GPS/acoustic Seafloor Geodetic Observations Near the Nankai Trough Axis

The GPS/acoustic seafloor geodetic observation system, which uses precise acoustic ranging and kinematic GPS positioning techniques, has been developed as a useful tool for observing seafloor crustal deformations associated with plate convergence and with earthquakes that occurred in ocean area including the 2011 Tohoku-oki Earthquake of Mw 9.0. Our research group installed eight seafloor benchmarks for this observation system in source areas of anticipated major interplate earthquakes along the Nankai Trough, off southwestern Japan. We have performed campaign measurement for 4-8 years until the end of 2012. The error of displacement rate is almost 5 mm/y through the monitoring for more than four years. At the northern-most potion of the Nankai (Suruga) Trough, the observed steady horizontal displacement rate is 45 mm/y toward west. Also no significant velocity difference is observed across the trough, indicating strong interplate locking up to the shallowest segments. On the other hand, site velocities are 40 mm/y in the direction of N75W at the central region of the Nankai trough, 70-90 km landward from the trough axis. Although this result is the strong evidence for interplate locking, with coupling ratios of 60-80% on the basis of the back-slip model, it has no resolution for the interplate locking at the most-shallowest segments whose depths are 0-10 km. In other words, seaward up-dip limit of locked zone is never resolved from the present seafloor benchmark network [Tadokoro et al., 2012] . Large co-seismic slips larger than 40-50 m on the shallowest interplate segment [Ito et al., 2011; Fujii et al., 2011; Iinuma et al., 2012] are the cause of the unexpected high tsunami that has attacked the pacific coasts of the Tohoku region during the Tohoku-oki Earthquake; it is, therefore, essential to understand slip deficit or strain accumulation condition near the trench axis, also for the anticipated mega-thrust earthquake at the Nankai Trough. For this reason, we installed a new seafloor benchmark in the vicinity of the Nankai Trough axis, about 15 km landward from the trough axis, on July 16, 2013. In addition, we plan to install another new benchmark on the other side of the Nankai Trough in August of this year, to directly measure the motion of the subducting Philippine Sea Plate. The coupling ratio is calculated from slip deficit divided by convergence rate. We can directly 'measure' the coupling ratio from crustal deformation at the newly-installed station on the Philippine Sea Plate, without using global plate motion models. Acknowledgments: We are grateful to the captain and crews of R/V "Asama" of Mie Prefecture Fisheries Research Institute, Japan. This study has been partly promoted by Ministry of Education, Culture, Sports, Science and Technology, Japanese Government.