INITIAL FIELD INVESTIGATION OF OFFSET LANDFORMS AND ASSOCIATED DEPOSITIONAL EVENTS ALONG THE MOJAVE SECTION OF THE SAN ANDREAS FAULT SYSTEM, CALIFORNIA

The Mojave segment of the right-lateral San Andreas strike-slip Fault (MSAF) is located between the Eastern California Shear Zone to the north and the Los Angeles Basin ~45 km to the southwest. As such, the MSAF has important implications for hazard assessment and earthquake potential in the greater Los Angeles area. Determining earthquake potential relies on knowing the frequency and magnitude of past seismic events on a fault system, and thus the slip history. Slip history can be reconstructed from geologic data by measuring and dating offset landforms and deposits across a fault combined with geodetic data derived from GPS (global positioning system) measurements of current motion, which can be extrapolated to develop an overall history of slip for a region. Apparent disparity along the MSAF between slip rates from geodetic data (14±5 mm/yr) and those from geologic observations (30±5 mm/yr) may indicate variation in slip rate over time, known as secular variation in slip. Testing the hypothesis that the MSAF has experienced secular variation in slip requires developing a Holocene slip history by collecting age and offset data along the fault. Preliminary remote analysis of 10 deflected and offset fluvial channels along the MSAF resulted in offset estimates between 50-250 meters. We are currently conducting detailed, field-based surficial geologic mapping and lithologic studies at two of these sites (Oakdale and Shoemaker Canyons) along the MSAF to test the remote reconstructions. At both sites we plan to excavate exploratory trenches in alluvial deposits northwest and southeast of the fault to correlate depositional events and support or refute offset estimates. Additional fault-parallel excavations will be completed to investigate the occurrence of paleochannel locations of the modern day drainage. Locating these paleochannels allows for an accurate representation of error associated with calculating the magnitude of offset. Correlating depositional events across the fault will be used to corroborate offset estimates. This investigation will also determine the type of dating technique to be used to quantify the age of observed offsets.