Refining the earthquake chronology of the last millennium along the Cholame segment of the San Andreas fault

Alana M. Williams, Ramon Arrowsmith, Thomas K. Rockwell, Sinan O. Akciz, & Lisa Grant Ludwig

Published August 14, 2018, SCEC Contribution #8609, 2018 SCEC Annual Meeting Poster #267

The Cholame section of the San Andreas Fault (SAF) is positioned between the Parkfield and Carrizo sections to the NW and SE, respectively. Motivated by its critical position along the SAF, we have developed the Annette site (20 km SE of Highway 46). In 2017, we studied 2 fault perpendicular excavations connected by a small fault parallel trench. We returned in 2018 and excavated 2 more perpendicular trenches to densify our dataset. Earthquake evidence spans a 5m-wide fault zone displaying horst and graben structures, colluvial wedges, and decreasing vertical offsets upsection. Stratigraphy consists of finely bedded sand, silt, gravel and bioturbated paleosols. We found evidence for 5 earthquakes (4 with good quality) and post-1857 deformation. We have numbered these events from youngest to oldest to correlate with the previous Annette trenches from 2017, in which we found 6 possible ruptures. E0 is a post-1857 ground shaking event and E1-E6 are interpreted as rupture events. Based on vertical offset, graben scale and stratigraphy, but without radiocarbon dates, the rupture evidence from 2018 can likely be correlated to E1, E2/3, E4, E5, and E6 from 2017 trenches. Post-1857 deformation is similar to evidence at the previous Annette trenches and the LY4 site 38 km southeast. Events with the most confidence overall are E1, E4, E5 and E6. Charcoal dates from 2017 excavations indicate that events E6-E1 occurred between 1300 A.D. and 1857, allowing for correlation of this earthquake sequence with the high-quality records of 6 ruptures at Bidart and Frazier Mountain (70 and 180 km SE, respectively) within a similar time period. OxCal derived dates of rupture sequences compared with southern San Andreas Fault paleoearthquakes from Scharer et al., 2014, show 4 of the 6 possible events could be attributed to Carrizo segment ruptures (E1, E2, E3, and E4/E5).

Key Words
paleoseismology, Cholame, San Andreas

Citation
Williams, A. M., Arrowsmith, R., Rockwell, T. K., Akciz, S. O., & Grant Ludwig, L. (2018, 08). Refining the earthquake chronology of the last millennium along the Cholame segment of the San Andreas fault. Poster Presentation at 2018 SCEC Annual Meeting.

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