Seismic structure of the northern end of the Ryukyu Trench subduction zone, southeast of Kyushu, Japan

Abstract

P -wave velocity models related to the Philippine Sea plate subduction have been obtained from seismic explorations at the northern end of the Nansei-Shoto (Ryukyu) Trench. Both multi-channel seismic reflection and ocean bottom seismographic records provide clear images of the rough seafloor topography and irregular P -wave velocity structure of the northwestern extension of the Kyushu-Palau Ridge and Amami Plateau below the landward trench slope. Low velocity ( V p ≤ 4–5 km/s) and thick (>7 km) materials above the plate boundary characterize the landward trench slope structure. An exception to this is found where the subducting plate contacts higher velocity ( V p > 5 km/s) materials of the landward plate at the estimated position of the seismic asperity of 1968 Hyuganada earthquake ( M w 7.5).