Estimation of interplate coupling at the Nankai Trough based on observation of ocean bottom crustal deformation at the Kumano Basin

At the Nankai Trough, the Philippine Sea plate (PH) subducts beneath the southwest Japan with a rate of about 4- 6 cm/yr, where great interplate earthquakes have repeatedly occurred every 100-200 years. A number of researchers have investigated crustal deformation caused by subduction of the PH based on geodetic measurements as represented by GPS observation. However it is difficult to infer the strength of the plate coupling in offshore areas, due to the poverty of offshore geodetic data. For this issue, we have conducted seafloor geodetic observation using GPS/Acoustic techniques around the Nankai Trough since 2004. In this system, we estimate the position of a surveying vessel by Kinematic GPS analysis and measure the distance between the vessel and the benchmark on the seafloor by Acoustic measurements. Next we determine the location of the benchmark and detected crustal movement of the seafloor. There are three seafloor sites in this region, and then the location of benchmark is determined within a precision of 2-3 cm at horizontal components for each observation (Tadokoro et al., 2006). As a result of observations from November 2005 to February 2007, one of the seafloor sites is moving at a rate of 6.96 +/- 4.03 cm/yr in the WNW direction with respect to the Amurian plate (AM). In this study, we investigate interplate coupling at the Nankai Trough using onshore GPS velocities derived from Geophysical Survey Institute of Japan and our estimated offshore GPS site velocity. We assume that observed GPS velocities are represented by the superposition of elastic deformation associated with subduction of the PH, rigid block motion of the overriding plate, and error. The plate interface along the Nankai Trough is represented by multiple rectangular faults. The strength of the plate coupling for each fault segment varies with depth according to a thermal model (Hyndman et al., 1995). Moreover relative plate motion of the PH-AM (Miyazaki and Heki, 2001) is assigned to the plate interface as a priori constraint.