Annual Meeting - SCEC.org

Group 1 – LAD | Poster Abstracts
predictions of an earlier model without LVVs by Becker & O'Connell (2001). Traction amplitude
variations can be induced by LVVs, and a major source of such variations underneath North
America is the presence of old continental lithosphere with underlying cratonic keels. Those keels
are inferred to be high viscosity and reach deep into the mantle, up to a depth of ~300 km. The
presence of the thick high viscosity keel, along with weak plate boundaries, likely creates large
LVVs, which will potentially have long-range effects on the deformation of the North American
plate. We quantify these effects by computing the basal tractions globally using a high resolution
finite element mantle convection code, CitcomS, that can take into account several orders of lateral
viscosity variations. The flow in the model is driven by density anomalies, as imaged by seismic
tomography. Since the relative viscosities of the stiff keel and the weak plate boundaries are poorly
constrained, a number of models with varying viscosities are tested. The stress fields associated
with these predicted tractions are then compared with stress observations such as the World Stress
Map and strain-rates from the Global Strain Rate Map. The ultimate goal is to construct a global,
spherical geodynamic model that is adapted to the North American plate. This background model
can provide information on how long-distance forcing affects regional tectonics, such as in
southern California. This will in turn provide a deeper understanding of how faults in this region
are loaded and how strain is localized on the plate boundary.
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CONSTRAINING THE EVOLVING ARCHITECTURE OF THE PLATE BOUNDARY
ZONE THROUGH 3D SEISMIC VELOCITY AND ANISOTROPY MAPPING Kosarian M,
Davis PM, Clayton RW, and Tanimoto T
The two main data sources for seismic anisotropy in Southern California, SKS splitting data and
surface wave data, show inconsistent patterns. The primary goal of this project is to understand the
source of this discrepancy and to obtain a seismic structure that satisfies both sets of data. The key
must be in the depth variations in anisotropy, as the two types of data have different depth
sensitivities. We formulated a scheme to invert surface waves and obtained S-wave velocity
anisotropy maps. We present results of crust and mantle anisotropy derived from measurements of
core–refracted phases (SKS and SKKS) recorded at stations in Southern California. We calculated
splitting parameters using a single layer anisotropy model. We have made new shear wave
splitting measurements for 126 seismic stations with best data (50 earthquakes out of 190
earthquakes) for the period of 1990-2008. On average, fast directions are east-west with about 1
second delay. For the surface wave anisotropy model we computed predicted SKS splitting times
from the mantle-lithosphere. We find that predicted splitting times are much less than SKS splitting
times. The surface wave fast axes directions are also different in that our results are mostly parallel
to the relative plate motion direction. Larger variations closer to the major faults also seem to be a
new observation. Anisotropic structure derived from surface waves clearly cannot explain SKS
splitting data. We suggest that the SKS waves are sensitive to the deeper parts of the upper mantle.
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ANELASTIC EARTH STRUCTURE FROM THE AMBIENT SEISMIC FIELD Prieto GA,
Beroza GC, and Lawrence JF
Cross correlation of the ambient seismic field is now routinely used to measure seismic wave travel
times; however, relatively little attention has been paid to other information that could be extracted
from these signals. In this paper we demonstrate the relationship between the spatial coherency of
the ambient field and the elastodynamic Green's function in both time and frequency domains.
Through measurement of the frequency domain coherency as a function of distance, we
sequentially recover phase velocities and attenuation coefficients. From these measurements we
generate 1D shear wave velocity and attenuation models for southern California. The ambient field
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