We applied three-dimensional (3-D) seismic tomography to both active-source data and local earthquake data recorded at onshore and offshore seismic stations in the Miyagi Prefecture forearc region of central northeastern Japan to obtain a detailed seismic velocity model of the mantle wedge. The velocity model showed a clear boundary that divides the mantle wedge into an oceanward high-velocity toe and a landward low-velocity region that extends trenchward from beneath currently active arc volcanoes. The top of the seaward dipping boundary is about 20-30 km landward from the coastline, and its bottom is approximately beneath the coastline. We consider that this velocity boundary represents the trenchward limit of corner flow and the high-velocity toe represents a stagnant mantle wedge. The high spatial resolution of the tomographic image we produced allowed us to identify a spatial correlation between velocity variations in the mantle wedge immediately above the plate boundary and the spatial extent of the rupture areas of the large interplate earthquakes that occur repeatedly in the region. Our results suggest that variations of the volume fraction of serpentinization in the mantle wedge cause the irregularity of rupture patterns of megathrust earthquake sequences. We also identified a swarm of active microseismicity in the mantle wedge, well above the seismogenic plate boundary. This activity may be related to dehydration of a detached seamount.
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