Exploring the transition between the Parkfield and Cholame sections of the San Andreas Fault

Alana M. Williams, Ramon Arrowsmith, Shreya Arora, Drew Cochran, Erik Jansen, Garrett Pontin, & Caroline Friedman

Submitted September 10, 2023, SCEC Contribution #13175, 2023 SCEC Annual Meeting Poster #090

The southern San Andreas fault (SAF) has one of the most well developed paleoseismic records in the world, which provides key data for evaluating spatiotemporal models of earthquake rupture. The Cholame section of the SAF located in south-central California has presented difficulties in evaluating earthquake hazards largely due to poor age control, difficult site access permissions, and challenging stratigraphy. Our previous paleoseismic studies in this region indicate that the central and southern Cholame section behaves similarly to the Carrizo section—that is with a mix of of full multi section ruptures like the M7.8 1857 earthquake and smaller single section M7 earthquakes. The northwestern Cholame section holds the key to understanding the regional seismic hazard over a transitional boundary in fault behavior along the SAF as it connects northwest to the Parkfield section with its mix of creep and M6 events. We opened 2 new fault perpendicular trenches, the first at a new site Twisselman Lake, ~9 km from the Parkfield boundary, and the second at a previous site, Miller’s Well, ~55 km from the boundary. Despite site locations being constrained by record-setting rainfall in southern California in 2023, we opened the farthest northwest trench thus far along the Cholame section to investigate the Parkfield-Cholame boundary. Priority sites were either still flooded or the moisture removed the soil cohesion and several trench attempts collapsed. We found evidence for 3 paleoseismic events at both successful trenches. The Twisselman Lake trench displays sag deposits, fissure fill and discontinuous soil development. The new Miller’s Well trench contains discontinuous stratigraphy, sags and apparent vertical offsets. Previous Cholame excavations resulted in 6-7 paleoevents. The drop in apparent events is likely due to a lack of sedimentation, extensive bioturbation, and clay rich soil that does not preserve event evidence, rather than a change in rupture behavior. Further excavations are needed in better locales along the northwestern portion of the Cholame section to further assess variations in rupture behavior between the Parkfield, Cholame, and Carrizo Plain sections of the SAF.

Key Words
San Andreas fault, Cholame, paleoseismology, California, Parkfield, strike slip,

Citation
Williams, A. M., Arrowsmith, R., Arora, S., Cochran, D., Jansen, E., Pontin, G., & Friedman, C. (2023, 09). Exploring the transition between the Parkfield and Cholame sections of the San Andreas Fault . Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)