Ocean Drilling Program, United States

During Leg 186, we will drill two sites off the northeast coast of Japan to monitor seismic and aseismic crustal deformation associated with subduction of the Pacific Plate beneath Japan. Both sites will be drilled into the deep-sea terrace on the landward side of the Japan Trench, a region that is part of the deforming edge of the overriding Eurasian plate. Borehole strainmeters, tiltmeters, and broadband seismometers will be installed at the bottom of the two holes to continuously monitor strain and seismic activity associated with plate motions. This will be the first attempt to establish long-term seafloor borehole observatories in one of the most active and most studied subduction zones in the world. These observatories will provide crucial near-field data that will quantify elastic and anelastic deformation occurring in the active plate boundary zone. 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.