Refining the Spatial and Temporal Signatures of Creep and Co-Seismic Slip Along the Southern San Andres Fault, Coachella Valley, California

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The southern segment of the San Andreas fault (SAF) has historically been the subject of geologic and geodetic slip rate and creep studies. Our investigation expands this research by focusing on small-scale offsets, potentially related to creep since the last earthquake, in addition to larger offsets due to paleoearthquakes which ruptured the Coachella Valley section of the SAF. Previous studies have utilized Structure from Motion methodologies and achieved sub-meter to decimeter resolution (Javernick et al., 2014, Westoby et al., 2012, etc.), whereas this study has acquired sub-centimeter resolution, allowing for the examination of decimeter to meter scale offsets. We conducted UAS surveys using a DJI Phantom 4 Pro flown at altitudes of 25 to 60 m above ground level over eight sections of the fault. We acquired a swath measuring 4 km long and 75 to 150 m wide resulting in 488,000 m² of aerial imagery. Imagery was processed using Agisoft PhotoScan, producing Digital Surface Models (DSMs) and orthomosaics from base imagery with a maximum resolution of 7.7 mm/px. DSMs were further analyzed in ArcGIS to create hillshade models, contour maps, and slope maps. Geomorphic offsets were measured in the field using standard techniques and compared to measurements extracted from the GIS coverages in ArcGIS. Observed offsets range from 15 cm to 85 m and reflect creep and multiple rupture events. This section of the SAF last ruptured in 1726 ± 7 years CE (Rockwell et al., 2018) and related studies have demonstrated a local creep rate of ~3 mm/yr (Lindsey et al., 2014) at our sites. We expect that any offset less than about a meter should be related to creep. Currently, all offsets less than 1 m are being evaluated with Gaussian Distribution analysis for variations in creep rate along the fault and to differentiate creep from co-seismic slip. The ultimate goal is a refined model of Late Holocene slip-per-event for the southern SAF.

Presenting Author: Chelsea M. Blanton

Authors