Complex and variable crustal and uppermost mantle seismic anisotropy in the western United States

The orientation and depth of deformation in the Earth is characterized by seismic anisotropy(1)-variations in the speed of passing waves caused by the alignment of minerals under strain into a preferred orientation. Seismic anisotropy in the western US has been well studied(2-11) and anisotropy in the asthenosphere is thought to be controlled by plate motions and subduction(6-9). However, anisotropy within the crust and upper mantle and the variation of anisotropy with depth are poorly constrained. Here, we present a three-dimensional model of crustal and upper mantle anisotropy based on new observations of ambient noise(12) and earthquake(13) data that reconciles surface wave and body wave(9) data sets. We confirm that anisotropy in the asthenosphere reflects a mantle flow field controlled by a combination of North American plate motion and the subduction of the Juan de Fuca and Farallon slab systems(6-9). We also find that seismic anisotropy in the upper mantle and crust are largely uncorrelated: patterns of anisotropy in the crust correlate with geological provinces, whereas anisotropy in the upper mantle is controlled by temperature variations. We conclude that any coupling between anisotropy in the crust and mantle must be extremely complex and variable.