SCEC Award Number 16202 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Testing the shorter and variable recurrence interval hypothesis along the Cholame segment of the San Andreas fault
Investigator(s)
Name Organization
Ramon Arrowsmith Arizona State University Sinan Akciz University of California, Los Angeles Thomas Rockwell San Diego State University
Other Participants Prof. Lisa Grant-Ludwig--Lisa is directly involved but not seeking support so we put her here as an other participant.
Alana Williams, Ph.D. pre-Candidate
School of Earth and Space Exploration
Arizona State University
SCEC Priorities 1a, 1d, 2a SCEC Groups EFP, Geology, SoSAFE
Report Due Date 03/15/2017 Date Report Submitted 03/15/2017
Project Abstract
 The Cholame section of the San Andreas Fault is between the Parkfield section with its creep and M6 earthquakes, and the locked Carrizo section. Although offset reconstructions exist for this ~75 km reach, rupture behavior is poorly characterized, limiting seismic hazard evaluation. We present new paleoseismic results from 2 fault perpendicular 26-m-long trenches connected by a 15 m fault-parallel trench. The site is ~20 km southeast of Highway 46. Site geomorphology is characterized by several ~50 m offset drainages, shutter ridges, sag ponds, and alluvial fans crossing the fault. Strata record evidence for 6 ruptures, and possible post-1857 deformation, similar to the LY4 site 38 km to the southeast. Events E6, E5, E4, and E1, the most recent earthquake (MRE), are well supported by evidence of decreasing vertical separation up-sequence, capped fissure fill, colluvial wedges, and horst and graben structures. Vertical separation ranges from 60 cm at the base to 12 cm near the MRE horizon within the ~4 m wide fault zone. E2 and E3 are less certain, supported by decreasing vertical separation, suggesting smaller displacements. The MRE horizon consists of silty clay sag capped by very fine, bedded sands and gravels. If the MRE is the 1857 event, this site has potential to correlate older earthquakes with high-quality rupture records at Bidart (BF) and Frazier Mountain (FM) (50km, 180 km southeast). This site contains abundant detrital charcoal and in situ burn horizons, of which we have processed 19 preliminary dates that support the correlation between the three sites.
Intellectual Merit Paleoseismic data from the Cholame section of the San Andreas Fault (SAF) addresses major earthquake system science questions for SCEC4 by attempting to resolve the timing of events and how the slip deficit along the southern SAF is released. The Cholame section does not have well defined recurrence interval, which significantly limits the ability to model seismic hazard in this area. This stretch of the fault is significant because it marks the transition between two segments of the SAF with vastly different rupture behavior (Carrizo segment and Parkfield segment). Our contribution attempts to fill a gap in the data for current hazard assessment in California, by placing geologic bounds on the character and frequency of multi-segment and multi-fault ruptures along the SAF. Advancing long-term earthquake rupture forecasts and the general constraints on earthquake simulator results depends on high quality and long records of earthquake recurrence. This site has abundant dateable material and excellent potential for a high-quality record of earthquake frequency spanning the last millennium.
Broader Impacts Along with the evident value of improving seismic hazard assessments, we were able to provide valuable practical training in paleoseismology and large scale project logistics. The earthquake geology field project was a large group effort spanning several institutions, including Arizona State University, University of California, Los Angeles, San Diego State University, Cal State Fullerton and the University of California, Irvine. We facilitated training in paleoseismic investigation for six undergraduates and 1 graduate student, five of whome were first time attendees at the 2016 SCEC Annual Meeting.
Exemplary Figure Figure 2. Correlation diagram for SSAF paleoearthquakes from Parkfield to Wrightwood showing location of the Annette Site. The black line and bracket indicate our preliminary interpretations from 2016. The orange lines represent the 3 poorly dated events at LY4. The rest of the figure is adapted from Scharer et al. 2015. Shaded curves are earthquake age probability distributions; vertical bars show 95% confidence range for each earthquake. Paleoearthquakes are labeled by the letter or number used in the original reference [Weldon et al., 2004; Pruitt et al., 2009; Akçiz et al., 2010; Scharer et al., 2011]. Grey numbers indicate horizontal displacement in meters (and vertical lowering for FM). Solid red lines show possible ruptures based on Scharer’s correlation of paleoearthquake ages among sites.
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