The high-resolution seismic velocity structure of the subducting slab beneath northern Honshu, Japan, has been obtained by double-difference tomography, capitalizing on the existence of two planes of seismicity. The upper plane lies in the region with average to high Vp/Vs ratios (1.72–1.85), which may correspond to the transformations of metabasalt and metagabbro to blueschist. The lower plane is associated with very low Vp/Vs ratios (1.6–1.7), in sharp contrast with high Vp/Vs ratios (∼1.8–1.85) in the region between the two planes. These features may be explained by forsterite-enstatite-H2O formation from serpentine dehydration in the lower plane and partial hydration of the region between the two planes. Our results provide direct constraints for petrologic modeling of the subducting slab.