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Poster Abstracts | Group 2 – FARM
each of several long segments, but abrupt changes between the segments (e.g., ~3.5 m in Cholame
segment, ~9.0 m in Carrizo segment). A similar study by Lienkaemper (2001) concentrated on the
Cholame segment. Although data were too sparse and ambiguous to resolve details of the 1857
slip, the slip amount was determined to be distinctively larger (~5 m) than previously estimated.
Both studies relied mainly on field observation and aerial photographs of offset landforms --mostly
ephemeral channels crossing the fault trace(s) at high angles.
We use “B4” LiDAR high resolution topographic data (resolution: 0.25 m) to determine the slip
distribution of the 1857 earthquake along the Cholame and Carrizo fault segments. After fault line
and offset linear features (i.e., fluvial channels) are traced at a site, we determine the position of
upstream and downstream profile. Then we back-slip one profile relative to the other in 0.1m
increments and calculate the elevation difference between both profiles. The amount of slip where
the difference between both profiles has its minimum is considered the optimal displacement
estimate. We measured offsets for channels identified by previous studies and utilized the
spectacular resolution of the LiDAR data to find additional geomorphic features that were offset in
1857 and prior earthquakes.
Although this work is still in progress, preliminary results show relatively uniform slip of ~4.6m
for the Cholame Segment. Inspection of offset gullies not previously identified along the Carrizo
Segment suggest that the ramping (increase in slip from 3 to 9m) proposed by Sieh (1978) may not
occur. In the Carrizo Plain, we found a number of gullies that were offset by only 4-6m, suggesting
that the observed 9m offsets may be the result of more than one earthquake.
2-045
TECTONIC GEOMORPHOLOGY OF THE SAN ANDREAS FAULT ZONE FROM
HIGH RESOLUTION TOPOGRAPHY: AN EXAMPLE FROM THE CHOLAME
SEGMENT Arrowsmith JR, and Zielke O
High resolution topographic data along fault zones are important aids in the delineation of recently
active breaks. A 15 km-long portion of the south-central San Andreas Fault (SAF) along the
southern Cholame segment preserves spectacular tectonic landforms such as benches, troughs,
scarps, and aligned ridges that indicate recurring earthquake slip. Recently acquired LiDAR
topographic data along the entire southern SAF (“B4” project) have shot densities of 3-4 m^2.
Computed from the LiDAR returns, Digital Elevation Models (DEMs) of 0.25 to 0.5 m resolution
using local binning with inverse distance weighting and 0.8 m or larger search radii depict the
tectonic landforms at paleoseismic sites well enough to rate them confidently. Mapping of recently
active breaks using a LiDAR-only based approach compares well with aerial photographic and
field based methods. The fault zone varies in width from meters to nearly 1 km and is comprised of
numerous en echelon meter to km-length overlapping sub parallel fault surfaces bounding
differentially moving blocks elongate parallel to the SAF. The semantic variations of what
constitutes “active” and the importance of secondary traces influence the breadth and complexity
of the resulting fault trace maps.
2-046
THE SAN ANDREAS FAULT ZONE IN SAN GORGONIO PASS: A GEOLOGIC-
FRAMEWORK PERSPECTIVE Matti JC
The pioneering work of C.R. Allen (1957) in San Gorgonio Pass (SGP) demonstrated three points
that have stood the test of time. First, a single throughgoing trace of the modern San Andreas Fault
(SAF) cannot be mapped confidently through the Pass; second, the architecture of the SAF zone in
the Pass is extraordinarily complex; third, any SAF rupture is likely to be complicated. For these
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