Testing the shorter and variable recurrence interval hypothesis along the Cholame segment of the San Andreas Fault

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

Submitted August 15, 2017, SCEC Contribution #7860, 2017 SCEC Annual Meeting Poster #113

The Cholame segment of the San Andreas Fault is positioned between the Parkfield and Carrizo segments to the NW and SE, respectively, 20 km southeast of Highway 46. Rupture behavior is poorly characterized for this ~75 km reach, limiting seismic hazard evaluation. We present paleoseismic results from 2 fault perpendicular trenches connected by a fault parallel trench. Site geomorphology is characterized alluvial fans crossing the fault, and offset drainages, shutter ridges, sag ponds. Fault zone stratigraphy consists of alternating finely bedded sand, silt, and gravel, and bioturbated soil horizons. The strata record up to 6 earthquakes (4 have good evidence) and possible post-1857deformation, similar to the LY4 site 38 km southeast. The events are numbered from youngest to oldest, with E0 being a ground shaking event and E1-E6 interpreted as rupture events. E6, E5 and the most recent earthquake (E1, MRE) are well supported by evidence of decreasing vertical separation up-sequence, capped fissure fills, and colluvial wedges, which created small horst and graben structures. Units display throw on individual fault splays ranging from 60 cm at the base to 12 cm near the E1 horizon, with associated small colluvial wedges and sag deposits within the ~4 m wide fault zone. We found abundant detrital charcoal in many of the units and burn horizons at or near event horizons providing great potential for bracketing ages of these paleoearthquakes. We collected 116 samples for 14C dating and 10 samples for Optically Stimulated Luminescence dating. Preliminary dates indicate that events E6-E1 occurred between 1300 A.D. and 1857, providing significant potential to correlate 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 but slightly longer time period. OxCal sequences compared with the correlation of southern San Andreas Fault paleoearthquakes from Scharer et al., 2015, show 4 of the 6 possible events could be attributed to Carrizo segment ruptures, (E1, E2, E3, and E4/E5), however more dates are required to refine this model. In addition to paleoseismic data, we performed a preliminary GPR investigation and found evidence for a channel at ~5 m depth (older than 1300 A.D.) on the SW side of the fault trace, which could provide a long-term slip rate for the Cholame segment. Further excavation of fault parallel trenches and the channel is scheduled for this fall.

Key Words
paleoseismology, San Andreas fault, recurrence,

Citation
Williams, A. M., Arrowsmith, R., Rockwell, T. K., Grant Ludwig, L., Akciz, S. O., & Gontz, A. M. (2017, 08). Testing the shorter and variable recurrence interval hypothesis along the Cholame segment of the San Andreas Fault. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology