STRUCTURAL CHARACTERIZATION OF THE SAN JACINTO FAULT FROM HIGH RESOLUTION UAV-BASED IMAGERY AND TOPOGRAPHY AT SAGE BRUSH FLAT, ANZA, CALIFORNIA

Increasingly, small unmanned aerial vehicles (UAV) are used for collecting high resolution imagery for geoscience applications. Structure from motion (SFM) photogrammetry processing of these data provides cm-resolution elevation models used in geomorphic analyses, geologic mapping and structural characterizations. Our research applies UAV SFM data to investigate the San Jacinto Fault Zone (SJFZ) at Sage Brush Flat (SGB) near Anza, California. Recent studies include a dense seismic array and Sharp’s geologic map of the SJFZ from 1967, interpreting 3D seismic structure at 10-100 m scale and regional fault structure respectively. These results indicate that the SJFZ at SGB consists of multiple faults with varying sense of slip within the study area. To test these hypotheses, we used sub-meter scale drone based imagery for detailed mapping, along with shallow excavations to characterize local geology, geomorphology, and structures. Comparisons of photogrammetry-derived 3D point clouds from two annual successive UAV flights (2016 and 2017) to B4 airborne lidar, have absolute difference mean values of 0.4 - 0.5 m. Larger differences (>0.3 m) observed in the point cloud maps are attributed primarily to vegetation growth. Use of B4 based ground control points (GCP) aided in correcting model warping, or “bowl” effect, common in SFM processing, which may be slightly exaggerated by the basin site morphology. All data sets were processed for ground classification producing digital terrain models (DTM) for detailed mapping of local geomorphic landforms and fault offset. LiDaCaoz channel offset analyses from SFM data measure 6.9 m of dextral offset, matching the multiple-event offsets cluster of the Clark Fault recorded by Salisbury et al. in 2012. We flew UAV surveys of fault core outcrops for 3D structural mapping, to augment and support site excavations of a prominent shutter ridge, exposing multiple fault strands with varying senses of slip. Results of site mapping illustrate differential landform distribution and morphology; steep slopes with small fans vs shallow slopes and broader fans on the east and west sides of the fault zone, respectively. These results will be further evaluated with geophysical and material properties to better define and characterize the complex structure of the SJFZ at SGB.