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In the period of Ocean Drilling Program Legs 186, 191, and 195, we successfully deployed a set of seismo-geodetic 195, we successfully deployed a set of seismo-geodetic sensors in deep seafloor boreholes in four locations (JT1, JT2, WP1, and WP2). The JT1 and JT2 borehole observatories are located in the landward slope of the Japan Trench, the Japan Trench, Japan Trench, where we expect seismic activity beneath the observatories. These borehole observatories gave us good platforms to look s to look to look at the dynamic process on the plate boundary in the the dynamic process on the plate boundary in the dynamic process on the plate boundary in the seafloor. The WP1 and WP2 are considered a part of a global a global global seismic network of observatories currently distributed mostly on land and ocean islands. For permanent seismic observation in the seafloor setting, installation of broadband seismic sensors in the seafloor borehole was recommended, , and candidate locations were chosen (Suyehiro et al., 2006). ., 2006). , 2006). The WP1 and WP2 sites are one of the first borehole observatories to implement such a permanent seafloor obser-permanent seafloor obser-seafloor observatory. We called these seafloor borehole observatories �NEREID� (�Neath Seafloor Equipment for Recording Earth�s Internal Deformation). The NEREID observatories are characterized by several key features that are thought to that are thought to to acquire optimum performance from the borehole sensors. optimum performance from the borehole sensors. Firstly, sensors are cemented at the bottom of the borehole. This is very important because long-period seismic and tilt/ strain sensors are sensitive to fluid flow around them, so it is them, so it is necessary to couple them to the ground very well. Cementing to couple them to the ground very well. Cementing them to the ground very well. Cementing to the ground very well. Cementing the ground very well. Cementing ground very well. Cementing sensors at the bottom of the borehole was able to fix sensors rigidly in place, and eliminated fluid around the sensors. in place, and eliminated fluid around the sensors. place, and eliminated fluid around the sensors. place, and eliminated fluid around the sensors. , and eliminated fluid around the sensors. eliminated fluid around the sensors. fluid around the sensors. the sensors. sensors. Cementing is necessary especially for strainmeters, because s, because , because the strain of the ground should be transmitted to the sensing cylinder of the strainmeter by some means. On land, it has means. On land, it has. On land, it has been widely practiced for that purpose. at purpose. purpose. Secondly, separate cables connect borehole sensors uphole for power feeding and seafloor data recording. This is to deal with possible failure of the sensors, cables, and , and and connectors in the borehole during and after the installation in the borehole. It worked very well, and we were able to , and we were able to and we were able to replace failed sensors after the...

Borehole resistivity images from ODP Leg 196 allow rapid and complete qualitative assessment of deformation within the toe of the Nankai prism, Japan. Borehole breakouts were common within the prism but prominent in the trench‐wedge unit around the frontal thrust, suggesting reduced sediment strength. Breakouts indicate consistent σ2 orientations (∼050°), compatible with northwesterly convergence. Deformation is dominated by discrete zones, including the frontal thrust and décollement zone. Prism fractures trend ∼NE–SW, consistent with convergence. The décollement shows minimal deformation and the dominant structural trend is ∼N–S. Prism deformation zones are characterized by high resistivity (compaction), whereas the décollement is apparently dilated, both with conductive fractures. Distribution of fracture orientations varies between log units confirming lithologic and rheologic influence. Pore pressure is elevated within the décollement and the misalignment of conductive fractures may reduce permeability.

We have developed a method for three-dimensional simultaneous velocity and density inversion using traveltimes of local earthquakes and gravity data. The purpose of this method is to constrain the velocity inversion and increase the spatial resolution of shallow velocity structures by introducing additional gravity data. The gravity data contributes to the P-and S-wave velocity models by imposing constraints between seismic velocities and density. The constraint curve is constructed so as to fit the data for porous rock samples, and deviations from the curve are taken into account in the inversion. The constraint is imposed at only the first layer, because density structure is well resolved at shallower parts and it is difficult to determine uniquely at greater depths. Synthetic inversion tests indicate that gravity data can improve the resolution of the velocity models for this layer. The method is applied to investigate the subsurface structure of Izu-Oshima volcano, Japan and velocity structures with high spatial resolution are obtained. The additional gravity data contribute primarily to improvement of the S-wave velocity model. At 0.25 km depth, a high velocity anomaly due to caldera-filling lava flows is observed. At 1.25 and 2.5 km depths, high velocity intrusive bodies are detected. A NW-SE trending high velocity belt at 1.25 km depth is interpreted as being caused by repeated intrusion of dikes.

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed fortyseven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15-20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15-20 km from the aftershock distribution.