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Poster Abstracts
2011 LONG BEACH/SIGNAL HILL, CA SEISMIC STUDY (B-076)
D.D. Hollis
Presented is an overview, maps and data of a recent active and passive seismic study recorded by a dense seismic network in
Long Beach, California. The network consists of approximately 5,000 short-period vertical sensors covering a 7 by 10 km area,
with an average station spacing of 100m. The network recorded continuously (24 hours/day) for a period of 6 months. The
primary goal of the survey was to collect 3D reflection seismic data over the Long Beach Oil Field. Another goal of the survey
was to collect passive seismic for basic earthquake and exploration seismology research.
NEAR REAL-TIME ESTIMATION OF VELOCITY GRADIENT TENSOR FIELDS FOR CONTINUOUS
MONITORING IN SOUTHERN CALIFORNIA (B-143)
W.E. Holt, G. Shcherbenko, and E. Caruso
We present test results of a geodetic network processing system designed to detect anomalous strain transients. This work is a
first step in the development of a continuous, near-real-time monitoring system for Southern California. The modeling
procedure consists of interpolating velocities within a region to estimate the velocity gradient tensor field. These velocities are
time-dependent estimates of velocity obtained from CGPS data using a simple moving average filter, while also removing
seasonal signal estimates. Time dependent velocities can be output at any regular time interval (e.g., daily, weekly) and the
associated velocity gradient tensor field for Southern California can then be estimated for each respective epoch.
Regularization of the solution for Southern California consists of obtaining the sharpest estimate of strain rates possible that
can be supported by the CGPS data. The smoothing of the strain rate solution is controlled through optimization of a
functional in a formal least-squares inversion. Both the transient detection exercise results, along with results from real CGPS
data, are presented. Results to date suggest that the method is well-suited for uncovering transients following past real
earthquakes, as well as transients within the blind test exercise that involve extended periods of slow slip over long-time
periods of several years. Presently further testing and refinement of the methodology is necessary to enable detection of
transients that occur over shorter time periods of 2 – 6 months.
NUMERICAL MODELING OF INTERSEISMIC EARTHQUAKE-INDUCED VERTICAL MOTION
ASSOCIATED WITH THE SAN ANDREAS FAULT SYSTEM (B-145)
B.P. Hooks, B.R. Smith-Konter, and G. Thornton
An accurate representation of vertical motion along active plate boundaries is a critical component of earthquake hazard
models, as the accumulation of strain and vertical displacement can greatly affect the strength of the crust, and therefore, the
associated seismic hazard. However, analysis of vertical deformation is rarely done due to the large uncertainty in the
geodetic datasets. Here we present 3D numerical models that reproduce the strain patterns associated with the San Andreas
and subsidiary faults. Our models utilize a commercial finite difference code (FLAC3D) that simulates the deformation of
Earth material using mechanical and thermal constitutive relationships. We use a temperature-dependent viscous model for
the lower crust (> 350 C) and plastic upper crust (Mohr-Coulomb with strain softening criteria). Model temperature controls
the thickness of the upper crust and the viscosity of the lower crust, and therefore the overall strength of the model.
Deformation, including the formation of large-scale fault systems, is driven by a basal shear consistent with the SCEC Crustal
Motion Model. The model reproduces first-order characteristic horizontal velocities, strain rates, and stress rates associated
with the San Andreas Fault System that compare favorably to reported measurements and previous modeling results.
Additionally, vertical displacement and velocity results reproduce first-order vertical offset marker and tide gauge datasets
and agree well with interseismic vertical deformation produced by a 3D semi-analytical model that applies geologic strike-slip
rates along active faults. Both models suggest three primary areas of the San Andreas Fault System where vertical motion is
significant: the Salton Trough, Death Valley, and the Big Bend. The Salton Trough and Death Valley are areas of subsidence
controlled by crustal transtension resulting in thinned and weakened crust with in a lower potential to accumulate large
stresses. The Big Bend region has uplifted due to a transpressional restraining bend. The substantial uplift and evolving
crustal stress conditions within this area would have thickened and strengthened the crust resulting in relatively larger stress
accumulation. The long-term effect of the non-linear feedbacks between vertical deformation, thermal advection, and crustal
rheology causes a dynamic link between geomorphic signal, strain accumulation and long-term seismicity.
178 | Southern California Earthquake Center