Uppermost mantle seismic velocity structure beneath USArray

We apply Pn tomography beneath the entire USArray footprint to image uppermost mantle velocity structure and anisotropy, as well as crustal thickness constraints, beneath the United States. The sparse source distribution in the eastern United States and the resulting longer raypaths provide new challenges and justify the inclusion of additional parameters that account for the velocity gradient in the mantle lid. At large scale, Pn velocities are higher in the eastern United States compared to the west, but we observe patches of lower velocities around the New Madrid seismic zone and below the eastern Appalachians. For much of the mantle lid below the central and eastern United States we find a moderate positive velocity gradient. In the western United States, we observe a moderate gradient in the region of the Juan de Fuca subduction zone, but no significant gradient to the south and east of this region. In terms of anisotropy, we find that the Pn fast axes generally do not agree with SKS splitting orientations, suggesting significant vertical changes in anisotropy in the upper mantle. In particular the circular pattern of the fast polarization direction of SKS in the western United States is much less pronounced in the Pn results, and in the eastern US the dominant Pn fast direction is approximately north-south, whereas the SKS fast polarizations are oriented roughly parallel to the absolute plate motion direction.