INTRODUCTIONThe scientific importance of establishing long-term geophysical stations in deep oceans has been acknowledged by earth sciences and Ocean Drilling Program (ODP) communities and is expressed in various articles (COSOD II, 1987; Purdy and Dziewonski, 1988; BOREHOLE, 1994; Montagner and Lancelot, 1995; Ocean Drilling Program, 1996). Indeed, more than a few important inferences on the dynamics of the Earth's deep interior have been inferred from seismic tomography images, such as existence of super plumes, deep continental roots, stagnation of subducting plates within the mantle, and variations in the thickness of the thermal boundary layer at the base of the mantle (e.g., Fukao, 1992). Detailed examination of active processes at plate boundaries have also brought about significant inferences on magma reservoirs at oceanic ridges and on the décollement structure of subducting plates.
In essence, we wish to understand active processes driving Earth's dynamics from a global to a regional scale. A major step forward will almost certainly be made by installing permanent observatories in the oceans, which constitute 71% of Earth's surface. Obtaining observations from oceanic areas is significant not only for global coverage, but because most of the plate boundaries exist beneath the oceans, particularly those boundaries where oceanic lithosphere is presently being generated and recycled.
The western Pacific area is ideal for addressing problems related to plate subduction. In particular, the Japan Trench area is where much effort has been made for monitoring seismic and geodetic motions on land for many years. Together with marine geological and geophysical investigations on both sides of the Japan island arc, this area probably is the best studied subduction area. The area can be characterized as having a fast subduction rate and being a seismically active and well-coupled area. We also have knowledge of the sedimentary and tectonic environment from previous drilling, which found the forearc area to be subsiding as a result of tectonic erosion with little accretionary prism development.
To monitor strain and seismic activity continuously and ultimately to understand how plate motion is accommodated across a subduction zone, we have installed two geophysical observatories on the landward side of the Japan Trench (Fig. 1). Coring and logging data collected during the cruise are also expected to provide a tectonic history that can be linked to the present dynamics, which will be inferred from the observatories.
Although not always the case, normal coring objectives and observatory objectives often overlap and are interrelated as in this leg or recent Circulation Obviation Retrofit Kit (CORK) legs. Once an observatory is established, ways and means to recover the data and to keep the station running become necessary. Such tasks are not easily undertaken even if a site only needs servicing once a year. A new fiber-optic cable owned by the University of Tokyo already exists and currently terminates near Site 1150. Once Site 1150 proves to be functioning, connections will be made to supply power, send commands, and retrieve data in real time on land. Furthermore, a 50 km cable extension is planned to connect Site 1151 as well. These stations will make invaluable additions to the existing geophysical network over the western Pacific. The data will eventually become accessible worldwide through the Internet.
Previous Drilling Along the Japan Trench
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