HR: 1340h
AN: S23A-1120    [Abstracts]
TI: Seismic Structure of the Northernmost Ryukyu Subduction Zone, Southward Offshore of Kyushu, Japan
AU: * Nishizawa, A
EM: azusa@jodc.go.jp
AF: Hydrogr. and Oceanogr. Dep., Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo, 104- 0045, Japan
AU: Oikawa, M
EM: mitsuhiro-oikawa@kaiho.mlit.go.jp
AF: Hydrogr. and Oceanogr. Dep., Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo, 104- 0045, Japan
AU: Kaneda, K
EM: kentaro-kaneda@kaiho.mlit.go.jp
AF: Hydrogr. and Oceanogr. Dep., Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo, 104- 0045, Japan
AU: Kasahara, J
EM: kasahara.j@tairikudana.com
AF: Japan Continental Shelf Survey Co. Ltd., NTC Building 3F, 1-11-2 Kyobashi, Chuo-ku, Tokyo, 104-0031, Japan
AB: The Philippine Sea plate subducts northwest beneath the Eurasian Plate along the Nankai Trough in the north and the Nansei-Shoto (Ryukyu) Trench in the south at a rate of about 50-60 mm/year. Magnitude 8-class earthquakes occurred repeatedly along the Nankai Trough in the northeastern neighborhood, while the largest earthquake ever recorded is at most 7.6 along the northernmost Ryukyu Trench in the southern neighborhood. We carried out five ocean bottom seismographic (OBS) and multi-channel seismic (MCS) profiles in the northernmost Ryukyu subduction zone and obtained seismic velocity structural models related to the characteristics of the earthquakes in this region. For each investigation in 2005 and 2006, we shot a tuned airgun array with a volume of 8,040 cubic inches at an interval of 200 m for the wide-angle seismic profiles and at 50 m for the MCS (480 channels, 60 folds) profiles. The OBSs were deployed at an average interval of 5 km, which provided us dense data of high quality. The OBS data were modeled by a tomographic inversion, two-dimensional ray tracing and synthetic seismograms. The obtained MCS records show distinct reflections from the subducting plate boundary beneath the landward slope of the Ryukyu Trench and low velocity (Vp < 4 km/s) and thick (~ 8 km) materials above and on the decollement zone are modeled from the OBS data. Both the MCS and OBS records give clear images of very rough sea bottom configuration of the northern extension of the Kyushu-Palau Ridge and Amami Plateau below the landward slope. Such strong undulations of the plate boundary may constrain the sizes and/or natures of the asperities in this region. In addition, the estimated position of the seismic asperity of 1968 Hyuga-nada earthquake (Mw 7.5) corresponds to high velocity materials ascending in our P-wave velocity model.
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting