Crustal deformation at the Nankai Trough estimated from seafloor geodetic observations

The Philippine Sea plate subducts beneath the southwest Japan along the Nankai Trough with a rate of about 4-6 cm/yr, where megathrust earthquakes have repeatedly occurred every 100-150 years. Because the region expected to be the hypocentral area of next Nankai and Tonankai earthquakes is almost located in offshore area, it is important to know the spatio-temporal variation of crustal deformation accompanied with plate interaction in high precision. For this issue, we have conducted seafloor geodetic observation at the Nankai Trough using a GPS/Acoustic technique 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. For the repeatability of this observation, the location of benchmark is determined within a precision of 2-3 cm at horizontal components. Several seafloor benchmarks are located at the Nankai Trough, which are individually operated by Japan Coast Guard, Tohoku University, and Nagoya University. In the Kumano Basin, we have three seafloor benchmarks located about 60-80 km away from the deformation front of the Nankai Trough. The observations from 2005 to 2011 have illustrated that those benchmarks are moving at rates of about 4 cm/yr toward west-northwest with velocity uncertainties of about 2 cm/yr relative to the Amurian plate. In this study, to explain the crustal deformation derived from seafloor geodetic observations especially at the shallower part of the Nankai Trough, we conduct numerical simulations using finite element method and then discuss the interplate coupling at the Nankai Trough.