In 2010, the first long-term borehole monitoring system was deployed at approximately 900 m below the sea floor (mbsf) and was assumed to be situated above the updip limit of the seismogenic zone in the Nankai Trough off Kumano (Site C0002). Four temperature records show that the effect of drilling diminished in less than 2 years. Based on in situ temperatures and thermal conductivities measured on core samples, the temperature measurements and heat flow at 900 mbsf are estimated to be 37.9°C and 56 ± 1 mW/m 2 , respectively. This heat flow value is in excellent agreement with that from the shallow borehole temperature corrected for rapid sedimentation in the Kumano Basin. We use these values in the present study to extrapolate the temperature below 900 mbsf for a megasplay fault at approximately 5,200 mbsf and a plate boundary fault at approximately 7,000 mbsf. To extrapolate the temperature downward, we use logging-while-drilling (LWD) bit resistivity data as a proxy for porosity and estimate thermal conductivity from this porosity using a geometrical mean model. The one-dimensional (1-D) thermal conduction model used for the extrapolation includes radioactive heat and frictional heat production at the plate boundary fault. The estimated temperature at the megasplay ranges from 132°C to 149°C, depending on the assumed thermal conductivity and radioactive heat production values. These values are significantly higher, by up to 40°C, than some of previous two-dimensional (2-D) numerical model predictions that can account for the high heat flow seaward of the deformation front, including a hydrothermal circulation within the subducted igneous oceanic crust. However, our results are in good agreement with those of the 2-D model, which does not include the advection cooling effect. The results imply that 2-D geometrical effects as well as the influence of the advective cooling may be critical and should be evaluated more quantitatively. Revision of 2-D simulation by introducing our new boundary conditions (37.9°C of in situ temperature at 900 mbsf and approximately 56 mW/m 2 heat flow) will be essential. Ultimately, in situ temperature measurements at the megasplay fault are required to understand seismogenesis in the Nankai subduction zone.

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 obserpermanent seafloor obserseafloor 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 install...