SCEC Award Number 12157 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Development of a new paleoseismic at Elizabeth Lake, central Mojave section of the San Andreas fault
Investigator(s)
Name Organization
Sean Bemis University of Kentucky Kate Scharer United States Geological Survey
Other Participants 1 Graduate student
1 Undergraduate student
SCEC Priorities 2a, 4c, 4b SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 Despite the recent development of several high-quality paleoseismic records for sites on the southern San Andreas fault , the distance between some of these sites remains too great to reliably test models of earthquake behavior and understand possible persistent segmentation of earthquake ruptures. Because the earthquake record from Bidart Fan and Frazier Mountain are very similar, but quite different from that of Pallett Creek, last year we began new paleoseismic investigations at Elizabeth Lake, CA. This location splits a 100 km gap between Pallett Creek and Frazier Mountain that is large enough to hold its own M7 earthquakes. Our SCEC-funded investigations during 2012 focused on two small transtensional pull-apart basins that pond these sediments. Our excavations revealed excellent cm- to dm-scale stratigraphy, including several organic-rich clayey layers that separate coarser-grained clastic units. Dating of five charcoal fragments recovered from these strata exposed in trench EL1 provides a preliminary chronology showing that the uppermost 2 m of sediment records ~900 years of deposition. Our investigations demonstrate that this site has the stratigraphic potential to provide important constraints on the extent and timing of ruptures along the Mojave section of the southern San Andreas fault over the past 1000 years.
Intellectual Merit The south-central portion of the San Andreas fault is one of the few places in the world where we are approaching a situation of adequate data coverage, both spatially along a fault and in time, where we can begin to test models of the recurrence of major earthquakes. Interpretations of paleoseismic data have produced a variety of earthquake recurrence models, notably the characteristic and variable-slip models. On this section, there is a remarkable difference in the earthquakes documented in trench exposures at Frazier Mountain and Pallett Creek over the last 1000 years. Since ca. A.D. 1400, six earthquakes occurred at Frazier Mountain while evidence for only three are preserved at Pallett Creek. These data clearly indicate that not all ruptures are like the 1857 earthquake, which spanned both sites, and bring into question long-standing segmentation models which place both sites on the Mojave segment. What remains unclear is the southeast termination of the “extra” Frazier Mountain earthquakes. Did they occur as ~50 km long, ~M7 “filler” earthquakes as predicted by the variable slip model, or part of larger (>M7.5) events with fairly long rupture lengths extending towards Pallett Creek? Significant spatial gaps remain that limit our ability to confidently develop rupture histories of the fault. Our investigations at the Elizabeth Lake paleoseismic site have demonstrated that we should be able to significantly refine the knowledge of rupture timing and extent for the southern San Andreas fault and apply this to improving our understanding of earthquake recurrence on this fault.
Broader Impacts This project supported an early-career investigator (Bemis) and provided an opportunity for hands-on training and experience to University of Kentucky (UK) graduate and undergraduate students in the techniques of paleoseismology. The students involved with this research propagated this experience to fellow UK students following the Elizabeth Lake work through collaborative research on additional student projects on the Denali fault and other nearby active faults in Alaska.

With the ongoing debate and uncertainty involving how to model long-term earthquake recurrence behavior on major fault zones, we anticipate that our ongoing work at this site will produce a significant contribution to seismic hazard planning for this fault zone, and for major strike-slip faults in general.
Exemplary Figure Figure 5. Locations of existing trenches (thick black lines) at Elizabeth Lake site. Trenches EL1 and EL2 were excavated by Bemis and Scharer in 2012, and ERT was excavated by Dolan in 1998; several other exploratory trenches excavated in 1998 near EL1 and EL2 are not shown for clarity. Open rectangles depict proposed new excavations. Large black arrows depict the trace of the SAF and highlight the transtensional step at this site.
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