资源环境科技发展态势分析平台

We performed observations of seafloor crustal deformation employing the Global Navigation Satellite System/Acoustic technique at two stations installed at about 45 and 70km from the axis of the Nansei-Shoto (Ryukyu) Trench, to the southeast off the Okinawa Main Island. The observations for 3- and 6-year survey periods indicate that the two stations moved landward, in the opposite direction to the trench, by 63 and 21 mm/year relative to the Ryukyu Arc, suggesting interplate coupling around the stations. The observational results reveal the strong coupling state on the plate interface with coupling ratios of 0.9-1.0 at the slab depths of 10-13 km or 0.7-0.8 at slab depth ranging from 13km up to the seafloor. The strongly coupled segment completely coincides with the source area of the 1791 tsunami event and does not overlap with the activity area of slow slip events.

Plain Language Summary The Nansei-Shoto (Ryukyu) Trench is a part of the subduction margins of the Philippine Sea Plate. The seismic potential at the trench, especially in the central portion, had been thought to be low. On the contrary, our seafloor crustal deformation measurements depicted a zone of strong coupling between the overriding and subducting plates at the trench, to the southeast off the Okinawa Main Island. The result indicates high seismic potential there, which is quite different from the previous understanding. The coupled zone is completely overlapped with the source area of the 1791 thrust event, and we need care about subduction earthquakes at the present region.