Measurement of anisotropy advances our understanding of mantle dynamics
by linking remote seismic observations to local deformation state
through constraints from mineral physics. The Pacific Northwest records
the largest depth-integrated anisotropic signals across the western
United States but the depths contributing to the total signal are
unclear. We used the amplitudes of orthogonally polarized P-to-S
converted phases from the mantle transition zone boundaries to identify
anisotropy within the ~400-700 km deep layer.
Significant anisotropy is found near slab gaps. Focusing of mantle flow
through slab gaps may lead to locally elevated stress that enhances
lattice preferred orientation of anisotropic minerals within the
transition zone, such as wadsleyite.