Abstract

Upper mantle anisotropy has been mapped beneath continents at high spatial resolution. Beneath the oceans, however, shear wave splitting constraints on upper mantle anisotropy are sparse, due to the paucity of seismic receivers. The technique that does not require the availability of seismic stations close to the region under study is differential PS-SKS splitting. Here, we use global wavefield simulations to investigate circumstances under which PS-SKS splitting can be applied, and then use this technique to measure upper mantle anisotropy beneath the Pacific Ocean basin. Our results demonstrate that upper mantle anisotropy in our study region mostly reflects shearing due to the Pacific plate. North of Fiji, we observe a rotation of fast polarization directions, away from the direction of absolute plate motion of the Pacific plate. We attribute this to far-field mantle flow effects associated with the subduction of the Australian plate beneath the Pacific.