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.