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

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

Submitted August 15, 2016, SCEC Contribution #6827, 2016 SCEC Annual Meeting Poster #128

The Cholame segment of the San Andreas Fault interacts with the Parkfield segment to the northwest with its creep and M6 earthquakes, and the locked Carrizo segment to the southeast. Although offset reconstructions exist for this ~75 km reach, rupture behavior is poorly characterized, limiting seismic hazard evaluation. Here we present new paleoseismic results from 2 fault perpendicular 26 m long trenches connected by a 15 m long fault parallel trench. The site is located south of the Parkfield segment 20 km southeast of Highway 46. Site geomorphology is characterized by several ~50 m offset drainages northwest of the trenches, small shutter ridges, sag ponds, and alluvial fans crossing the fault. Fault zone stratigraphy consists of alternating finely bedded sands, silts, and gravels, and bioturbated soil horizons. The strata record 3-4 earthquakes and possible deformation post-1857, similar to the LY4 site 38 km southeast. E4, E3 and the most recent earthquake (MRE) are well supported by evidence of decreasing vertical offset up-sequence, capped fissure fill and colluvial wedges, which create small horst and graben structures. Units display vertical offsets ranging from 60 cm at the base to 12 cm near the MRE horizon, small colluvial wedges, and sag deposits within the ~4 m wide fault zone. E2—the penultimate--is less certain, supported only by the decreasing offset in the stratigraphic sequence. The E4 event horizon is a gradational clayey silt sag deposit capped by discontinuous gravel, 18 cm at its thickest point and extending 4.8 m across the fault zone. The E3 and E2 event horizons are capped by thin bedded silty clay, and bounded by discontinuous burn horizons. The MRE horizon extends 6 m across the main fault zone, and consists of a silty clay sag deposit capped by very fine, bedded sand and coarse gravel, 22 cm at its thickest point and overlying a burn horizon. If the MRE is indeed the 1857 event, it has significant potential in correlation with the high quality rupture records at Bidart (70 km southeast), and Frazier Mountain (180 km southeast). This site contains abundant detrital charcoal in many of the units and burn horizons at or near event horizons providing great potential for bracketing the age of these paleoearthquakes.

Key Words
Cholame, recurrence, paleoseismology, San Andreas

Citation
Williams, A. M., Arrowsmith, R., Akciz, S. O., Rockwell, T. K., Grant Ludwig, L., & Branscomb, S. (2016, 08). Testing the shorter and variable recurrence interval hypothesis along the Cholame segment of the San Andreas Fault. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)