We employ seismic tomography to estimate detailed 3D seismic velocity structures in the focal area of an intraslab earthquake (M7.1), which occurred on April 7, 1 month after the 2011 Tohoku earthquake (M9.0) near its source area. The results show a low-velocity zone around the focal area of the M7.1 event, and that the aftershock activity is limited to the upper 15 km of the oceanic mantle. The lateral extent of the low-velocity zone is comparable to the distribution of aftershocks, suggesting a concentration of fluids in the aftershock area. The angle between the aftershock alignment and the dip of the slab surface is estimated to be ∼60°, which is consistent with the dip of an oceanward-dipping normal fault observed at the outer-trench slope. These observations suggest that the M7.1 intraslab event occurred as a result of reactivation of a buried hydrated fault that formed prior to subduction. The upper ∼15 km of the oceanic mantle may be locally hydrated by bending-related tensional faulting at the outer-trench slope.