SCEC Award Number 21138 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Earthquake behavior at a major transition in San Andreas Fault behavior: Dating paleoseismic events at the Parkfield-Cholame boundary
Investigator(s)
Name Organization
Ramon Arrowsmith Arizona State University
Other Participants Alana Williams (AS), Gordon Seitz (CGS), Tom Rockwell (SDSU)
SCEC Priorities 2a, 1a, 1d 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/Parkfield boundary of the San Andreas Fault. The northwestern site is located just north of Highway 46. Site geomorphology is characterized by an abandoned floodplain and a deeply incised offset channel. This location was chosen as part of an overpass construction project led by the California Department of Transportation. We found evidence for at least 4 ground-rupturing events, numbered E1-E4 by increasing relative age. Supportive evidence for these events include shear fabric, significant uplift and sagging, and growth strata. Optically Stimulated Luminescence (OSL) dates are in progress.
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. 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, San Diego State University, California Geological Survey and Caltrans. This data was used to evaluate the seismic hazard that an overpass will be expected to sustain.
Exemplary Figure Figure 1 is nice to show the setting.