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
ramon.arrowsmith@asu.edu
Dallas D. Rhodes
Department of Geology Whittier College
Whittier, CA 90608
(562) 907-4220 phone (562) 693-6117 fax
DRhodes@Whittier.edu
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.
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.
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).
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.
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.
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.
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.
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).
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.
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.
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.