Tomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. High-quality digital seismic data obtained from seismic stations on land, for example, have been used to identify zones of anomalous velocity and anisotropy in the mantle, and from these, to determine patterns of mantle flow. In particular, Tanimoto (1988) has demonstrated the existence of a strong pattern of deep (>550 km) high-velocity anomalies in the western Pacific, suggesting complex interaction between subducting slabs and the surrounding mantle, whereas more recent studies in areas of dense seismic coverage have provided crude images of subducting plates extending to the 670-km discontinuity (van der Hilst et al., 1991; Fukao et al., 1992) and of deep velocity anomalies extending beneath ridges (Zhang and Tanimoto, 1992; Su et al., 1992).

One of the critical problems facing seismologists who wish to improve such tomographic models is the uneven global distribution of seismic stations. Few seismic stations are located on the 71% of the Earth's surface covered by oceans, and this problem is particularly acute in large expanses of ocean such as the Pacific. The scientific importance of establishing long-term geophysical observatories at deep ocean sites to understand the dynamic processes occurring in the Earth's interior through seismic imaging has long been recognized by the earth science and ocean drilling communities (COSOD II, 1987; JOI-ESF, 1987; Purdy and Dziewonski, 1988; JOI/USSAC, 1994; Montagner and Lancelot, 1995). The International Ocean Network (ION) project has identified the western Pacific and the Philippine Sea as a particularly important gap in the global seismic network. By installing long-term borehole seismic observatories in the seafloor in this region, the ION project is attempting to fill this gap so high-resolution tomographic images of slab-mantle interaction zones can be obtained at great depth in the most active system of subduction complexes in the world. To this end, borehole seismic observatories were installed at ODP Sites 1150 and 1151 (Sites JT-1 and JT-2 in Fig. F14) on the inner wall of the Japan Trench during Leg 186 (Suyehiro, Sacks, Acton, et al., 2000) and another observatory was successfully installed at Site 1179 in the western Pacific (Site WP-2 in Fig. F14) during ODP Leg 191 (Kanazawa, Sager, Escutia, et al., in press).

A major objective of the ION project was to establish a borehole seismic observatory at a quiet site in the middle of the Philippine Sea on the upper plate of the Mariana subduction system to determine whether the Pacific plate is penetrating into the lower mantle below the 670-km discontinuity under the Mariana Trench but not under the Izu-Ogasawara (Bonin) Trench. A high-quality digital seismic observatory was thus installed during Leg 195 at Site 1201 in the West Philippine Basin west of the Palau-Kyushu Ridge between existing stations at Inuyama (IMA) and Taejon (TJN) to the north, Minami-Torishima (MCSJ) and Chichi-jima (OGS) to the east, Ponapei (PATS) and Jayapura (JAY) to the south, and Ishigaki (ISG) and Baguio (BAG) to the west (Fig. F14). The observatory is designed as a stand-alone system with its own battery pack and recorder at the seafloor so that it can be serviced and interrogated by an ROV. Like the borehole observatories installed at Sites 1150 and 1151, however, there is a coaxial transoceanic telephone cable (TCP-2) near Site 1201 that can be used eventually for data recovery and power. There are plans to connect data, control, and power lines to the TPC-2 cable, which is owned by the University of Tokyo, after confirmation of data retrieval. This will be done under the auspices of the Ocean Hemisphere Network Project, an ongoing national program in Japan. The data will eventually become accessible worldwide through the Internet.