Splitting constraints in regions with sparse coverage: uniformly processed PS measurements from a massive global data set

SUMMARY Shear wave splitting, caused by seismic anisotropy, provides insights into convective processes in the mantle. While upper mantle anisotropy beneath the continents is well-resolved with high lateral resolution, its characterization beneath the oceans remains limited due to the paucity of seismic stations. In this study, we leverage the sensitivity of P waves that convert to S upon reflection at the surface (PS) to infer seismic anisotropy near their bounce point. We measure PS splitting automatically from a global data set that includes all earthquakes with magnitudes $\ge 5.9$ from January 1995 to present, collected from 25 datacenters, totaling approximately 5800 events and 18 million three-component seismograms. After careful quality control, we obtain 889 PS splitting measurements from this data set, mostly for regions without other splitting constraints. These regions include the South China Sea, the Philippine Sea, the western Pacific Ocean, the western Atlantic Ocean near the Caribbean and the South Atlantic Ocean. Where independent SKS constraints are available, these are in general agreement with our PS results. Comparisons of PS fast directions to azimuthal seismic anisotropy derived from surface waves and the direction of absolute plate motion show a dominant contribution of asthenospheric deformation to the measurements. This agreement is particularly pronounced beneath the oceans, away from subduction zones, for instance, across much of the Pacific basin. In contrast, splitting patterns near subduction zones are generally more complex: fast directions are most often trench-parallel or oblique, and only rarely trench-perpendicular, suggesting that a trench-parallel component of shear deformation is commonly generated in subduction settings, as commonly inferred from SKS splitting. We make all measurements publicly available as a data product, along with detailed metadata to enable future work.