IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS016 Poster presentation 2352
Seafloor borehole seismic observatories in the western Pacific and upper
mantle and crustal structure beneath the northwestern Pacific basin
Dr. Masanao Shinohara
Earthquake Research Institute University of Tokyo IASPEI
Tetsuo Fukano, Toshihiko Kanazawa, Eiichiro Araki, Kiyoshi Suyehiro, Masashi
Mochizuki, Kazuo Nakahigashi, Yuka Kaiho, Tomoaki Yamada
Geophysical networks provide data for better understanding of the dynamics in the Earth. However, a
great limitation on existing networks is the uneven distribution of stations, especially lack of stations in
the ocean such as the Pacific. For uniform distribution of stations on the Earth, we need to construct
observatories in the sea. The western Pacific area has been selected for installation of ocean-bottom
observatories because it is ideal for problems related to plate subduction. Boreholes are considered to
give best environment for geophysical observations in the sea. Four borehole geophysical observatories
at three areas have been installed during ODP Leg186, 191 and 195. Two stations (JT-1 and JT-2) are
located on the landward side of the Japan Trench. 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. At present,
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.
Geomagnetic studies in the WP-2 area indicate the crust was formed 129 Ma. The seismic experiments
with ocean bottom seismometers (OBSs), the WP-2 and airguns were performed around the WP-2 in
July 2001, July 2002 and July 2005. To detect a seismic anisotropy in the uppermost mantle, the
experiments had four profiles with different directions. Broadband seismic records for 436 days in total
were retrieved from the WP-2. Reflecting low noise environment, many teleseismic events were
recorded. The purposes of the present study are to obtain a detail seismic structure of crust and
uppermost mantle including a seismic anisotropy from the seismic surveys and to estimate a upper
mantle structure including depths of discontinuities using the WP-2 records. Shallow seismic velocity
models just below OBS were derived from using tau-p method and records of a hydrophone streamer.
The obtained seismic models were confirmed using the ODP logging data. Deep structures were
estimated by forward modeling using a two dimensional ray tracing method. The estimated crustal
structures beneath each line are almost identical. In a sediment layer, P-wave structure is 1.6km/s, S-
wave is about 0.2km/s, the thickness of this layer is approximately 0.4km. The layer 2 is divided into
two layers which have different vertical velocity gradients (layer 2A and layer 2B). The P- and S-wave
velocities at the top of the layer 2A are 4.6km/s and 2.7km/s, respectively. The layer 2B has P- and S-
wave velocities of 5.3km/s and 3.1km/s at the top of the layer, respectively. Total thickness of the layer
2 is about 1.4 km. The data from the WP-2 were useful to estimate the detailed structure of the layer 2.
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. To explain large amplitudes of Pn phase, a crust-mantle transition layer is required
at the bottom of the crust. The Pn velocities are different in each profile. Average velocities of Pn and