SCEC Award Number 17017 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Refining the earthquake chronology of the last millennium along the Cholame segment of the San Andreas fault
Name Organization
Ramon Arrowsmith Arizona State University Sinan Akciz California State University, Fullerton Lisa Grant Ludwig University of California, Irvine Thomas Rockwell San Diego State University
Other Participants Alana Williams (Arizona State University)--this is the core part of Alana's Ph.D.
SCEC Priorities 5b, 5c, 5d SCEC Groups WGCEP, SAFS, Geology
Report Due Date 06/15/2018 Date Report Submitted 07/12/2018
Project Abstract
The Cholame segment of the southern San Andreas fault (SAF) accommodates the interaction between the creeping Parkfield segment to the northwest and the locked Carrizo segment to the southeast. This ~75 km long reach is known to have ruptured during the M7.8 1857 Fort Tejon earthquake, however a complete record of rupture behavior has not been well-characterized. Classical models of recurrence for this section of the SAF expected repeats of the 1857 event with smaller superposed M7 Cholame-only events.

We have returned to the Annette Site along the north-central Cholame segment and excavated two fault-perpendicular trenches with the intent to obtain additional event evidence and an older record. We found evidence for 4 or 5 ground rupturing events and 1 ground shaking event. We anticipate that the ages obtained from radiocarbon will corroborate our previous findings at the Annette site (5 or 6 ground rupturing events over the last millennium and one post-1857 ground-shaking event) and compare and possibly correlate with Carrizo Plain rupture records documented at Bidart (50 km southeast) and Frazier Mountain (180 km southeast). These data will be used to test earthquake recurrence models for the largest fault in California, which has been widely hailed as the likely source for the next great earthquake in southern California. Thus, gathering additional data will place useful bounds on the character and frequency of multi-segment ruptures along the SAF, which can then be used as essential input data for earthquake hazard forecasts.
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 and collect more evidence for our previous interpretations, 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.

The strata record evidence for up to 6 earthquakes, of which 4 are of high quality. There is also possible post-1857 deformation, similar to that recognized at the LY4 and Bidart sites (Young, et al., 2002; Akciz, et al., 2010), which may represent a strong ground-shaking event. In 2018, we excavated two fault-perpendicular trenches several meters southwest of the 2016 trenches, in an attempt to bolster our earlier evidence. The strata recorded evidence for five ground rupturing events (four of good quality) that could be correlated to the six events seen previously. We infer that E1 in all four fault-perpendicular trenches was the 1857 earthquake. It is represented by several fault splays capped by silty clay sag deposits that fill a depression in the fault zone that would have been produced by the 1857 surface rupture.
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 field earthquake geology project was the result of a large group effort spanning several institutions, including Arizona State University, “University of Potsdam”, San Diego State University, Cal State Fullerton and the University of California, Irvine. We facilitated training in paleoseismic investigation for two undergraduate and 1 graduate student.
Exemplary Figure Figure 3. Previous work location in the Cholame segment from 2016 and 2018 excavation is shown with logging examples. A) B4 lidar of Annette Site with the fault trace mapped in red, box inset of B. B) Structure from Motion model of the Annette site with the fault zone shaded. Trenches T2A and T2C are visible from 2016 excavation, T2D and T2E outlines from 2018 are shown in red. C) Partial log of T2C NW wall displaying the key units and the event horizons (red stars) E0 through E6. D) Inset box showing the detailed hand logging with color for reference. See Table 1 for list of event dates, event horizon evidence, and rankings.

Williams, et al., in prep.