Abstract
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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