Journal Article

Upper Mantle Anisotropy and Flow Beneath the Pacific Ocean Revealed by Differential PS‐SKS Splitting

Wolf, Jonathan, Long, Maureen D.

2023

Overview

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. A 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. This may reflect far‐field mantle flow effects associated with the subduction of the Australian plate beneath the Pacific. Plain Language Summary Earthquakes cause seismic waves whose speeds sometimes depend on their polarization and propagation direction. This material property, called seismic anisotropy, can be used to infer the direction of flow in Earth's upper mantle. Seismic anisotropy is straightforward to measure directly beneath a seismic station, but harder to study if station coverage is sparse. We use a technique that allows us to infer upper mantle seismic anisotropy beneath the Pacific Ocean in places without nearby seismic stations. Our measurements show that while seismic anisotropy varies laterally beneath the Pacific Ocean, in most cases it can be explained by the movement of the Pacific tectonic plate, leading to horizontal shearing of the underlying mantle. North of Fiji, we can observe the effects that the subduction of the Australian beneath the Pacific tectonic plate has on upper mantle flow. Key Points We test the robustness of differential PS‐SKS shear‐wave splitting measurements to characterize anisotropy near the PS bounce point We use this technique infer seismic anisotropy beneath the Pacific Ocean A majority of our measurements can be explained by plate motion induced shearing beneath the Pacific plate

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Anisotropy / Direction / Earth mantle / Earthquakes / mantle flow / Material properties / Ocean basins / Oceans / P-waves / Pacific Ocean / Plate motion / Plate tectonics / Plates / Plates (tectonics) / Polarization / S waves / Seismic activity / seismic anisotropy / Seismic waves / Shearing / shear‐wave splitting / Spatial discrimination / Spatial resolution / Splitting / Subduction / Subduction (geology) / subduction zones / Upper mantle