Seismic Velocity structures in Northern Izu-Bonin arc derived from passive OBS observations

HR: 0800h
AN: T41C-0701 [Abstracts]
TI: Seismic Velocity structures in Northern Izu-Bonin arc derived from passive OBS observations
AU: * Obana, K
EM: obanak@jamstec.go.jp
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Kamiya, S
AF: DONET/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Kodaira, S
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Suetsugu, D
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Takahashi, N
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Takahashi, T
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Tamura, Y
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Sakaguchi, H
AF: IFREE/JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AB: The Izu-Bonin Island arc is an oceanic island arc, where the Pacific plate subducts beneath the Philippine Sea plate. Recent active seismic surveys in the Izu-Bonin arc show significant variations in thickness of the middle crust along the volcanic front [Kodaira et al, 2007]. To understand the crustal evolution in the oceanic island arc, we have to clarify structures in the mantle wedge along the arc in addition to the oceanic island arc crust. We conducted seismicity observations to investigate structure variations in northern Izu-Bonin arc using natural earthquakes. A temporal ocean bottom seismograph (OBS) network consists of 40 pop-up type OBSs was deployed in April 2006 between Tori-shima and Hachijo-jima islands. These OBSs were retrieved in July after about 80-day observations. We used continuous seismic data at 36 OBSs and three F-net and Hi-net seismic stations on Hachijo-jima and Aoga-shima islands operated by National Research Institute for Earth Science and Disaster Prevention. During the OBS observations, about 1600 earthquakes were located. These earthquakes clearly show double seismic zone along the subducting Pacific plate. We estimated 1D and 3D P- and S-wave seismic velocity structure using arrival time data of these earthquakes. The 1D velocity model shows that a layer with low Poisson’s ratio of 0.24 and high Poisson’s ratio of 0.28 corresponds to middle and lower crust, respectively. The low Poisson’s ratio layer suggests the granitic middle crust with Vp of ~6km/s. The high Poisson’s ratio layer agrees with the gabbroic lower crust as suggested by Kodaira et al. [2007]. Three-dimensional Vp and Vs structures were estimated by 3D tomographic inversion method by Kamiyra and Kobayashi [2000] using the 1D model as an initial model. The estimated 3D model shows structure variations along the volcanic front. We will discuss relationships between the seismic velocity variations and the island arc crust structures in northern Izu- Bonin arc.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: 2007 Fall Meeting