USING CREEP-RELATED FRACTURES AND GEODESY TO HELP DEFINE THE LOCATION, SLIP RATE, AND GEOMETRY OF THE HIDDEN SPRING FAULT ZONE IN SOUTHERN CALIFORNIA

The slip rate between the North American and Pacific plates is ~50mm/yr and only ~ 22 mm/yr is accommodated on the Southern San Andreas Fault (SSAF), because of this, strain must be accommodated by other faults. Geodetic models suggest that, in addition to the faults to the west, faults east of the San Andreas Fault may also accommodate right-lateral motion. We investigate the role of the Hidden Spring Fault Zone (HSFZ), and related structures east of the SSAF in accommodating some of this slip. We hypothesize that the Hidden Spring Fault Zone forms a ladder structure south of the projected intersection with the Salton Creek Fault, which can better be considered a voluminous fault zone up to 10 km wide, and accommodates a small portion of tectonic slip from the SSAF. Field studies, image analysis, and geodesy all show that the Hidden Spring Fault is linked with adjacent structures in the west foothills of the Orocopia Mountains and Salt Creek drainage basin. The northern half of the fault zone contains mostly right-lateral faults that cut Plio-Pleistocene Palm Spring, Pleistocene Ocotillo formations, and undated Quaternary deposits. Preliminary analyses of UAVSAR data, using edge detection methods, which detect phase jumps often associated with triggered fault slip, paired with geologic observations reveal the HSFZ consists of a series of cross faults bound by two faults, such as the HSF to the west, and potentially the Hot Spring Fault to the east. These two faults bound a complex mesh of right and left-lateral structures, NE-striking cross faults, and incipient fault folds. Some en echelon faults of the Hot Spring Fault displace modern sand dunes along linear vertical fractures that we interpret as a record of a small amount of creep or triggered slip, this is also evidenced in the left-lateral structures. The varied datasets allow us to create an integrated model displaying the precise location of the HSFZ, the amount and nature of slip distribution on the faults associated with the SSAFZ, reveals their stress orientations, as well as allows us to conduct a kinematic analysis of the HSFZ. We examine how faults in the region experienced triggered slip from distant earthquakes, and are establishing a baseline analysis of deformation in a region where large seismic ruptures very likely will occur.