INTRODUCTION
The western Pacific, where plate-consuming boundaries are concentrated, is very well suited to study the dynamics of plate subductions, the formation and evolution of island arcs and marginal seas, and their relation to mantle convection (e.g., Fukao, 1992) (Fig. 1). In particular, the Japan Trench region (Fig. 2) has both high seismicity and plate convergence rates compared to other trenches. Dense regional geophysical networks have been expanded in the land area across Japan over the years. Data accumulated from these have made it possible to precisely locate the two proposed sites in seismically active (JT-1C) and inactive (JT-2G) areas about 10 km immediately above the interplate seismogenic zone (Fig. 3).
Even though the Japan Trench subduction zone is heavily monitored by dense land-based seismic and geodetic networks, it lacks near-field data that are crucial in quantifying the elastic and anelastic behavior of the active plate boundary zone. The sites we propose to drill will become observatories that will start to fill this void in data coverage. Borehole strainmeters, tiltmeters, and broadband seismometers will be installed at the bottom of the holes to continuously monitor strain and seismic activity associated with plate motions.
Secondary objectives, which rely on coring at both sites, include (1) determining the forearc subsidence and deformation history, (2) placing constraints on tectonic erosion within the subduction zone, and (3) obtaining a record of arc volcanism over the past 3 m.y.