HR: 1340h
AN: S23A-1108    [Abstracts]
TI: Seafloor borehole seismic observatories in the western Pacific and upper mantle and crustal structure beneath the northwestern Pacific basin
AU: * Shinohara, M
EM: mshino@eri.u-tokyo.ac.jp
AF: ERI, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Fukano, T
AF: ERI, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Kanazawa, T
AF: ERI, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Araki, E
AF: JAMSTEC, 2-15, Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan
AU: Suyehiro, K
AF: JAMSTEC, 2-15, Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan
AU: Mochizuki, M
AF: IIS, Univ. Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo, 153-8505, Japan
AU: Nakahigashi, K
AF: ERI, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Mochizuki, K
AF: ERI, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Yamada, T
AF: ERI, Univ. Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AB: Geophysical networks provide data for better understanding of the dynamics in the Earth. For uniform distribution of stations on the Earth, we need to construct observatories in the sea. The western Pacific area had been selected for installation of ocean-bottom observatories because it is ideal for problems related to plate subduction. Boreholes give best environment for geophysical observations in the sea. The observatories WP-1 and WP-2 are effectively located to complete a 1000-km span network in the western Pacific area. The WP-1 site is in the west Philippine Basin west of the Kyushu-Palau Ridge. The WP-2 observatory is situated in the northwestern Pacific Basin. Both the stations fill gap for global seismic networks. The WP-1 observatory was activated in March 2002 using an ROV and long-term observation started. In June 2006, an ROV dived to the WP- 1 (fourth visit) and recovered the data. At this visit, data recording was discontinued. Seismic records of 692-days (Mar. 2002 - Feb, 2004) have been obtained from the WP-1. The WP-2 observatory was activated in October 2000 using an ROV. In June 2005, an ROV made fourth visit to the WP-2 and recovered the data. Recording at the WP- 2 has been suspended from the fourth ROV visit. In total, 436-days data (Oct. 2000 - Jan. 2001, Aug. 2001 - July 2002) were retrieved. A detailed structure of an oceanic plate is important information to consider a dynamics of oceanic plate. The seismic experiments with ocean bottom seismometers (OBSs), the WP-2 and airguns were performed. To detect a seismic anisotropy in the uppermost mantle, the experiments had four profiles with different directions. Reflecting low noise environment, the WP-2 recorded many teleseismic events. A detail seismic structure of crust and uppermost mantle including a seismic anisotropy was obtained by seismic surveys and an upper mantle structure including depths of discontinuities is estimated using the WP-2 records. A thickness of sedimentary layer is approximately 0.4km. The layer 2 is divided into two layers which have different vertical velocity gradients (layer 2A and layer 2B). Total thickness of the layer 2 is about 1.4 km. The uppermost Layer 3 has P- and S-wave velocities of 6.8km/s and 3.8km/s, respectively. The layer 3 is about 5km thick. The Pn velocities are different in each profile. Average velocities of Pn and Sn are 8.2 km/s and 4.7 km/s, respectively. The velocity variations are about 5% for P-wave and about 3.5% for S-wave. From this result, a seismic anisotropy of the uppermost mantle is suggested, and the fast direction seems to be perpendicular to the magnetic lineations. Therefore the anisotropy below the WP-2 is suggested to originate in a preferred orientation of olivine crystals in the uppermost mantle. To perform receiver function analysis, we selected 16 events with a magnitude greater than 6 from the WP-2 data. After the theoretical Ps-P times are calculated, travel times for large amplitudes of the receiver function corresponding to 410 km and 660 km discontinuties were read. Averaging the reading times, conversion depths of 416 km and 666 km were obtained.
DE: 3025 Marine seismics (0935, 7294)
DE: 3094 Instruments and techniques
DE: 7218 Lithosphere (1236)
DE: 7220 Oceanic crust
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting