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Poster Abstracts
propagation problems. It has been observed, however, that its accuracy deteriorates badly with increasing contrast between
the P- and S-wave velocities.
By using a dual-based formulation, we develop a method that is robust within the entire range of an elastic solid medium with
moderate to high contrast of the P- and S-wave velocities. By solving a constrained variational problem with the Lagrange-
Multiplier Method, we developed the dual-based (mixed) formulation with pressure and displacement as the variables.This is
similar to earlier mixed displacement-pressure formulations, except that we introduce the methodology as a global one
applicable to different domains rather than being limited to certain extreme cases. The variational problem makes a wider
range of interpolation functions permissible for pressure including discontinuous functions across element boundaries.
We illustrate the validity of the method by solving a range of idealized problems with moderate to high contrast of P- to S-
wave velocity, and compare the performance of different types of elements. We verify the method and illustrate the general
principles with an application in Southern California of the 1994 Northridge earthquake.
FAULT ARCHITECTURE OF THE SALTON SEA THROUGH MULTI-SCALE SEISMIC REFLECTION
SURVEYS (B-133)
A.M. Kell, N. Driscoll, G.M. Kent, A. Harding, and R. Baskin
Two sets of seismic reflection images collected in the Salton Sea, California in May 2010 and April 2011 highlight a
longstanding episode of extension-related deformation within the Salton Sea pull-apart system. These data are part of a
continued multi-scale network of seismic studies of the faults within the Salton Trough. In 2010, we collected ~350 line-km of
data using a 75-m-long, 24-channel streamer and a 1.6kJ “sparker” source fired at 1.2 sec intervals. These images document a
series of south-east dipping normal faults that are related to the current pull-apart geometry; this configuration appears to
persist for only the past 20-40 ka. Newly acquired low fold images (~150 line-km) collected using a 300-m-long, 48-channel
streamer and a Generator Injector (GI) airgun source firing at 1 min intervals show that the same structures seen in the higher
resolution (2010) data as well as high-resolution seismic CHIRP images collected in 2007 (Brothers et al., 2009, 2010) continue
to depths of >2.5 km. From this deeper imagery, we infer that the structures seen in the very shallow CHIRP data are throughgoing
to seismogenic depths and play a dominant role in strain partitioning from the Imperial Fault to the San Andreas Fault
through the Brawley Seismic Zone. The 2011 reflection and refraction data are part of a larger collaborative project involving
Cal Tech, Virginia Tech, the USGS, University of Nevada, Reno and Scripps Institution of Oceanography. Within this study we
seek to understand the mechanisms of how crustal thinning and rifting develops. The fault dip imaged at both scales is ~50-
60° and show vertical offsets (sub-meter to tens of meters) distinguishable to the limits of our imaging resolution. These multiscale
data offer a unique opportunity to calculate the timing and mode of motion in the most actively deforming portion of the
Salton Trough. The insights gained through these data allow a greater understanding of the tectonics and seismic hazards
inherent in Southern California.
NEW CHRONOLOGY AND ESTIMATED RATES OF VERTICAL UPLIFT FOR THE GAVIOTA COAST
RAISES ISSUES RELATED TO THE EARTHQUAKE HAZARD (A-127)
E.A. Keller, D.E. DeVecchio, L.D. Gurrola, and L.A. Owen
Marine terraces are useful for quantifying surface uplift and thus earthquake hazard. Surprisingly, numerical dates are few
along the Gaviota Coast, which extends westward for 50 km from Ellwood (west of Santa Barbara) to Point Conception.
Previously estimated rates of vertical uplift of marine terraces along the Gaviota Coast and elsewhere in Southern California
tend to be relatively low at 0.1 to 0.3 m/ka. An exception is within the Santa Barbara and Ventura fold belts, where uplift rates
are 1 to 2 m/ka and 8 m/ka, respectively. Our research in the Santa Barbara fold belt (SBFB) indicates the rate of uplift
increases toward the west from More Mesa toward Isla Vista and Ellwood, where the first emergent terrace, correlated to
Marine Isotope Stage 3a (MIS3a), is at an elevation of 10 m above mean sea level; for this marine terrace to be present requires
a rate of uplift exceeding 2 m/ka. Existing chronology in southern California consists largely of amino acid racemization
(AAR) ages from 3 decades ago. We have dated and estimated terrace age at More Mesa, Isla Vista, and Ellwood using
multiple techniques (OSL, U-series, 14C, and oxygen isotope ratios on small mollusks). The MIS3a terrace appears continuous
along much of the Gaviota coastline, where the first emergent terrace has been previously interpreted to be MIS5a. If our
chronology is correct, this calls in question the previous chronology and rates of uplift from Ellwood west across the south
branch of the Santa Ynez fault. Low marine terraces at UCSB Point (and other locations) are ~ 2+ m above mean sea level. Two
hypotheses have been presented to explain these low marine terraces. The terraces either represent a mid-Holocene maximum
sea level highstand or in an active tectonic setting like the SBFB the terrace may represent coseismic uplift, which we prefer.
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