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)
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.