SCEC Award Number 14172 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Exploration of two new paleoseismic site locations in the Carrizo Plain National Monument land for testing the variable slip/variable magnitude earthquake hypothesis along the northern section of the southern San Andreas Fault
Investigator(s)
Name Organization
Sinan Akciz University of California, Los Angeles Lisa Grant Ludwig University of California, Irvine J Ramon Arrowsmith Arizona State University Katherine Scharer United States Geological Survey
Other Participants Emily Kleber (ASU, graduate student); UCLA undergraduate volunteer to be named
SCEC Priorities 2a, 1d, 4c SCEC Groups WGCEP, Geology, EFP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Differentiating between characteristic, uniform slip, and variable earthquake rupture models requires a string of proximal paleoseismic sites that provide tightly constrained age control, so that both the rupture extent of individual earthquakes and the offset distribution along the fault can be evaluated. One of the most significant challenges in testing the variable slip hypothesis along the south-central San Andreas Fault (SAF) is the distance between sites. Bidart Fan, Frazier Mountain, and Pallett Creek individually provide robust earthquake ages for the Carrizo, Big Bend, and Mojave sections of the SAF, but ~100 km (the equivalent of a M7.3 earthquake) separates each site. In 2013, we proposed to open trenches at two new locations along the Cholame/Carrizo sections of the San Andreas Fault. In June 2014, to fill the gap between Bidart and Frazier Mountain, we excavated two exploration trenches along the San Andreas Fault near the southern boundary of the Carrizo National Monument. Using evidence such as filled fissures, folded sediments with overlying angular unconformities, colluvial wedges, and fault-bounded deposits, we identified a minimum of three earthquakes, but up to eight possible horizons in the walls of the northeastern trench (T1). The top-most stratigraphic layers, with abundant liquefaction evidence, are not cut by a fault and contain pieces of wooden planks, and thus likely indicate a post-1857 ground-shaking event. Radiocarbon dates indicate these 3-8 earthquakes occurred in the last 2000 years. Exploration of the Cholame site will be concluded in 2015.
Intellectual Merit . Bidart Fan, Frazier Mountain, and Pallett Creek individually provide robust earthquake ages for the Carrizo, Big Bend, and Mojave sections of the SAF, but ~100 km (the equivalent of a M7.3 earthquake) separates each site. Although existing earthquake ages permit some rupture correlation between sites, the distances between sites significantly limit our ability to infer how many prehistoric ruptures spanned the sites or the general spatial distribution of the inferred ruptures. In order to differentiate between , uniform slip, and variable earthquake rupture models requires a string of proximal paleoseismic sites (<100 km apart) that provide tightly constrained age control, so that both the rupture extent of individual earthquakes and the offset distribution along the fault can be evaluated. The southern Carrizo sagpond site is ideally located to provide such critical data, and so is the Cholame site we are continuing to obtain permits for excavation.
Broader Impacts The Southern Carrizo Plain research project included many student volunteers. Kevin Coffey, a second-year graduate student and Kyler Boyle-Pena, a 3rd-year undergraduate student, joined us from UCLA. Robert Leeper, a Master's student at CSU Fullerton (now a Ph.D. student at UC Riverside) was a USGS volunteer. Crystal Wespestad, was an NAGT intern with the USGS. All of the students were involved with every aspect of a paleoseismic field site preparation, trench wall cleaning, gridding, logging and organic sample finding and collection.
This project would not have been possible without the generous permissions we were able to obtain from BLM who manage the Carrizo National Monument.
Exemplary Figure Figure 3. Trench log of the east end of T1. Possible earthquake horizons are noted with green lines.