Fault Zone Geometry and Historic Displacement Along the Cholame Segment of the San Andreas Fault, Southern California

STONE, ELIZABETH M., Department of Geology, Arizona State University, Tempe, AZ 85287-1404, emstone@asu.edu; ARROWSMITH, J RAMON, Department of Geology, Arizona State University, Tempe, AZ 85287-1404, ramon.arrowsmith@asu.edu; RHODES, DALLAS D., Department of Geology and Geography, Georgia Southern University, Statesboro, GA 30460, DRhodes@GaSoU.edu; GRANT, LISA B., Department of Environmental Analysis and Design, University of California, Irvine, 262 Social Ecology I, Irvine, CA 92697-7070.


The Cholame segment of the San Andreas Fault (SAF) is the transitional zone between the Parkfield segment to the north (containing both creeping and locked zones) and the locked Carrizo segment on its southern end (Fig. 1). Unlike both the Parkfield and Carrizo segments where the SAF is typically a single well defined trace, the geometry of the Cholame segment fault strands is variable on a kilometer scale. If segment length scales with depth, the rupture pattern of the Cholame segment may be an important indicator in differences of the downdip fault surface continuity.


Figure 1. Important sites along the Parkfield, Cholame, and Carrizo segments of the SAF. Note the 73 km between the existing paleoseismic sites in the Carrizo Plain and the Watertank site of Sims [1987]. The Bitterwater Canyon, Las Yeguas and South Cholame sites, among others, will be investigated for their suitability for paleoseismic investigation (green). James E. Freeman surveyed township boundaries from township 24 to 32 in 1855 and 1856 (shown in blue), and Grant and Donnellan [1994] recovered original monuments from that survey spanning the SAF in the Carrizo Plain near Wallace Creek. The intersections between the state highways in the area and the SAF are shown. The background is from the 1:750,000 scale state geologic map [Jennings, et al., 1977].

There are sparse data for the paleoseismic history of this little-studied fault segment. In contrast, the recent rupture histories of the Parkfield and Carrizo segments have been fairly well documented. Locating optimal trench sites for earthquake recurrence studies requires careful geologic and geomorphic mapping in order to understand the context of the sites (Fig. 2). Careful determination of surface offset is also important to understand the fault properties (Fig. 3).


Figure 2. Photo mosaic and geologic map of southern Cholame segment of the San Andreas Fault. Linework from field mapping on the aerial photos was heads-up digitized in Imagine and cleaned in Arc/INFO. Coverages were compiled into the final map using ArcView and the USGS program A-la-Carte provided symbology. The photos for the aerial photo base were provided by the Fairchild Collection at Whittier College. They were rectified against the topographic maps in ERDAS Imagine.

The landscape development that we infer along the San Andreas has resulted in an excellent example of the distinctive topography which occurs as structural and geomorphic processes act together along active strike-slip fault zones. On the smallest spatial scale, steps in the individual fault strands can produce active domes and basins oriented parallel to the fault. On the largest spatial scale, the central Cholame segment landscape is dominated by the classic "Rift Zone" expression which results from the interaction between the tectonic processes of fault slip and local deformation and the geomorphic processes of fluvial erosion and landsliding.


Geologic and geomorphic mapping were conducted on three separate trips (March, May and August of 1998) for a total of 18 field days. The aerial photographs were provided courtesy of the Fairchild Aerial Photograph Collection, Whittier College. These photographs were rectified and mosaicked using ERDAS Imagine. Linework on maps is an Arc/INFO GIS. The topographic maps of the three trench sites were surveyed using a total station and plotted using LisCad.


Fault zone geometry

  • Discontinuous on a kilometer scale
  • Fault zone contains areas with 2 recently active traces and no active faults conspicuous (central portion of map area)
  • Fault strands may strike up to 10 degrees away from general fault strike for this area (N40W)

    Paleoseismic data

  • Small offset channel (~25m) on Whaleback
  • Large offset valley (~400m) on Whaleback
  • Offset ridge (~40 m)
  • Three potential trench sites: Bitterwater Canyon, Las Yeguas and South Cholame

    Landscape development

  • Active doming (Whaleback) and resulting incision of Bitterwater Creek
  • Landsliding at a variety of scales

    2-D Dislocation modeling


    Figure 3. Offset data for the Cholame and the Carrizo segments compared to offsets modeled with variable stressdrops on the two segments. The curve for TCholame/TCarrizo=1 shows the effect on the slip distribution of the deepening fault zone alone. As the strength of the Carrizo segment relative to that of Cholame increases, the segment boundary becomes more clearly defined by the change in offset. The model does not fit the data on the southeast end of the Carrizo segment because the San Andreas fault trace geometry changes as it enters the Big Bend and the loading geometry changes (unlike the constant strike of the of the modeled fault surface). Strike-slip offset would be expected to decrease in this area characterized by an increase in the compressive normal traction and a decrease in the shear traction along the fault surface. This frictionless model was run using DIS3D Erikson, 1987) and used a constant stressdrop for each fault segment. Modeling mesh size was 2km x 2km.

    This modeling compliments the research efforts of Lisa Grant, who is working to determine the far field fault-parallel displacement using the changing lengths of surveyed corner markers (Fig. 4,5).


    Figures 4 and 5. Preliminary analysis of data from 1855 and subsequent surveys in several locations across the SAF indicate that several meters displacement occurred in 1857 over a 1 mile wide zone spanning the Cholame segment. There are large uncertainties in the reliability and precision of the measurements. Additional analysis and paleoseismic data are needed to reduce the uncertainty.


    Erikson, L. L., DIS3D: A three dimensional dislocation program with applications to faulting in the earth,
    unpublished Masters Thesis, Stanford University, Stanford, CA, 1987.
    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.
    Grant, L.B., and K.E. Sieh, Paleoseismic evidence of clustered earthquakes on the San Andreas Fault
    in the Carrizo Plain, California, Journal of Geophysical Research, 99, 6819-6841, 1994.
    Jennings, C. W., Strand, R. G.; Rogers, T. H., Geologic map of California. Calif. Div. Mines and
    Geol., Sacramento, Calif., USA. 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 1957 earthquake, Bulletin of
    the Seismological Society of America, 68, 1421-1448, 1978b.


    Funding for research provided by the Southern California Earthquake Center. Thanks to George Hilley for help with modeling and GIS. Thanks to Vince Matthews for his expertise and insights in the field and to Gavin also for his field assistance. Thanks to the ASU Active Tectonics group for your support in the lab.