Annual Meeting - SCEC.org

Group 2 – SoSAFE | Poster Abstracts
material or longer travel times for the detrital material. The most interesting difference is for the
older layers (P52-P35), where most of the AMS dates are younger than the original dates. Because
the AMS dates are for individual charcoal pieces, they provide maximum age estimates for the
layer, suggesting that the original dates are older than the layer age. An important consequence of
the AMS dates is younger ages for earthquakes F and I, and because layer 52 is the lower bounding
layer for events N and R, these earthquakes ages also become younger. As a result, the AMSderived
earthquake chronology appears less clustered than the original, although pending AMS
dates including several near P52 may alter this initial conclusion. Finally, we also present a
comparison of the earthquake ages in Biasi et al. (2002) with OxCal models of the original and new
AMS derived dates.
2-053
SLIP RATE SITE ON THE SAN ANDREAS FAULT NEAR LITTLEROCK, CA Weldon
RJ, Scharer KM, Sickler RR, Pruitt AH, Gilleland CL, and Garroway J
Work at the Littlerock site has focused on two offsets that have been interpreted to yield slip rates
from about 2-4 cm/yr for time intervals of ~3500 and ~450 years (3 earthquakes based on the
nearby Pallett Creek chronology). In response to a 2007 SoSAFE field review we addressed three
key points to complete the project. For the older ~3500 year old offset, we confirmed that the offset
“yellow gravel”, previously only dated and exposed well on the north side of the fault, really exists
on the south side and that there are no older deposits on the south side that could have been
scoured from the north side (thus allowing for an older age and slower slip rate). Our second
objective was to extend the younger offset channel to the fault to confirm that it was cleanly
truncated (and thus did not flow along the fault and have less offset). Finally, we collected
additional C-14 samples from deposits associated with both offsets.
We dug two large benched pits into the main tributary channels on the south side of the fault and
found 6-8 m of an alluvial deposit that is visually identical to the “yellow gravel” (re-exposed on
the north side for comparison). Seven stratigraphically consistent dates from these pits and the
original T1 exposure show that the “yellow gravel” accumulated continuously between about
~3500 and ~1000 BP. We infer that the major incision event that set the geomorphology of the ~130
m offset ended just before our oldest date in the “yellow gravel”, yielding a slip rate of just under
3.7 cm/yr. While it is possible that offset of the deep channel preceded onset of deposition of the
“yellow gravel”, we found no older deposits, and the switch from deep downcutting to
aggradation is the most likely event to set the geomorphology of the ~130 m offset.
To test the younger offset we progressively excavated between Trench 16 and the fault to trace the
channel to the fault. The thalweg and the western margin of the channel were traced to the Juniper
Hills bedrock directly above a bedrock fault, providing clear evidence that the channel was cut by
movement of the shutter ridge. Based on the outlet on the north (opposite) side of the fault, the
channel is offset between 9 and 20 m, with a preferred value (exits straight out the outlet) of 17-18
m. C-14 dates above and below the channel suggest an age of 1440 to 1660 AD, consistent with
offset caused by the last 3 earthquakes documented at Pallett Creek and a slip rate of ~3.6 cm/yr.
2-054
LONG-TERM SLIP RATES OF THE ELSINORE-LAGUNA SALADA FAULT,
SOUTHERN CALIFORNIA BY U-SERIES DATING OF PEDOGENIC CARBONATE IN
PROGRESSIVELY OFFSET ALLUVIAL FAN REMNANTS Fletcher KE, Sharp WD, and
Rockwell TK
2008 SCEC Annual Meeting | 167