Depth dependent azimuthal anisotropy in the western US upper mantle

We present the results of a joint inversion of long period seismic waveforms and SKS splitting measurements for 3D lateral variations of anisotropy in the upper mantle beneath the western US, incorporating recent datasets generated by the USArray deployment as well as other permanent and temporary stations in the region. We find that shallow azimuthal anisotropy closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle (70-150. km depth), whereas at depths greater than 150. km, it is dominated by northward and upward flow associated with the extension of the East Pacific Rise under the continent, constrained to the east by the western edge of the North American craton, and to the north, by the presence of the East-West trending subduction zone. In particular, the depth-integrated effects of this anisotropy explain the apparent circular pattern of SKS splitting measurements observed in Nevada without the need to invoke any local anomalous structures.