Seismic Anisotropy in the Mantle Wedge Revealed by Tectonic Tremors Beneath Bungo Channel, Southwest Japan

Abstract Tectonic tremors, which are dominated by shear‐wave ( S ‐wave) energy, provide a valuable tool for measuring seismic anisotropy. We performed continuous S ‐wave splitting analysis using seismic data from stations above the tremor belt in the Nankai Trough subduction zone. The derived splitting parameters (lag times of ∼0.3 s and WNW–ESE fast polarization directions) closely match those from the direct S phases of slightly deeper intra‐slab earthquakes. The lag times are an order of magnitude greater than those associated with the S phases of crustal earthquakes. Furthermore, the fast polarization direction is subparallel to the subduction direction of the Philippine Sea Plate, suggesting a likely serpentinized mantle wedge origin, influenced by slab‐derived fluids. This finding offers new insights into fluid‐related processes in subduction zones. The results demonstrate that tremor‐based S ‐wave splitting analysis is a robust tool for measuring seismic anisotropy in the mantle wedge of the Nankai Trough subduction zone.