A detailed investigation of the hypocenter distribution beneath Kanto, Japan, reveals a NW-SE-trending linear alignment of seismicity within the subducted Pacific slab. We estimate the 3D seismic velocity structure in the Pacific slab to understand the factors controlling the genesis of such intraslab earthquakes. A narrow low-velocity zone is imaged within the subducted slab over a length of ~150 km, which partly penetrates into the mantle portion of the slab. The low-velocity zone correlates in space with the NW-SE-trending earthquake cluster. A reactivation of hydrated fracture zone formed prior to subduction is probably related to the low-velocity anomaly. Dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might lower the seismic velocity along the fossil fracture zone, accompanied by intraslab earthquakes. These observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes.