HR: 14:55h
AN: S43D-05    [Abstracts]
TI: Inferences on Upper Mantle Seismic Velocity and Anisotropy in Western North America From Surface Wave Analyses
AU: * Beghein, C
EM: cbeghein@asu.edu
AF: Arizona State University, School of Earth and Space Exloration Box 871404, Tempe, AZ 85287, United States
AU: Snoke, J A
EM: snoke@vt.edu
AF: Virginia Polytechnic Institute and State University, Department of Geosciences 4044 Derring Hall (0420), Blacksburg, VA 24061, United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Arizona State University, School of Earth and Space Exloration Box 871404, Tempe, AZ 85287, United States
AB: In this study, we examine surface wave dispersion to determine seismic velocity variations in the crust and upper mantle beneath the Great Basin. This will improve our understanding of the relationship between lithospheric and asthenospheric processes in the western United States, and what caused the relatively recent extension episode in the Basin and Range. We employ a two-station method using the excellent lateral and good azimuthal coverage enabled by USArray Transportable Array (TA) broadband seismic stations. We use data from TA stations to generate Rayleigh wave dispersion curves between periods of 16s and 170s, which enable modeling of seismic shear wave velocities to ~300km depth. In order to reduce trade-offs between structure in the crust and upper mantle, we supplement our measurements with published results of surface wave phase velocity data between 8s and 40s determined from ambient seismic noise analysis. Constraints on Moho depth are incorporated using the routinely determined receiver function results (http://www.seis.sc.edu/EARS). To model upper mantle shear wave velocities from the measured dispersion curves, we apply a forward modeling technique which allows us to determine quantitative model uncertainties and parameter trade-offs. As of summer 2007, we have found 22 events with high quality Rayleigh wave dispersion curves for ~900 station pairs with inter-station distances between ~70 and 400km. We combine phase velocity dispersion measurements for several sub-regions within the Great Basin and calculate an average and standard deviation for each group. We find a clear increase per group in phase velocities from north to south for periods above ~35s. In the northern part of the Great Basin (N41°- N42.5° latitude), dispersion curves for measurements along E-W paths show a well-defined reduction in phase velocity with respect to a slightly modified Tectonic North America (mTNA) shear wave velocity model between periods of ~35s and 100s. Dispersion curves for southern regions (between N39° and N41° latitude) in the E-W direction do not differ from the predictions of mTNA, and phase velocities for periods longer than 35s are generally larger than those for the northern paths. This increase in phase velocity from N to S is much less clearly visible for paths that are not in the E-W direction, most of which exhibit phase velocities lower than predictions from model mTNA around 35s-100s. Given that the sensitivity to shear wave velocities of 50s Rayleigh waves peaks at about 70km depth, these results suggest a reduction in shear wave velocities compared to mTNA, located below a thin (<100 km) lithospheric lid across the region. Our finding of a likely isotropic phase velocity structure in the northern Great Basin is similar to what is observed in most of the High Lava Plains to the north. Our results also suggest azimuthal anisotropy in the southern Great Basin with a fast direction aligned approximately E-W, which is generally consistent with regional shear-wave splitting results, though new splitting results within the Great Basin show shear wave splitting complexity that is not clearly manifested in the surface wave data. These variations are likely the result of extension across the Great Basin combined with (or the result of) well-organized asthenospheric flow across the region.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: 2007 Fall Meeting