Background | Table of Contents
INTRODUCTION
Tomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. The greatest problem in improving 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 observation network 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 northwest Pacific (Fig. 1).
Site WP-2, situated in the Northwestern Pacific Basin (Figs. 1, 2), is slated to become a long-term borehole seismic observatory, where it will be bounded by the Petropavlosk (PET) station (the station is beyond the boundaries of Fig. 1A but is located at 53.0235°N and 145.7430°E) to the north, by many Japanese stations to the west, by Minami-Torishima (MCSJ) Island station to the south (Fig. 1A), and by the proposed Midway Island station to the east. Owing to its location, this site will provide unique seismic observations from the seaward side of the Japan Trench (Fig. 2). Unlike other existing (Sites 1150 and 1151; Suyehiro, Sacks, Acton, et al., in press) and planned (Site WP-1) oceanic borehole observatories, there are no nearby coaxial transoceanic telephone cables to utilize for data recovery and power, so the Site WP-2 installation is designed as a stand alone system with its own batteries and recorder. Thus, once instruments are installed in the hole, they must be serviced for data analyses, distribution, and archiving. 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 this means data recovery will be costly and the data will not be available in real time, the scientific importance of the site to the ION concept makes this worthwhile.
Site WP-2 is also important because it will provide samples representative of the Cretaceous crust of the northwest Pacific. Results from this site will augment those from Leg 185, which characterized material being subducted into the Mariana and Izu-Bonin Trenches to better understand geochemical fluxes at convergent plate boundaries (Plank, Ludden, Escutia, et al., in press). Results from this site will also add to our knowledge of Pacific crustal structure, geochemistry, plate tectonics, magnetic lineations, sedimentation, and microbiology.
Site SR-1, atop the ocean plateau known as Shatsky Rise (Figs. 2, 4), was chosen for engineering tests because it offers a bare basalt outcrop at a relatively shallow depth in a convenient location. A basalt outcrop is needed because one of the main purposes of the hammer drill system is to emplace a casing string directly into such rocks. If the hard rock reenty system (HRRS) tests are successful, the Ocean Drilling Program (ODP) will be able to begin holes in geologic environments that have previously frustrated coring attempts. If basalts are recovered during the engineering tests from Shatsky Rise, they will provide valuable data about the age and composition of that large igneous province (LIP).
Tests of the technical feasibility and modes of operation of the HYACE (Hydrate Autoclave Coring Equipment) System are also scheduled at Shatsky Rise. The tests will be conducted at Site SR-3, tenatively located near (~15-20 km) Site SR-1, where the oceanic plateau is covered with thick (~900 m) sediments. Tests of the HYACE tools during Leg 191 are dependent on ODP Site Survey Panel (SSP) site approval and successful results from land tests.