Annual Report, 1997: Historic and paleoseismic behavior of the south-central San Andreas Fault between Cholame and the Carrizo Plain

Ramón Arrowsmith

Department of Geology Arizona State University

Tempe AZ 85287-1404

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

[email protected]

Dallas D. Rhodes

Department of Geology Whittier College

Whittier, CA 90608

(562) 907-4220 phone (562) 693-6117 fax

[email protected]

Note that this progress report summarizes work done by Arrowsmith and Rhodes that was done in collaboration with Lisa Grant who is submitting a separate progress report on this project.

Introduction

We have initiated a two year project to provide new data on the historic and paleoseismic behavior of the south-central San Andreas Fault (SAF; Figure 1). In the first year, we analyzed 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 also identified a suitable paleoseismic site in this area for possible excavation in the next funding cycle.

Logistical efforts

Access to private property along the Cholame segment of the San Andreas fault

Property along the Cholame segment of the SAF is all privately owned. The landowners are mostly ranchers who are sympathetic to the scientific investigations associated with earthquake geology along the SAF. Documentation of endangered species and illegal infiltration of their property by hunters and others has discouraged them from granting permission to access their property. A major effort in the initial stages of our 1997 was spent on establishing contacts and repor with the landowners of the region. We were successful in communicating to them our serious and limited interest in the geology and geomorphology along the San Andreas fault zone. The investment of time to cultivate these contacts should pay off in future work in the area (and did so in our permission to bring the 20 or so participants of the SCEC paleoseismology field workshop to the sites we investigated).

SCEC paleoseismology field workshop and SCEC annual meeting

We presented the highlights of our 1997 field research to the group participating on the SCEC paleoseismology field workshop in September 1997. Arrowsmith attended the SCEC annual meeting and presented a poster entitled "Investigation of historic and paleoseismic behavior of the south-central San Andreas Fault between Cholame and the Carrizo Plain" co-authored with Grant and Rhodes.

Coordination with other Cholame segment investigations

Arrowsmith visited the US Geological Survey in Menlo Park in August 1997 to discuss this research with Carol Prentice and Jim Lienkaemper. Prentice worked with Kerry Sieh and Charlie Rubin in the mid 1980s at an overly bioturbated paleoseismic site along the Cholame segment. Lienkaemper's review of offset landforms is largely unpublished [aside from the detailed study in Lienkaemper and Sturm, 1989]. We concluded that we should not repeat his efforts, but strive to document other offset landforms with new topographic mapping.

Reconnaissance of entire Cholame segment

Limited roads, private property, and rugged terrain make the Cholame segment of the SAF difficult to access. This area has not received much geologic investigation aside from that associated with [Vedder and Wallace, 1970] and [Sieh, 1978]. For that reason, we perused and scanned 41 aerial photographs from flight C-1260 and 16 aerial photographs from flight C-15340 of the Fairchild Aerial Photography Collection at Whittier College (of which Rhodes is the Director). We used the photographs in our evaluation of paleoseismic sites and so-far unmeasured offset landforms.

Evaluation of paleoseismic sites

We have spent the most time in reconnaissance at two sites: the Bitterwater Canyon and Still Lake sites (Figure 1). We have not excavated at either. Based upon our investigations and site review by participants on the SCEC paleoseismology field workshop in September 1997, we favor the Bitterwater Canyon site for further study.

Still Lake

The Still Lake site is a depression within a right step along the SAF. Its lowest portion contains evaporites and salt tolerant plants indicating that it is a sag pond. John Sims and colleagues surveyed the site as a potential paleoseismic candidate. Arrowsmith and Rhodes mapped a 2 km reach of the SAF centered on Still Lake in summer 1997; a portion of that mapping is overlain on the aerial photograph in Figure 2. We also surveyed a offset channel about 2 km northwest of Still Lake (Figure 2). It is offset 6 m, but a second SAF strand about 200 m northeast may accommodate more slip (although no offset landforms were evident along it).

Bitterwater Canyon

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). 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 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 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. We propose to excavate the two 20 m-long trenches shown in Figure 3.

Investigation of historic land surveys

In 1855 and 1856, James E. Freeman surveyed Township and Range lines in the region of the Carrizo Plain and northward. Grant and Donnellan [1994] recovered original monuments from that survey spanning the SAF in the Carrizo Plain near Wallace Creek and determined that the 1857 slip along the SAF between those monuments was 11 +/- 2.5 m. While most of the monuments have been moved, destroyed, or otherwise lost, the possibility of recovering a few more along the Cholame segment is conceivable. We have found several in the field and Lisa Grant has collected an extensive history of numerous monuments (Figure 4). Those that we found were typically resurveyed by the Kern County surveyor and it was the benchmark associated with that survey that gave us confidence in the identification. We inspected ten corners in the 1997 field season. These observations complemented Grant's review of office records. Depending on the completeness of those records, we expect that at least several more monuments can be located in the 1998 field season. To test our ability to measure the distance between the corners (which should have been 1 mile with error in the 1855 survey), we measured the distance along the east side of T28S R18E Sec 6 using an electronic total station with three set ups and a 1285 m backsight horizontal error of less than 16 mm. We measured a length of 1511.87 m (4958.93 feet) which is only 2.2 feet different than the 1893 chained survey and is approximately the same magnitude of measurement error that Grant and Donnellan [1994] found when they resurveyed some of the 19th century lines in the Carrizo Plain. Note that this line length is shorter than a mile because the standard surveying practice was to accommodate accumulated error in the eastern sections of the townships.

References

Grant, L.B., and A. Donnellan, 1855 and 1991 surveys of the San Andreas Fault; implications for fault mechanics, Bulletin of the Seismological Society of America, 84 (2), 241-246, 1994.

Jennings, C.W., R.G. Strand, and T.H. Rogers, Geologic map of California; 1:750,000, California Division of Mines and Geology, Sacramento, CA, 1977.

Lienkaemper, J.J., and T.A. Sturm, Reconstruction of a channel offset in 1857(?) by the San Andreas Fault near Cholame, California, Bulletin of the Seismological Society of America, 79 (3), 901-909, 1989.

Sieh, K.E., Slip along the San Andreas fault associated with the great 1857 earthquake, Bulletin of the Seismological Society of America, 68, 1421-1448, 1978.

Sims, J.D., Late Holocene slip rate along the San Andreas Fault near Cholame, California, in Geological Society of America Abstracts with Programs, pp. 451, 1987.

The Working Group on California Earthquake Probabilities, (WGCEP), Probabilities of large earthquakes occurring in California along the San Andreas fault; USGS Open File Report 88-398, 62 pp., 1988.

Vedder, J.G., and R.E. Wallace, Map showing recently active breaks along the San Andreas and related faults between Cholame Valley and Tejon Pass, California, U.S. Geol. Survey Misc. Geol. Inv. Map I-574, Washington, D. C., 1970.