Interplate Coupling State at the Nansei-Shoto (Ryukyu) Trench, Japan, Deduced From Seafloor Crustal Deformation Measurements

Abstract

We performed observations of seafloor crustal deformation employing the Global Navigation Satellite System/Acoustic technique at two stations installed at about 45 and 70 km 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 13 km 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.