Anomalous deepening of a belt of intraslab earthquakes in the Pacific slab crust under Kanto, central Japan: Possible anomalous thermal shielding, dehydration reactions, and seismicity caused by shallower cold slab material

A belt of intraslab seismicity in the Pacific slab crust parallel to iso-depth contours of the plate interface has been found beneath Hokkaido and Tohoku. Hypocenter relocations have shown that this seismic belt does not run parallel to but obliquely to the iso-depth contours beneath Kanto, deepening toward the north from similar to 100 km to similar to 140 km depth. The depth limit of the contact zone with the overlying Philippine Sea slab is located close to and parallel to this obliquely oriented seismic belt, suggesting that the deepening of the seismic belt there is caused by the contact with the overlying slab. The contact with this cold slab hinders the heating of the Pacific slab crust by hot mantle wedge, which would cause delay of eclogite-forming phase transformations and hence deepening of the seismic belt there. The depth limit of the subducting low-velocity crust also deepens toward the north, supporting this idea.