EARTHQUAKE SLIP, HURRICANES, AND RILL FORMATION – DRONE-BASED DEMS AND ORTHOMOSAICS FROM VERY HIGH-RESOLUTION SFM IMAGERY ALONG THE SOUTHERN SAN ANDREAS FAULT, COACHELLA VALLEY, CALIFORNIA

The southern segment of the San Andreas fault (SAF) has historically been the subject of studies involving geologic and geodetic slip rates and creep. Our investigation expands this research by focusing on small-scale offsets potentially related to creep after the last earthquake as recorded by major precipitation events and rill formation affecting the Coachella Valley. Previous studies have utilized Structure from Motion (SfM) methodologies and achieved sub-meter to decimeter resolution, whereas this study has acquired sub-centimeter resolution, allowing for the examination of decimeter to meter scale offsets. We conducted drone surveys using a DJI Phantom 4 Pro near an open pit mine in Thermal, CA along the southernmost SAF at an altitude of 25 m. We collected approximately 41,000 m² of aerial imagery spanning 500 m along the fault and an 85 m swath width. Imagery was processed using Agisoft PhotoScan SfM, resulting in DEMs and orthomosaics from base imagery with a maximum resolution of 7.5 mm/px. Geomorphic offsets were measured in the field using standard techniques, and field measurements were compared with measurements extracted from digital imagery with ArcGIS and MATLAB GUI LaDiCaoz v2.1. While the SAF is active, this section of the fault last ruptured in 1725 ± 7 years. Related studies have demonstrated a local creep rate of 3 mm/yr, thus we would expect that any offset less than 85 cm should be related to creep. The potential creep signal in the geomorphology-based analysis of small offset rills may be tied to historical precipitation events. For instance, several rills appear deflected by approximately 50 cm along the principal strand of the fault. Assuming that the 50 cm offset represents slip since a major erosion event, this implies an age of about 150 years. In 1858 (160 years ago) a hurricane made landfall in San Diego, likely carrying precipitation to this arid environment and potentially creating rills and therefore resetting the geomorphic offsets. Furthermore, an atmospheric river hit southern California for 2 months in 1862 (156 years ago). Forthcoming work will evaluate the slip per event for the last several earthquakes at this site and assess paleoearthquake behavior along this segment of the SAF by subsequently removing creep from the measured offsets of this high-resolution dataset.