Ramón Arrowsmith, Ph.D.

Elizabeth M. Stone

Department of Geology, Arizona State University

Tempe AZ 85287-1404

(602) 965-3541 phone (602) 965-8102 fax

ramon.arrowsmith@asu.edu

Category: Science

Working Group: Earthquake Geology (Group C)

Major emphasis: Earthquake Potential

SCEC Task C1: Paleoseismic research along the San Andreas Fault

Progress report: Arrowsmith progress report accompanies this proposal.

Proposed start date: February 1, 1999

Requested amount of support: $30,733

We propose to continue a project to provide new data on the historic and paleoseismic behavior of the south central San Andreas Fault. We will continue to analyze offset landforms and historic survey data along the northern portion of the 1857 rupture. This will improve our understanding of the rupture potential of the San Andreas Fault in the Parkfield-Cholame-northern Carrizo Plain area, and provide data for evaluating models of fault behavior and seismic hazard in Southern California. We have identified a promising paleoseismic site at which we propose to excavate two 20 m-long trenches in order to test the site’s suitability for determining earthquake timing. We have permission to trench at that site. We will strive to identify effects of the 1857 event, including displacement measured from historic surveys, and to date the penultimate event. Please note the text and figures in our accompanying progress reports to illustrate our progress and the potential for this continued effort. We expect that the results of this work can be completed and published by the end of year 2001.

The principal task is to assess the suitability of the Las Yeguas (LY4) site for large-scale paleoseismic investigation (See figures 1, 2, and 3 of Arrowsmith progress report). Interpretation of the geomorphic evidence from the site suggests that it may preserve an important record of the earthquake history of the Cholame segment of the San Andreas Fault.

At the LY4 site, the right-stepping SAF cuts an alluvial fan that abuts a 100-m-long ridge. A 1-m-high scarp is evident along the most recently active trace. We expect that the low relief surface may permit the growth of grasses that should have been buried by the relatively rapid sedimentation from the active fans at portions of the site. The deposition of alluvial material should be rapid enough to minimize the bioturbation that would probably be seen on other portions of the surface. This site is similar in setting to the Bidart site [Grant and Sieh, 1994], and careful site selection among the different portions of the fan surface (with different ages) will permit us to optimize our exposure of the buried paleoseismic ground distortion. Note that we have selected the LY4 site based upon field investigations and significant mapping along the fault zone in this region (See figure 2 of Arrowsmith progress report). However, before committing to that site completely, we will review all accessible sites (including the South Cholame site, Figure 3 Arrowsmith progress report) with Grant and other available earthquake geologists to add confidence to our site selection and to improve our investigation plan.

These investigations will be accomplished by excavating a pair of fault perpendicular trenches that are approximately 20-m long and about 4 m deep. Standard logging methods will be employed to document the stratigraphy and structure preserved on the trench walls. We will experiment with photographic documentation of trench wall features using a USGS-style "Trenchomatic." Our primary goal for the trenching will be to demonstrate the continuity of the stratigraphic record at the site, to find the 1857 event, and to date the penultimate event.

This project and associated geologic and geomorphic mapping will constitute the masters thesis of ASU student Beth Stone. Arrowsmith and Grant will supervise her in the field. Grant will provide technical review of the research plan, initial results, and final interpretations, as well as lead the investigation of the historic surveys. This will entail several trips to the field and office consultations. The field portions of the proposed work will be completed in spring 1999. We will present preliminary findings of our work at the annual SCEC Meeting. The results of this work will be presented in at least one journal article.