SCEC Award Number 21136 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title San Andreas Fault earthquake geology: North Cholame
Investigator(s)
Name Organization
Ramon Arrowsmith Arizona State University
Other Participants Alana Williams (ASU)
SCEC Priorities 5c, 5b, 1a SCEC Groups SAFS, Geology, SDOT
Report Due Date 03/15/2022 Date Report Submitted 02/27/2024
Project Abstract
We present new paleoseismic results from 2 fault perpendicular long trenches along the Cholame segment of the San Andreas Fault. The northwestern site is located south of the Parkfield segment 11 km southeast of Highway 46. Site geomorphology is characterized by linear depressions, several sag ponds, and small offset stream channels. The southeastern site was an additional excavation at the Miller’s Well Paleoseismic site in the northwesternmost Carrizo Plain. We were limited in our trenching locations by months of rain in spring 2023 that left our original sites underwater, or the soil moisture too high to prevent trench collapse. Neither site had ideal stratigraphy. The northern site contained clay rich soils and a lack of event horizon evidence but displayed repeating graben-like formations that indicate at least 3 separate rupturing events, numbered E1-E3 by increasing relative age. Supportive evidence for these events include significant vertical soil material change, aligned gravels, and clay filled sags. The southern site had massive thick sand units and lacked variable stratigraphy to determine multiple event horizons for dating. However, the stratigraphy displayed apparent vertical separation of discontinuous large boulder and gravel units.
Intellectual Merit The Cholame segment of the San Andreas Fault is significant because it records the interaction between the creeping + M6 events of the Parkfield segment to the northwest and the locked Carrizo segment to the southeast. The Cholame segment has poorly characterized rupture history which places significant limitations on evaluating seismic hazard. Large gaps in the paleoearthquake record and various interpretations of past earthquake offset pose problems for modeling past fault behavior.
Broader Impacts Our understanding of the earthquake history of the Cholame segment of the SAF is incomplete (no maximum likelihood recurrence interval estimate; Biasi, 2013; UCERF3 Appendix H). New paleoseismic data on earthquake recurrence will lead to a better understanding of the transitional rupture behavior between Parkfield and Cholame.

Along with improving our understanding of Southern California’s earthquake hazards, our field efforts included a collaboration between Arizona State University and Bates College. We introduced and facilitated training in field geology and paleoseismic investigation for nine undergraduate students, one of which will complete their senior thesis on this project.
Exemplary Figure Figure 5 is nice because it shows the undergraduates working in the trench