Seismic evidence for thermally-controlled dehydration reaction in subducting oceanic crust

We perform travel-time tomography to estimate detailed seismic velocity structures in the crust of the Pacific slab from northeastern (NE) Japan to the Kanto district, Japan, and reveal that the depth extent of the low-velocity (hydrated) oceanic crust varies along the arc. The low-velocity oceanic crust is subducting to depths of 120-150 km beneath Kanto, which is 40-70 km deeper compared to NE Japan. Such deeper preservation of the low-velocity oceanic crust beneath Kanto can be explained by lower-temperature conditions in the Pacific slab as a result of the subduction of the Philippine Sea slab immediately above it. These observations suggest that dehydration reactions accompanied by large velocity changes are controlled principally by temperatures, not by pressures. We also find spatial correspondence between intensive seismicity in the oceanic crust and the disappearance depth of the low-velocity oceanic crust, suggesting that breakdown of hydrous minerals triggers earthquakes in the oceanic crust.