Anisotropy in subduction zones: Insights from new source side S wave splitting measurements from India

This study presents 106 splitting and 40 null measurements of source-side anisotropy in subduction zones, utilizing direct S-waves registered at two stations sited on the Indian continent, which show null shear wave splitting measurements for SKS phases. Our results suggest that trench-parallel anisotropy is dominant beneath the Philippines, Mariana, Izu-Bonin and edge of the Java slab, while plate motion parallel anisotropy is observed beneath the Solomon, Aegean, Japan and Java slabs. Results from Kuril and Aleutian regions reveal trench-oblique anisotropy. We chose to interpret these observations primarily in terms of mantle flow beneath a subduction zone. While the two-dimensional (2D) slab entrained flow model, offers a simple explanation for trench-normal fast polarization azimuths (FPA), the trench-parallel FPA can be reconciled by extension due to slab rollback. The model that invokes age of the subducting lithosphere, can explain anisotropy in the sub-slab, derived from rays recorded at the updip stations. However, when downdip stations are used, contributions from the slab and supra-slab need to be considered. In Japan, anisotropy in the sub-slab mantle shallower than 300 km might be associated with trench parallel mantle flow resulting in the alignment of FPA in the same direction. Anisotropy in the deeper part, above the transition zone is probably associated with 2D flow resulting in trench-normal FPA. Anisotropy in the Mariana trench might be associated with trench parallel mantle flow in the supra-slab region, with similar deformation in the upper mantle and the transition zone.