Complex upper mantle anisotropy in the Pacific Northwest: Evidence from SKS splitting

•Mantle anisotropy varies laterally and vertically.•Anisotropy near slab is perpendicular to slab.•Anisotropy east of slab parallels mantle seismic structure.•Finite frequency effects influence SKS splitting in northern Oregon. We use a dense network of observations and an automated method of analysis to investigate complex patterns of seismic anisotropy in the Pacific Northwest. We present SKS splitting results for approximately 220 broadband seismic stations, including 33 stations from the new Wallowa2 array deployed between 2016 and 2018 in northeast Oregon. Our data set contains approximately 3300 splitting measurements. Our anisotropy measurements indicate that anisotropy in the Pacific Northwest can be divided into two domains: near the subduction zone, anisotropy is perpendicular to the slab; and inboard of the subduction zone anisotropy tends to parallel the velocity structure and is roughly oriented with “absolute” plate motion. However, splitting analysis performs poorly in northeast Oregon, and results are not consistent with uniform or layered anisotropy. We argue that the complex splitting behavior in northern Oregon is a result of spatially variable upper mantle anisotropy, related to the complex upper mantle seismic structure. Interpreting the mantle anisotropy in this region requires methods not limited to standard ray theoretical shear wave splitting analysis.