Timing of Landform Displacements along the Mojave Section of the San Andreas Fault: A Comparison of Field-based and Remote Reconstructions at Two Sites

Determining the Holocene slip rate of the Mojave section of the San Andreas Fault (MSAF) is key for assessing the earthquake hazard that this ~150-km-long section of fault poses to the Los Angeles metropolitan area, which is located ~45 km to the southwest. Possible temporal variations in slip rate along the MSAF are suggested by an apparent discrepancy between geologically and geodetically determined slip rates, with rates from geologic observations reported to be up to twice as fast as those reported from geodetic data. This apparent variability could be the result of changes in slip rate over time, which is known as secular variation in slip. To test the hypothesis that the MSAF exhibits variability in slip rate over time requires establishing not just a Holocene-average slip rate, but a Holocene slip history. Previous work along the MSAF using remote, virtual-reality based analysis of B4 LiDAR topographic data and pilot field observations identified ~60 potential slip-rate sites with landform offsets between 30 and 300 m, 10 of which are particularly promising. We are currently conducting detailed, field-based studies at two of these 10 sites (Oakdale and Shoemaker Canyon), with an emphasis on collecting age and offset data to determine both Holocene-average slip rates and constrain slip-history analysis. Initial offset estimates were made by remote analysis using 3D visualization software with 1-meter resolution LiDAR (Light Detection and Ranging) data. We plan to excavate exploratory, fault-parallel trenches both northwest and southeast of the fault to constrain the ages of offset landforms, correlate depositional events across the fault, and test the offset estimates that were determined remotely. Upon establishing the stratigraphic relationships of lithologic units within the trenches and correlating this stratigraphy across the fault, we plan to employ geochronologic techniques to quantify the age of depositional events. The nature of the deposits will determine the type of geochronology that we will utilize. Both locations are moderately vegetated and likely to yield organic-rich deposits ideal for radiocarbon dating. If sand units are present, we may also use optically stimulated luminescence (OSL) dating, because of the potential for multiple dating techniques to strengthen the age determinations.