The depth distribution of mantle anisotropy beneath the Tonga subduction zone

Abstract

Shear-wave splitting recorded by a temporary deployment of broadband seismometers located above the Tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the Tonga back-arc region. Splitting parameters obtained for local S and teleseismic SKS phases were modeled using a ray-based method that predicts shear-wave splitting on individual phase paths and determines the best-fitting values of anisotropic strength, orientation and depth extent. Splitting in teleseismic SKS phases is identical to that in S phases from local earthquakes that occur at the base of the transition zone, demonstrating that there is no significant splitting due to anisotropy in the lower mantle. Splitting times for local S phases do not vary significantly with depth, and for models in which the strength of anisotropy is laterally uniform, they rule out anisotropy in the transition zone. Finally, the observed splitting indicates that anisotropy of more than 0.8% exists in the upper mantle, with a fast symmetry axis at 60°W, roughly parallel to the absolute plate motion vector of the subducting Pacific lithosphere.