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Perris2: .17, .16, .20, .23
Poster Abstracts
PRELIMINARY PALEOSEISMIC RESULTS FROM SOUTHERN CLARK FAULT, SAN JACINTO FAULT
ZONE, SOUTHERN CALIFORNIA; COMPARISON TO THE HOG LAKE PALEOSEISMIC RECORD (A-148)
M.T. Buga, T.K. Rockwell, and J.B. Salisbury
We present preliminary results from a new paleoseismic site on the Clark strand of the San Jacinto Fault Zone in Clark (Dry)
Lake, western Salton Trough, southern California. Based on morphological evidence, previous workers mapped multiple
strands of the fault through Clark Lake. We excavated trenches across two of the prominent lineaments and found only minor
cracking but no faults. In contrast, a trench across the most prominent lineament with a surface scarp exposed a major fault
with clear evidence of recurrent activity. Based on these observations, it appears that most late Holocene slip along the Clark
fault at Clark Lake is localized within a narrow zone. The main strand of the fault juxtaposes mid-Holocene lake deposits
against late Holocene, inter-bedded lake and alluvial deposits. We identified past surface ruptures by the presence of filled
fissures, upward fault terminations, angular unconformities, presence of growth strata, and abrupt vertical separations, from
which we identify evidence for six surface ruptures that have occurred in the past 1600 years, yielding an average recurrence
interval of 250-300 years. The two most recent events are likely the November 1800 and ca 1550 earthquakes, and correlate to
events 1 and 2 at Hog Lake, ~50 km to the NW, based on mapping of small geomorphic offsets along the Clark fault. The 3
earliest events are constrained to have ruptured between about 400 and 1250 AD and likely correlate to events 6, 7 and 8 at
Hog Lake. Thus, we recognize only 1 event at Clark Lake that corresponds to the cluster of 3 events at Hog Lake, suggesting
that some of the ruptures in the Hog Lake cluster correspond to rupture of the northern part of the zone, similar to the 1918
rupture.
SURFACE-EXPOSURE DATING OF SAN ANDREAS EARTHQUAKES (A-022)
W.B. Bull
Precise, accurate dating of prehistoric earthquakes provides verification of 40 years of stratigraphic dating at trenches
excavated across the southern San Andreas fault. Radiocarbon dating of trench organic material, created before or after times
of surface ruptures, is much improved – thanks to being able to date many small-size samples, and to Bayesian modeling.
Limits to what can be done were noted by Biasi and Weldon (2009, p. 496) who said “Dating precision alone is unlikely to ever
be adequate to demonstrate correlations, especially among events relatively close in time.” But surface-exposure dating gives
us a breakthrough to easily distinguish between two seismic shaking events in a decade, with a dating precision of ± 5 years.
We now can date "seismically induced landslides (that are) induced at extraordinary distances" (Keefer, 1984; Harp, et al, 1993).
Freshly exposed block and outcrop surfaces are colonized by lichens with known growth rates. I measure the largest lichen on
many rockfall blocks at sites susceptible to seismic shaking, but not snow avalanches and rainstorms. Rockfalls 400 km away
in Yosemite date to the times of southern San Andreas earthquakes of 1857 and 1812 AD. Previous Sierra Nevada work
defined growth rates for four lichen genera that were used to date rockfall events in diverse altitudes and climatic settings.
These four genera grow at the same rate in the San Jacinto, San Gabriel, and San Bernardino Mountains.
Icehouse Canyon (San Gabriel Mountains) lichen sizes cluster nicely in separate peaks that date back to ~815 AD. Most of
these seismic shaking events emanated from earthquakes that have been C 14 dated at eight trench sites between Carrizo and
Indio. Closely spaced, distinct lichen-size peaks date as ~1503 and ~1490 AD (Also C 14 dated at Thousand Palms as ~1503 and
at Wrightwood as ~1487 AD).
Future work will collect data from many sites. Seismic shaking, and rockfall abundance, decrease away from an earthquake
epicenter. Regional data modeling will map prehistoric seismic shaking of San Andreas events >Mw 7, and may assess the
relative importance of earthquakes generated by the San Jacinto and Transverse Range fault zones.
CONSEQUENCES OF THE ASSUMPTIONS REGARDING SEGMENTATION VERSUS NONSEGMENTATION
OF THE SHORELINE FAULT WHEN ESTIMATING EARTHQUAKE POTENTIAL AND SEISMIC LOAD FOR
DIABLO CANYON NUCLEAR POWER PLANT (DCNPP) (B-097)
A. Bykovtsev and M. Kasimov
The CALIFORNIA ENERGY COMMISSION held a day-long workshop “California Nuclear Power Plant Issues” in
Sacramento on July 26, 2011. One of the main questions for discussion on Section 1 “Seismic/Tsunami Scenarios and
Uncertainties for Diablo Canyon…” was “A recent USGS study in April 2011 concluded that, “There’s no objective evidence
for any discontinuities or segmentation of the Shoreline Fault,” in contrast to PG&E’s conclusion in January 2011 the Shoreline
2011 SCEC Annual Meeting | 149