Anisotropy in the Great Basin from Rayleigh Wave Phase Velocity Maps

Azimuthally anisotropic phase velocity maps at 16 s and 38 s period. The background colors represent the isotropic phase velocity anomalies. The black lines show the fast direction of propagation for Rayleigh waves (2Ψ anisotropy). The reference phase velocity, calculated using the reference mTNA model defined by Beghein et al. (2010), and the average measurement uncertainty are given on top of each map.
The Great Basin region, Nevada, is characterized by a semi-circular shear-wave splitting pattern around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached.
</p><p>In order to obtain better constraints on the origin of this intriguing anisotropy pattern, we performed fundamental mode Rayleigh-wave dispersion measurements using data from the USArray Transportable Array. We then inverted these measurements to obtain azimuthally anisotropic phase velocity maps at periods between 16 s and 102 s.
</p><p>At periods of 16 s and 18 s, which mostly sample the crust, we found a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Combined with recent crustal receiver function results, these short period phase velocity maps are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. We calculated that our short period phase velocity anisotropy can only explain ~30% of the SKS splitting times, which implies that the origin of the regional shear-wave splitting signal is complex and must have both lithospheric and sublithospheric origins.
</p><p>At periods between 28 and 102 s, which sample the uppermost mantle, we detected a significant reduction in isotropic phase velocities across the study region, and found more uniform fast directions with a E-W fast axis. We attributed the transition in phase velocities and anisotropy to the lithosphere-asthenosphere boundary at depths of ~60 km. We interpreted the longer periods fast seismic directions in terms of present-day asthenospheric deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere.
</p><p>Beghein, C., Snoke, J.A., and Fouch, M.J., Depth Constraints on Azimuthal Anisotropy in the Great Basin from Rayleigh Wave Phase Velocity Maps, Earth Planet. Sci. Lett., 289, 467-478, 2010.
</p><p>Acknowledgements: This research was partially funded by NSF grants EAR-0548288 (MJF EarthScope CAREER grant).</p>


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