1998 Proposal for scientific research to the Southern California Earthquake Center

Historic and paleoseismic behavior of the south-central San Andreas Fault between Cholame and the Carrizo Plain--Continuation

Ramón Arrowsmith

Department of Geology

Arizona State University

Tempe AZ 85287-1404

(602) 965-3541 phone

(602) 965-8102 fax

ramon.arrowsmith@asu.edu

Lisa Grant

Division of Natural Sciences

Chapman University

333 N. Glassell

Orange, CA. 92666

phone 714-744-7697 fax 714-532-6048

lgrant@chapman.eduCategory: Science

Working Group: Earthquake Geology (Group C)

Major question: What is the distribution of earthquake slip in space and time along the Cholame and northwestern Carrizo segments of the San Andreas fault?

The proposed research addresses the following:

SCEC Question 1: Determining the size and extent of past ruptures along the San Andreas Fault.

SCEC Question 2: Formulate and test hypotheses of fault segmentation.

SCEC Question 3: Geological investigations of the detailed slip distributions of recent earthquakes.

Improving our knowledge of the past history of large earthquakes along the southern half of California's master fault (one of Earthquake Geology group research foci).

Progress report: Arrowsmith progress report accompanies this proposal. Grant's SCEC progress reports are provided to SCEC under separate cover.

Proposed start date: May 1, 1998

Requested amount of support: $33,389 (ASU) and $5,112 (Chapman University)

Summary

We propose to continue a project to provide new data on the historic and paleoseismic behavior of the south-central San Andreas Fault. In the second year, 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 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.

Project plan

The principal task is to assess the suitability of the Bitterwater Canyon site for large scale paleoseismic investigation (See figures 1 and 3 of Arrowsmith and Rhodes 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.

The Bitterwater Canyon site is located on a fluvial terrace in a wide portion of the typical gorge-like Bitterwater Canyon (Figures 1 and 3 of Arrowsmith and Rhodes progress report). The deposits there are interfingering fluvial deposits from Bitterwater Creek and alluvial deposits from the drainage to the NE. The aerial photographs and our field mapping indicate that the SAF formed a 0.5 to 1 m high moletrack along here in 1857 (see the lower photograph, geomorphic sketch map, and topographic map on Figure 3 of Arrowsmith and Rhodes progress report for a detailed look at the site). Portions of the mole track have been buried by the alluvial material. If previous ruptures were similar, the distortion and vertical displacement of the ground surface may be preserved by the burial from the fluvial and alluvial fan materials. We expect that the relatively low relief surface may permit the growth of grasses and possbily peats that should have been buried by the relatively rapid sedimentation. The deposition of alluvial material should be rapid enough to minimize the bioturbation that would probably be seen on other portions of the terrace surface.

The preliminary investigations will be accomplished by excavating a pair of fault perpendicular trenches that are approximately 20 m long and about 4 m deep (Figure 3 of Arrowsmith and Rhodes progress report). 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.

Project Management Plan

We expect to work together on all aspects of this project. This project and associated geologic and geomorphic mapping will constitute the masters thesis of ASU student Beth Stone. She will be supervised in the field by Arrowsmith and Dallas D. Rhodes. Grant will provide technical review of the research plan, initial results, and final interpretations, as well as lead the investigation of the historic surveys. That will entail several trips to the field and office consultations. The field portions of the proposed work will be completed in summer 1998. 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.