Interplate coupling along the central Ryukyu Trench inferred from GPS/acoustic seafloor geodetic observation

The Ryukyu trench is a major convergent plate boundary where the Philippine Sea plate is subducting at a rate of about 8 cm/yr. Large earthquakes have not been reported along the Ryukyu subduction for the last 300 years. Because the rate of release of seismic moment in the Ryukyu Trench over the last 80 years is 5% in consideration of the plate convergence rate, interseismic coupling in the trench is assumed to be weak.The GPS measurements by Japan Geographical Survey Institute also show the southward motion (2.5 cm/yr) of Ryukyu arc relative to the Amurian plate, which is due to extensional rifting of Okinawa Trough. Backslip by the interplate coupling between the subducting Philippine Sea plate and the overriding Eurasian plate cannot have been detected in the GPS network along the Ryukyu Islands. We have started the GPS/acoustic seafloor observation to detect the inter-plate coupling in the central Ryukyu trench. For this measurement, we used a system capable of performing two main tasks: precise acoustic ranging between a ship station (observation vessel) and seafloor transponders, and kinematic GPS positioning of observation vessels. The seafloor reference point was set at about 33 km landward from the axis of the Ryukyu trench (southeast of Okinawa Island). A set of three acoustic transponders has been installed on the seafloor, at a depth of about 2900m. The transponders are placed to form a triangular. Five campaign observations were carried out for the period from January 2008 to November 2009. Each epoch consists of three observation days. The coordinates of the seafloor benchmark were calculated using the least-squares technique (Ikuta et al., 2008); this technique minimizes the square sum of acoustic travel-time residuals. The RMS of travel time residuals for each campaign analysis is about 70 micro-seconds. The result shows that the benchmark moved to northwest direction for two years at a rate of 4 cm/yr relative to the Amurian plate. Then we estimated the length and width of interplate coupling area using observed movement of the benchmark. The movements of the GPS stations on the Ryukyu Islands and the benchmark are described as the combination of the block rotation of the Ryukyu arc (Nakamura, 2004) and the displacement by the backslip in the coupled area. The results show that the estimated width of the interplate coupling area is 40-50 km from the Ryukyu trench. The results also show that the length of the coupled area is over 40 km. Since the calculated displacements are not sensitive to the change in the length of the coupled area, the accurate length is uncertain. These suggest that the interplate coupling occurs up-dip of the seismogenic zone in the Ryukyu subduction zone. The tsunami earthquake (M8.1) occurred near the south Ryukyu Trench in 1771. This suggests the interplate coupling near the Trench would be the cause of the tsunami earthquakes.