This group's mission is to establish long-term observatories on the sea floor and link them with land observatories in order to better understand the dynamic behavior of the Earth's interior with emphasis on the role of plate subduction concentrated in the Western Pacific. With the link between the sea floor and land observatories a long-term broadband seismic network of 1000 km station spacing is planned to be established and to have accumulated about a year-long data by the end of the program. Success, if achieved, will lead us into continued mode of operation. The large lateral heterogeneity of the uppermantle requires us to conduct temporary ( but with a longer duration than conventional experiments ) seafloor observations utilizing array observations of <~100-km scale spacing.
This group aims to obtain crucial seismological and geomagnetic data that will help in understanding how the subduction process transfers heat, recycles material, stimulates deep mantle processes, and create arcs and marginal seas. Our observations will greatly aid in increasing the resolving power of global earth tomography by filling in part of the "oceanic gap". We will also start experimental observations on crustal deformation on the sea bottom to better understand the dynamic processes occurring in the close vicinity of subduction boundaries. During the first three years, we will develop and test instruments to be installed into deep ocean boreholes for stable and quiet observation. As of this writing ( in the middle of the second year ), we have conducted some land borehole tests of the broadband seismometer and the volumetric strain meter ( during the summer of 1997 ). Both of these instruments are at the most advanced level of currently available technology. Simultaneous high-resolution temperature and pressure measurements will be made to further improve the signal-to-noise ratio. During the first three years, data to study mantle conductivity structure and seismic velocity and Q structure will be obtained by mobile array observations, the first of which is planned this year. During the fourth year ( 1999 ), we expect that the Ocean Drilling Program will coordinate with us to install instruments at four sites ( Figure 4 ) and establish truly long-term observatories. These sites, with the exception of the northwest Pacific Basin site, may be connected to nearby cables for future real-time data recovery and power supply. Mobile array observations will aim to study the relation between surface features and deep earth processes. The numerous bathymetric highs in the western Pacific of which the most pronounced feature is the Ontong Java Plateau, are of great interest. Their creation and fate in relation to plume and subduction are the subject of hot debate by earth scientists.