HR: 1340h
AN: S53A-0171    [Abstracts]
TI: The Seismicity and Structure of Izu-Bonin Arc Mantle Wedge at $31\deg$N Revealed by Ocean Bottom Seismographic Observation
AU: * Sato, T
EM: satot@earth.s.chiba-u.ac.jp
AF: Department of Earth Sciences, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AU: Ishimura, C
AF: Department of Earth Sciences, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AU: Kasahara, J
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Maegawa, K
AF: Japan Metrological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo, 100-0004 Japan
AU: Tatetsu, H
AF: Japan Metrological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo, 100-0004 Japan
AU: Tanaka, M
AF: Japan Metrological Agency, 1-3-4 Otemachi, Chiyoda-ku, Tokyo, 100-0004 Japan
AB: 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 ($>$400km). Other unique characteristics of this subduction zone include the existence of serpentine seamounts exposed along the forearc slope and the presence of low-velocity ($<$7.3km/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 earthquakes 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. (3) The low mantle velocity area in the mantle wedge terminates about 140km west of the trench axis. (4) The subducting slab has a dip of $55\deg$ 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 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.
DE: 7230 Seismicity and seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting