We determined a fine 3-D P-wave velocity structure of the subducting Pacific slab by inverting a large number of high-quality P-arrival times to better understand the genesis of the 26 May 2003 Miyagi-oki earthquake (Mw 7.0) and its aftershock sequence, which occurred within the subducting Pacific slab beneath northeast (NE) Japan. Lateral and depth-ward heterogeneities are imaged up to several tens of kilometers beneath the upper boundary of the subducting slab. The main shock and its aftershocks occurred in a distinct zone characterized by a lower velocity anomaly than its surroundings within the slab. The reduced velocity anomaly within the slab is attributed to the process of dehydration embrittlement resulting from the dehydration of hydrous minerals in the subducting Pacific slab, which may have induced the main shock and its aftershock sequence by enhancing pore pressures along the pre-existing faults/fractures in the subducting slab beneath the NE Japan forearc region.