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Poster Abstracts | Group 2 – FARM
over time. These latest Pleistocene rates, however, are still substantially higher than rates inferred
from elastic block modeling of geodetic data.
2-051
SPATIAL AND TEMPORAL SLIP RATE VARIABILITY ON THE SAN JACINTO
FAULT Le KN, Oskin ME, Rockwell TK, and Owen LA
The role of the San Jacinto fault in accommodating plate boundary motion is pivotal to
understanding fault system behavior in southern California. We present new slip rate results from
alluvial fans displaced by two parallel strands of the southern San Jacinto fault zone: the Clark fault
and Coyote Creek fault. Alluvial fans were mapped in the field with aid of ‘B4’ LiDAR imagery
and dated using cosmogenic 10Be exposure. We find that slip rates 1) varied synchronously by at
least a factor of two over the past ~35 kyr and 2) change significantly along strike as slip is
transferred southeastward from the Clark fault to the Coyote Creek fault. Latest Pleistocene (~35
kyr) to present dextral slip rates for the Clark fault are 4.6 ± 1.6 mm/yr at the Rockhouse Canyon
and 1.5 ± 0.4 mm/yr near the southern Santa Rosa Mountains. For a comparably aged displaced
alluvial fan at Ash Wash along the Coyote Creek fault, we find a slip rate of 3.1 ± 1.0 mm/yr.
Combined, these yield an average rate of slip along the San Jacinto fault system of 7.7 ± 2.6 mm/yr
over the past ~35 kyr. Mid-Holocene to present rates are significantly faster along both fault
strands. Displaced ~5 ka alluvial fans show that the Clark fault slips at a rate of 7.7 ± 1.8 mm/yr at
Rockhouse Canyon and 3.9 ± 1.4 mm/yr at the southern Santa Rosa Mountians. At Ash Wash, the
Coyote Creek fault may have slipped at a rate as high as 12.4 ± 3.5 mm/yr over the past ~2 ka.
Overall these Holocene rates are comparable to geodetic slip-rate estimates of 15 to 21 mm/yr for
the San Jacinto fault zone, and imply that presently it may dominate plate boundary motion in
southern California. The apparently synchronous variation of slip-rate along both strands of the
San Jacinto fault suggests that the rate of loading across the fault zone has varied significantly over
the past ~35 kyr.
2-052
OLD EARTHQUAKES, NEW DATES: AMS 14C DATES ON CHARCOAL FROM
PALLETT CREEK Scharer KM, Gerard TL, and Weldon RJ
The Pallett Creek paleoseismic record holds a keystone position in most attempts to develop
rupture histories for the southern San Andreas fault. Unlike the other long records on the fault,
however, Pallett Creek radiocarbon ages were determined by decay counting methods rather than
accelerator mass spectrometry (AMS) methods. The organic layers at Pallett Creek contain a
heterogeneous mix of partially decomposed, in-situ plant material, charcoal and wood detritus,
and invasive roots, all of which would contribute (in competing ways) to the dates of the large
samples required for decay counting methods and the resulting earthquake ages.
We present 40 new AMS dates from the Pallett Creek site and compare these to the original layer
ages of Sieh et al. (1989) and Sieh (1984) and earthquake ages recalculated by Biasi et al. (2002).
Careful examination of samples revealed that charcoal fragments are a common component of the
organic-rich layers. To take advantage of small sample sizes dateable by AMS, 22 dates are from
individual charcoal pieces and 4 dates are from composite charcoal samples (when pieces were too
small for separate dates). The remaining 14 dates are for bulk samples from layers where the
charcoal was too finely disseminated to pick out and include analyses for both AAA and humic
pretreatments.
In general, the AMS dates compare very well to the original large-sample dates. As expected, some
AMS charcoal pieces are >100 cal years older than others in the same layer, indicating old source
166 | Southern California Earthquake Center