GEOLOGIC AND GEODETIC ANALYSES TO DEFINE THE LOCATION, CREEP, TRIGGERED SLIP, SLIP RATE, AND GEOMETRY WITHIN A VOLUMINOUS HIDDEN SPRING FAULT ZONE, SOUTHERN CALIFORNIA

A strain deficit of ~10 mm/yr exists across the San Andreas Fault (SAF) Zone on the boundary between the North American and Pacific plates, but it is unknown where that additional strain is accommodated. Geodetic models suggest that, in addition to faults west of the SAF, faults to the east may accommodate right-lateral motion. We investigate the role of the Hidden Spring Fault Zone (HSFZ) and related structures, east of the San Andreas fault in the Mecca and Durmid Hills, in accommodating some of this slip. We use several remote sensing techniques informed by previous field work and UAVSAR studies. Field studies suggest that the HSFZ forms an ~10-km-wide ladder structure south of the Salton Creek Fault, and anastomosing right-lateral faults farther north. UAVSAR data independently show that the southern HSFZ is voluminous and consists of a series of cross faults bounded by two NNW-striking right lateral fault zones: the HSFZ to the west, and left-stepping faults that include the Hot Spring Fault to the east. These two master faults bound a complex mesh of right and left-lateral structures, NE-striking left-lateral cross faults, and incipient fault folds. We compiled data from historic and modern aerial and satellite imagery and LiDAR, to map truncated Late Quaternary beds, fault scarps, ground cracks, springs, oases, beheaded streams, and vegetation lineaments. Preliminary field work verified consistent right-lateral slip sense within the HSFZ along the eastern margin of the Mecca Hills. The UAVSAR data allows us to examine ground motion over time, and after large regional earthquakes, revealing regions of creep often triggered by distant seismic activity. Comparing the results of edge detection in the UAVSAR datasets before and after the El Major earthquake with the interpretations from remote mapping identifies the more active faults in the larger fault mesh. By combining these different remote sensing methods, we have identified dozens of new faults in the HSFZ. These data emphasize the orientations and potential fault linkages that are vital for a kinematic analysis of the Hidden Spring Fault Zone and for creating a baseline analysis of deformation in a region where large multi-fault earthquake sequences are likely to occur.