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.

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