ASSESSMENT OF SLIP AND DEFORMATION ALONG THE SANTA SUSANA FAULT, SOUTHERN CALIFORNIA, USING HIGH RESOLUTION TOPOGRAPHY

The Santa Susana fault (SSF), located in the Western Transverse Ranges of southern California, is a 38-km-long active thrust fault. The trace of the SSF is near the northern margin of the Los Angeles metropolitan area and connects other active faults along the Ventura and San Fernando basins. The reported slip rates for the SSF in the UCERF-3 earthquake hazard forecast range from 10 mm/yr as determined from balanced cross-sections to as low as 0.5 mm/yr based on a subdued geomorphic expression. The goal of this work is to determine a more accurate slip rate for the late Quaternary using high-resolution topographic (HRT) data applied in three ways. In the area with the best offset geomorphic features, airborne lidar data are used to measure displaced landforms and determine dip-slip rates when combined with field-based mapping, surveying, and sampling for ³⁶Cl cosmogenic surface dating. Second, channel steepness analysis of the lidar data is used to estimate relative changes in uplift rate along strike from the location where vertical separation and slip rates can be directly measured. Third, HRT data are used to measure fault deformation along the eastern SSF where the landscape is modified by modern urban development, using DEMs created from aerial photos dating back to the 1920s that predate development. Observed displacements of fan surfaces and correlation based on soil color and clay accumulation to a nearby dated soil chronosequence indicate that the late Quaternary slip rate for the SSF is likely no greater than 6 mm/yr and may be much lower. Samples for ³⁶Cl cosmogenic nuclide depth profile dating currently being processed will constrain the age of abandonment of these offset surfaces, and thus the slip rate. Preliminary channel steepness results indicate abrupt changes in uplift along strike, suggesting that the spatial pattern of uplift of the Santa Susana Mountains is controlled by other structures in addition to the SSF. Integration and development of these HRT datasets can provide information on deformation rates at a local scale as well as along the broader extent of the SSF. This study suggests a recent decrease in slip rate for the SSF in the late Quaternary compared to long-term geologic rates and provides insight on possible interaction of the SSF with other structures within the broader Western Transverse Ranges.