IntroductionTomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. Perhaps the greatest problem facing seismologists who wish to improve such tomographic models is the uneven distribution of seismic stations, especially the lack of stations in large expanses of ocean such as the Pacific. The International Ocean Network (ION) project, an international consortium of seismologists, has identified "gaps" in the global seismic net and is attempting to install digital seismometers in those locations. One of the highest ION priorities is to install a station beneath the deep seafloor of the Philippine Sea (Fig. 1).
Site WP-1B, situated in the west Philippine Basin west of the Kyushu-Palau Ridge (Fig. 2), is slated to become a long-term borehole seismic observatory, which will be neighbored by stations at Ishigaki, (ISG) and Tagaytay (TAG) to the west, by many Japanese stations to the north, by Minami-Torishima (MCSJ) Island station to the east, and by the stations at Ponpei (PATS) and Port Moresby (PMG) to the south (Fig. 1). A seismic station at the center of the Philippine Sea plate is an essential addition to the surrounding stations and, together with existing land stations, will aid in understanding the global dynamics operating in the western Pacific (Fig. 2). Like other existing oceanic borehole observatories (Sites 1150 and 1151; Suyehiro, Sacks, Acton et al., in press), there is a nearby co-axial transoceanic telephone cable (TCP-2) to utilize for data recovery and power. However, the Site WP-1B installation is designed as a stand-alone system with its own batteries and recorder. Thus, once instruments are installed in the hole, they will be serviced for data analyses, distribution, and archiving. We plan to connect data, control, and power lines to the TPC-2 cable owned by the University of Tokyo after confirmation of data retrieval. This will be done under the auspices of an ongoing national program within Japan (Ocean Hemisphere Network Project). Initially, power will be supplied to the observatory by a battery pack, and data will be retrieved by a remotely operated vehicle (ROV; Fig. 3). The data will eventually become accessible worldwide through the Internet. Although data recovery will be costly and the data will not be available in real time until the system is connected to the TCP-2 cable, the scientific importance of the site to the ION concept is such that this is worthwhile.
Proposed Site WP-1B is also important because it will provide samples representative of the Eocene/Paleocene crust of the northern west Philippine Basin. Results from this site will augment those obtained on Deep Sea Drilling Project (DSDP) Legs 31 and 59, which were the first legs to sample and estimate the age of basement in the region and to confirm that the seafloor formed by backarc spreading. Results from this site will also add to our knowledge of backarc crustal structure and geochemistry, microplate tectonics, magnetic lineations, and sedimentation. Because core quality and dating techniques have vastly improved since these early legs, it is also anticipated that drilling at Site WP-1B will provide better age control on backarc spreading, as well as detailed records of Northern Hemisphere climate change, aeolian transport, and arc volcanism in the region during the Tertiary.
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