Tomographic evidence for hydrated oceanic crust of the Pacific slab beneath northeastern Japan: Implications for water transportation in subduction zones

We estimate detailed seismic-velocity structure around the Pacific slab beneath northeastern Japan by double-difference tomography. A remarkable low-velocity zone with a thickness of ∼10 km, which corresponds to much hydrated oceanic crust, is imaged coherently along the arc at the uppermost part of the slab. The zone gradually disappears at depths of 70-90 km, suggesting the occurrence of intensive dehydration reactions there. The concentration of intraslab earthquakes at these depths supports dehydration-embrittlement hypothesis as a mechanism for generating intraslab earthquakes. A low-velocity zone imaged immediately above the slab at depths >70 km probably reflects a hydrous layer that absorbs water expelled from the slab and carries it to deeper depths along the slab. Our observations suggest that an along-arc variation in arc volcanism might be related to that in the development of the hydrous layer above the slab.