Seismic Evidence for Water Transportation in the Forearc off Northern Japan

The water cycle plays an essential role in arc volcanism, earthquake generation, mantle rheology, and thermal structure of subduction zones. Previous seismic studies have revealed strong structural heterogeneities in the megathrust zone in Northeast Japan and Hokkaido. However, water transportation in the forearc region remains poorly understood due to the lack of long-term seismic observatories at the seafloor. Using high-quality data recently recorded by the permanent ocean-bottom-seismometer network (S-net) in the Pacific Ocean off Northern Japan, we study the fine three-dimensional P wave velocity (V_p) structure of the crust and upper mantle beneath the Kuril and Tohoku forearc region. Our results reveal high velocities in the mantle-wedge corner, implying a low degree of serpentinization there. We suggest that the forearc mantle in the study region is cold and anhydrous. The slab interface under the forearc area may have a low permeability, which controls the fluid flux to the mantle wedge and the overriding plate. A low-V_p layer atop the subducting Pacific plate is interpreted as the subducted oceanic crust. Dehydration of the subducted oceanic crust occurs at depths of 80–120 km, providing a large volume of water to the overriding mantle wedge to produce arc volcanoes, and part of the water may migrate upward to the shallow area. Large megathrust earthquakes (M_w ≥ 6.0) mainly occurred around low-V_p patches in the megathrust zone. Destruction of the low-permeability slab interface would result in fluid flow upward, which may trigger large megathrust earthquakes and seismicity in the mantle wedge under the forearc.