The seismicity and structure of Izu-Bonin arc mantle wedge at 31°N revealed by ocean bottom seismographic observation

The Izu-Bonin arc system is the subduction zone forming the plate boundary between the downgoing Pacific plate and the overriding Philippine Sea plate. Seismicity along the Izu-Bonin subduction zone is very different in character from other western Pacific subduction zones. Few large earthquakes have occurred at shallow depths (0–100 km), but many large earthquakes have occurred at greater depths (>400 km). Other unique characteristics of this subduction zone include the existence of serpentine seamounts exposed along the forearc slope and the presence of low-velocity (<7.3 km/s) material between the two plates in a zone extending from the forearc seamounts to the mantle wedge. To investigate these unique characteristics, we carried out an ocean-bottom seismic experiment in 1999 to estimate the hypocenter distribution and the structure of the mantle wedge simultaneously by performing 3D event locations employed within different velocity models. We obtained the following results: (1) No earthquake occurred on the upper surface of the subducting plate and some were located as far as 20 km away from the upper surface. Most events occurred within the mantle of the subducting slab. (2) There were no earthquakes in the mantle wedge above the subducting slab. (3) The low mantle velocity area in the mantle wedge terminates ∼140 km west of the trench axis. (4) The subducting slab has a dip of 55° to the west. From these results we suggest that the low-velocity material between the plates is chrysotile, a low-temperature, low-strength, low friction phase of serpentine, which may act as a lubricant on the plate boundary. The western boundary of the low mantle velocity region in the mantle wedge, which is about 140 km west of the trench axis and at a depth of about 20 km, coincides with the temperature-controlled transition from chrysotile to antigorite (the high-temperature phase of serpentine) along the plate boundary. Our results suggest that chrysotile may migrate upward and eastward along the plate boundary, while antigorite may move downward with the subducting slab.