Analysis of Offset Stream Channels – Deconstructing Creep and Coseismic Slip Components Using Very High Resolution SfM Imagery, Southern San Andreas Fault, Coachella Valley, California

Chelsea M. Blanton, Thomas K. Rockwell, Allen Gontz, & Josh Kelly

Submitted August 7, 2018, SCEC Contribution #8313, 2018 SCEC Annual Meeting Poster #226

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 and larger offsets due to paleoearthquakes which ruptured the Coachella Valley section of the SAF. Previous studies have utilized Structure from Motion (SfM) 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 drone surveys using a DJI Phantom 4 Pro at several sites along the southernmost SAF at altitudes of 25 to 60 m. We collected approximately 370,000 m2 of aerial imagery along five sections of fault for a total of 3 km of the fault over an 85 to 150 m width. Imagery was processed using Agisoft PhotoScan SfM software, 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 (Zielke et al., 2010). This section of the SAF last ruptured in 1726 ± 7 years (Rockwell et al., 2018). Related studies have demonstrated a local creep rate of 2-4 mm/yr (Lindsey et al., 2014), thus we would expect that any offset less than about a meter should be related to creep. Offsets observed in the DEMs range from approximately 50 cm to 100 m and reflect creep and multiple rupture events along this segment of the SAF. All offsets likely represent minimum values of slip for displacement in past earthquakes because multiple fault strands are present along much of the southern SAF, whereas discrete offsets are typically measured across a single strand. Forthcoming work will evaluate the long-term creep rate along with slip per event for the last several earthquakes, allowing for an assessment of paleoearthquake behavior along this segment of the SAF.

Key Words
geomorphology, creep, San Andreas fault

Citation
Blanton, C. M., Rockwell, T. K., Gontz, A., & Kelly, J. (2018, 08). Analysis of Offset Stream Channels – Deconstructing Creep and Coseismic Slip Components Using Very High Resolution SfM Imagery, Southern San Andreas Fault, Coachella Valley, California . Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology