The Izu-Bonin Trench (IBT) has characteristics different from other trenches in many aspects. A chain of serpentine seamounts is exposed at the forearc slope 40-50 km west of the Izu-Bonin Trench axis. In this area, few large earthquakes (>M7.0) have occurred at shallow depths (0-100 km) and many large earthquakes have occurred at greater depths (>400 km). To reveal the mechanism of serpentine diapiring and the relation between heterogeneity in earthquake occurrence and plate subduction, seismic refraction/reflection studies at the forearc slope, with two lines perpendicular and parallel to the Izu-Bonin Trench axis, were carried out in 1998. Detailed seismic velocities were obtained for both lines by seismic tomography using refracted first arrivals and reflected arrivals. On the E-W line, the subducting plate interface and the Moho of the subducting oceanic plate were recognized. The velocity of the mantle wedge is lower than that of the average velocity of oceanic mantle. Along the subducting plate, a layer between the two plates (Plate Boundary Layer (PBL)) with a low-velocity appears just beneath the serpentine diapir and forms a sill shape, which becomes thicker with increasing velocity from the diapir toward the west, and connects to the mantle wedge. The low-velocity materials in the PBL are interpreted as chrysotile, which is a low-temperature phase of serpentine produced by the hydration of peridotite by water supplied by the subducting slab. The chrysotile in the PBL might act as a lubricant and decrease seismic activity along the subduction zone, and this can explain the characteristics of seismicity in the Izu-Bonin subduction zone (IBSZ). In contrast to the E-W line, the crustal structure along the N-S line is rather homogeneous.
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