Poster #023, San Andreas Fault System (SAFS)

Investigating the Earthquake Chronology of the Last Millennium along the Cholame Section of the San Andreas Fault

Alana M. Williams, Ramon Arrowsmith, & Thomas K. Rockwell
Poster Image: 

Poster Presentation

2020 SCEC Annual Meeting, Poster #023, SCEC Contribution #10750 VIEW PDF
The Cholame section of the SAF is located between the Parkfield section and the locked Carrizo section. This portion of the fault is significant because it marks a behavioral transition between Parkfield in the north with its creep and numerous M6 earthquakes, and Carrizo in the south with its century-long recurrence intervals.
We present results from 4 fault perpendicular trenches at the new Miller’s Well site (MW) and 2 fault perpendicular trenches from the previously studied LY4 site. The MW site is 3 km north of Hwy 58. Site geomorphology is characterized by a SW draining alluvial fan ~50m wide depositing against a ~10 m high NE-facing SAF-bounded scarp. Stratigraphy was heavily b...
ioturbated, and interpretable only on the east side of the main fault zone, with massive soil and rare sandy blocks on the west. We collected samples for 14C dates from the continuous stratigraphic units bounding interpreted earthquake horizons and the massive soil to determine the approximate age of the most recent earthquake (MRE) and penultimate event. The ages we received were 1332±70 cal BP, 983±60 cal BP, bracketing the penultimate event and 941±50 cal AD, and 1005±30 cal AD as a lower bound for the MRE. The lack of preserved and distinctive sediment markers makes for a poor earthquake event record at MW, however it provided interesting data for studying the geomorphic features and fault geometry near the transition from the Cholame to Carrizo sections.

We opened two new fault perpendicular trenches at the LY4 paleoseismic site 10 km northwest of MW. This site is located east of linear ridges bounding a 20th century earthen-dammed stock pond that has since been drained. The site has several alluvial fans depositing up against the SAF-bounded NE-facing scarp. Stratigraphy consists of alternating finely bedded sand, silt, and gravel layers, and bioturbated soil horizons. The fault zone is marked by a ~4m wide graben. The bottom of the trench contains larger well-sorted gravels and small channels. There is evidence for 5-7 ground rupturing events, including the MRE (1857), compared to the 3-4 events previously documented at this site. E1 is likely the 1857 surface rupture. Four of the event horizons have excellent evidence indicating that a surface rupture occurred, whereas the evidence for 3 events is less convincing. This site has significant potential to correlate this earthquake sequence with Annette and high-quality records of Carrizo sites.