Paleoseismic Results from a Trench across the Casa Loma Strand of the San Jacinto Fault Zone in San Jacinto, California

Robert Givler, Ross Hartleb, Chris D. Kemp, Mark Szymanski, Scott C. Lindvall, Brian Gray, Thomas K. Rockwell, & Robert Bell

Published August 15, 2019, SCEC Contribution #9753, 2019 SCEC Annual Meeting Poster #103

The San Jacinto fault zone is one of the most seismically active structures in southern California and has produced 10 historical M>6 earthquakes. The San Jacinto fault zone comprises several fault strands, including (from N to S) the Claremont, Casa Loma, Clark, Coyote Creek, Superstition Mountain, and Superstition Hills faults. The Casa Loma fault bounds the SW margin of a 2.5-km-wide extensional step-over at Mystic Lake in San Jacinto Valley and is collinear with the Clark fault to the SE. Overlapping ages of prehistoric earthquakes interpreted at paleoseismic sites on the Claremont strand (Mystic Lake site) and Clark strand (Hog Lake site) suggest that some larger-magnitude events (≥M7.3) ruptured across the step-over, involving the Casa Loma fault. Prior to this study, conducted to characterize a pipeline fault crossing, no paleoseismic data existed for the Casa Loma fault.

Here we present results from a trench across the central Casa Loma fault in San Jacinto, CA, where the fault is expressed as a degraded, NW-striking, NE‐facing scarp that is ~45 m wide and 4–5 m high. Units exposed in the trench walls include sandy fluvial deposits and fine-grained overbank deposits of the San Jacinto River along with buried soil horizons (weak A horizons) separating major stratigraphic packages. 11 AMS radiocarbon dates indicate the main fault juxtaposes late Holocene deposits on the E against latest Pleistocene to early Holocene deposits on the W. The main fault is an ~6-m-wide zone of anastomosing strands with up to 1.5 m vertical relief in late Holocene deposits. The main fault zone is coincident with the middle of the scarp and was generated by a combination of E-side-down faulting, non-tectonic subsidence, and post-10 ka San Jacinto River incision followed by late Holocene deposition.

We identify three (possibly four) events since ~1,371 cal yr BP (~580 AD), based on displaced/warped deposits and upward fault terminations. The Mystic and Hog lake sites record more events over this period. Thus, either the recurrence interval is much longer for the Casa Loma fault or our radiocarbon ages include a detrital component such that the samples are significantly older than the deposits from which they were collected. The youngest event may be related to non-tectonic settlement and/or the 25 December 1899 earthquake centered in San Jacinto, CA. More work is required to compare timing of ruptures on the Casa Loma fault with those at Mystic and Hog lakes.

Key Words
San Jacinto fault, Casa Loma fault, Paleoseismology,

Citation
Givler, R., Hartleb, R., Kemp, C. D., Szymanski, M., Lindvall, S. C., Gray, B., Rockwell, T. K., & Bell, R. (2019, 08). Paleoseismic Results from a Trench across the Casa Loma Strand of the San Jacinto Fault Zone in San Jacinto, California . Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology