and the San Jacinto fault in the Vallecitos and Fish Creek Mountains area. Some of these smaller faults also have indications of late Quaternary activity, with scarps in bedrock and alluvium. Orientations of these active faults are grouped into two primary directions and compared to the average orientations of the Elsinore and San Jacinto faults. One of these groups trends close to parallel to the Elsinore and San Jacinto faults, whereas the other is roughly orthogonal. In contrast, more westerly striking faults of the Hot Springs fault zone (also called the San Felipe fault zone of Steely et al., 2009) do not exhibit evidence of late Quaternary activity, and the Lancaster fault segment appears to have been inactive for at least several hundred thousand years. The observed geomorphological evidence along with the fault orientation data suggest a model in which the ATEV system has superseded the older, Hot Springs fault system, which is likely Pliocene in age, and now acts as the primary conduit in actively transferring slip from the northern Elsinore fault to the southern San Jacinto fault. The Palomar-Agua Tibia Mountain and Vallecito-Fish Creek Mountain uplifts are interpreted as large pressure ridge systems that accommodate this transfer from the Elsinore fault to the ATEV fault, and then to the southern San Jacinto fault.

2013 SCEC Annual Meeting