Earth Planets Space, Vol. 61 (No. 8), pp. e37-e40, 2009E-LETTER
Azusa Nishizawa, Kentaro Kaneda, and Mitsuhiro Oikawa
Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo 104-0045, Japan
(Received June 12, 2009; Revised September 4, 2009; Accepted September 6, 2009; Online published September 30, 2009)
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 (Vp ≤ 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 (Vp > 5 km/s) materials of the landward plate at the estimated position of the seismic asperity of 1968 Hyuganada earthquake (Mw 7.5).
Key words: Marine seismics, Ryukyu (Nansei Shoto) Trench, Kyushu-Palau Ridge, Philippine Sea plate subduction.