High-resolution subducting-slab structure beneath northern Honshu, Japan, revealed by double-difference tomography

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.