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Poster Abstracts | Group 2 – Seismology
2-124
PALEOCLIMATE CONSTRAINTS ON CHANNEL INCISION AND EARTHQUAKE
SLIP AT THE BIDART FAN SITE, SAN ANDREAS FAULT, CALIFORNIA Noriega GR,
Grant Ludwig LB, Akciz SO, and Arrowsmith JR
Laterally offset channels are one of the main geomorphological features that are commonly used to
infer slip rates, estimate the magnitude of these paleoearthquakes and their recurrence intervals.
Difficulty in constraining the age of channel incision, however, generally introduces large
uncertainties into the calculations. We combine paleoclimate data from southern California with
paleoseismologic data from the Carrizo section of the San Andreas Fault (SAF) to better interpret
the age of some of the ephemeral stream channels that are offset by paleoearthquakes that ruptured
this section of the SAF. In this study, we assume that runoff from precipitation is the main driving
force of channel incision in the Carrizo Plain, and by identifying the extreme wet and dry periods
based on the readily available climate data, particularly precipitation data, we narrow down the
timing of initiation of channel incision. This allows us to examine the rate of channel incision across
the San Andreas fault and compare it with the rate of seismic events. We identified several extreme
climate events (wet years) as likely dates of channel incision events in the Carrizo Plain. The
findings reveal new constraints on incision of two channels in the Bidart Fan site. We propose that
a channel (NW channel) that has been offset 14.6 to 18.4 m, could have been incised during an
extreme wet year in either A.D. 1366 or A.D. 1418. A second channel (SE channel) that is offset 7 or
8 m probably incised in A.D 1642. Grant and Sieh (1994) indicated the 14.6 to 18.4 m offset of the
NW channel is due to the 1857 and one or two previous earthquake(s). Grant and Sieh (1994)
inferred 7 or 8 m offset of the SE channel is due to the 1857 earthquake alone. The new climate
inferred dates of channel incision, along with earthquake dates from Akciz et al. (submitted)
suggest the NE channel may have been offset by three or four earthquake events, and the SE
channel has been offset by the 1857 and possibly by a previous earthquake.
2-125
CRUSTAL ANISOTROPY MEASURED NEAR THE CALICO FAULT IN THE
EASTERN CALIFORNIA SHEAR ZONE Colella H, and Cochran ES
We present preliminary results from a study of crustal anisotropy near the Calico Fault located in
the Eastern California Shear zone. Shear wave splitting is used to approximate the in situ stress
field and/or shear fabric near the fault and examine whether proximity to the main slip plane
affects the observed anisotropy. Between June and November 2006, approximately 65 seismic
stations were deployed in a grid spanning 4 km across and 1.5 km along the Calico fault to explore
seismic properties of a fault that has not experienced a major earthquake in hundreds to thousands
of years. Here we examine shallow crustal anisotropy determined using the 37 earthquakes that
occur within the shear wave window, with an angle of incidence less than 45 degrees, during the
experiment period. Splitting parameters are determined using an automated cross-correlation
method that determines the fast direction and delay time for each station-event pair. Preliminary
results show significant scatter in fast directions and delay times, but suggest a fast direction
between N15°W and N15°E. Future work will examine the relationship between observed shear
wave splitting and event-station path.
2-128
EVOLVING AFTERSHOCK SEISMICITY USING RATE-STATE EQUATIONS IN A 3D
HETEROGENEOUS STRESS FIELD Smith DE, and Dieterich JH
We combine 3D models of stress heterogeneity [Smith, 2006; Smith and Heaton, 2008] with a
formulation for seismicity based on rate-state friction [Dieterich, 1994; Dieterich, et al, 2003] to
208 | Southern California Earthquake Center