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

[1] 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 similar to 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.