Anisotropy andVp/Vsin the uppermost mantle beneath the western United States from joint analysis ofPnandSnphases

Abstract

Pn and Sn phases are valuable for resolving velocity structure in the mantle lid, as they propagate horizontally right below the Moho. Relatively few Sn tomography attempts have been made compared to Pn, because Sn is often highly attenuated or buried in P wave coda. USArray has greatly increased data coverage for regional phases, and both Pn and Sn are routinely picked by network analysts. Here we jointly invert Pn and Sn arrival time residuals with a modified time-term analysis and a regularized tomography method and present new maps of crustal thickness, uppermost mantle P velocity perturbations, Vp/Vs ratios, and azimuthal anisotropy strength and orientation beneath the western United States. The results indicate partially molten mantle below the Snake River Plain and the Colorado Plateau. The seismic structure of the top ∼40 km of the mantle below the Colorado Plateau differs from that seen at greater depths in other studies, such as surface wave or teleseismic body wave tomography, whereas the Snake River Plain anomaly just below the Moho is comparable to structures seen at about ∼200 km depth. Pn fast axes provide complementary information to SKS shear wave splitting observations, and our analysis indicates that in several regions in the western United States the orientation of azimuthal anisotropy changes with depth in the upper mantle. However, we have so far been unable to resolve shear wave splitting directly in Sn waveforms, which seem to be dominated by Sn-SV energy.