Annual Meeting - SCEC.org

Poster Abstracts
THE MECHANICS OF EARTHQUAKES AND FAULTING IN THE SOUTHERN GULF OF CALIFORNIA (B-
032)
D.F. Sumy, J.B. Gaherty, W.-Y. Kim, and T. Diehl
Accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted
continental margins. The Pacific-North America plate boundary within the Gulf of California (GoC) provides an excellent
opportunity to explore such rifting, as continental extension in the north transitions to seafloor spreading in the south. From
October 2005 to October 2006, an array of fourteen four-component ocean-bottom seismographs were deployed in the GoC to
record earthquakes and other natural seismic signals as part of the Sea of Cortez Ocean-Bottom Array (SCOOBA) experiment.
By combining the data from this deployment with that from the on-shore NARS-Baja array, we detect and locate ~700
earthquakes (Mw 2.5-6.6) mainly located on the NW-SE striking transform faults that delineate the plate boundary. The
earthquakes occur mainly during short-term swarm events that cluster predominantly around the inside corner of ridgetransform
intersections, with events occurring on both the strike-slip and normal faults within the system. We use the
waveforms of 31 events with high signal-to-noise long-period Rayleigh waves (Mw 3.45-4.92) to calculate regional deviatoric
moment tensors and determine the focal mechanisms of these earthquakes. Most of these events exhibit right-lateral strike-slip
focal mechanisms along the transform faults; however, several oblique and rotated mechanisms reveal a more complex
structure within the gulf. In addition, we capture a swarm of events on Baja California along an apparent right-lateral NW-SE
striking fault, which may be evidence of an extension of the Carmen fracture zone across the Baja California peninsula. A
calculation of seismic coupling using data from global earthquake catalogs combined with our dataset illuminates regions of
active deformation along broad regions of the gulf and individual transform segments, respectively. The combination of highresolution
earthquake locations, with the broad distribution of event focal mechanisms and an estimate of seismic coupling
improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern
GoC.
USING STRUCTURAL CONSTRAINTS ON THE LITTLE SAN BERNARDINO MOUNTAINS TO
PETROLOGICALLY AND GEOCHEMICAL ANALYZE THE CRUSTAL VELOCITY MODEL OF SOUTHERN
CALIFORNIA. (B-130)
C. Symcox and A. Barth
Recent years have seen great advances in subsurface velocity model data stemming from an increased density and
sophistication of global seismic networks, resulting in improved models of crustal structure. Crustal velocity models are
critical for accurately calculating earthquake hypocenters and understanding how crustal architecture influences the
development of faults. Using a tilted crustal cross section in the Little San Bernardino Mountains, we aim to compare what we
see petrologically and geochemically in the region to the CVMH, a crustal velocity model created by the Southern California
Earthquake Center (SCEC) and Harvard. Our focus is predominantly two-fold: to finding the “average” rock that best
describes the crust in the region by comparing the data provided in the CVMH to what we see on the ground knowing the
convenient structural architecture of the region, and to search out inconsistencies between what is seen in the velocity model
and what geologists see in outcrop. Other focuses in the coming weeks may include analyses of crystal structure and
composition, anisotropy, and radiogenic heat production to compare what we see geologically in the rock to what the CVMH
tells us what we should see.
REAL TIME GEODETIC EARLY WARNING SYSTEM FOR EARTHQUAKES (A-037)
H. Tahtinen, Y. Bock, B.W. Crowell, D. Melgar, and M.B. Squibb
Every second, new positions are calculated for all GPS stations within the California Real Time Geodetic Network
(http://sopac.ucsd.edu/projects/realtime/). When a medium or large earthquake does occur in the region, the displacements will be
quite visible in the recorded data in all three directions (north, east, up or x, y, z). We report on a redesign of our processing
methodology by using Delaunay Triangulation to break up the large network of GPS sites (~100) can be broken into subnetworks
each of which contain one reference station and two moving stations; in conjunction with the network redesign. The
principal components of 2-D strain rate are computed for each triangle. If strain exceeds a pre-set threshold, a network
adjustment is performed to reconstitute the entire network; this process detects and extracts outliers and references the
positions of all sites with respect to the coordinates of a single CRTN site. The study area contains two networks, one in
northern California including BARD (http://seismo.berkeley.edu/bard/) and PBO (http://pboweb.unavco.org/?pageid=107) sites, while
the other is in southern California. The reason to have a network that covers such a large spatial extent is because when an
earthquake initiates, the rupture will be able to be observed from its start to end along many possible faults, in particular the
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