Recent seismic structural studies in trench‐outer rise regions have shown that V_p within the incoming oceanic plate systematically decreases toward the trench, probably owing to bending and fracturing of the plate. To understand the mechanisms acting to reduce V_p, V_s is critical because the V_p/V_s ratio is a sensitive indicator of lithology, porosity, and the presence of fluid. In the outer rise region of the Kuril trench, we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in V_p, V_s, and V_p/V_s. Our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending‐related fractures at the top of the oceanic crust, consistent with the seawater percolation. Our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones.