Assessment of the paleoseismic potential of the Ferrum site along the southernmost San Andreas fault using new UAV imagery, GPR data, and preliminary trenching

William C. Buckley, Thomas K. Rockwell, Katherine M. Scharer, Allen M. Gontz, Paula M. Figueiredo, & Zoe A. Morgan

Published August 4, 2021, SCEC Contribution #11148, 2021 SCEC Annual Meeting Poster #056 (PDF)

We initiated studies along the southern San Andreas fault at Ferrum, CA, where a double releasing bend has ponded Holocene alluvium against an uphill-facing scarp. Ferrum is located along the east side of the Salton Sea between Bombay Beach and North Shore, about 2 km north of Salt Creek. We constructed high-resolution orthomosaics and digital surface models (DSMs) generated from Structure-from-Motion-Multiview Stereo (SfM-MVS) techniques from uncrewed aerial vehicle (UAV) imagery. The images, along with field mapping, were used to identify and measure fault-related offsets of geomorphic features that have been preserved through the past two to three inundations of ancient Lake Cahuilla. The most recent event appears to have produced between 2 and 3.5 m of lateral slip, with lower overall lateral slip values where multiple strands are present. We also identify offset channels with up to ~7 meters of lateral deflection, which are interpreted as representing multiple (probably 2) events. A cluster of small-scale offsets was identified, generally with displacements of less than a meter but a few up to ~1.5 meters, which probably reflect displacement from creep that postdate the most recent lake and large earthquake event. Along with the UAV imagery interpretations, we acquired ground penetrating radar (GPR) data to map the upper several meters of the subsurface architecture of the fault zone and ponded alluvial sediment to examine stratigraphy and locate faults and fault-related deformation. Following the geophysical analysis, we excavated two 1.5-2 m deep trenches to collect samples for radiocarbon dating to constrain ages on shallow sediments, identify and date lake events, and to verify findings from the GPR surveys. The trenches revealed excellent stratigraphy and evidence for preservation of three or four lake deposits, along with several minor fault splays. The primary fault is located beneath thick dune deposits. We collected over 50 radiocarbon samples, all gastropod shells, representing nearly all stratigraphic units exposed in the trench walls to develop the site chronology and assess the site’s potential for dating earthquake events.

Key Words
geomorphology, slip-per-event, subsurface fault imaging, san andreas fault

Citation
Buckley, W. C., Rockwell, T. K., Scharer, K. M., Gontz, A. M., Figueiredo, P. M., & Morgan, Z. A. (2021, 08). Assessment of the paleoseismic potential of the Ferrum site along the southernmost San Andreas fault using new UAV imagery, GPR data, and preliminary trenching. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology