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US - Japan Collaborative 3-D Seismic Investigation of the Nankai Trough Plate-Boundary Interface and Shallowmost Seismogenic Zone N L Bangs 1 (512-471-0424; nathan@utig.ig.utexas.edu) A Taira 2 S Kuramoto 3 T H Shipley 1 G F Moore 4 K Mochizuki 2 S S Gulick 1 Z Zhao 1 Y Nakamura 2 J-O Park 5 B L Taylor 6 S Morita 3 S Ito 3 D J Hills 4 S C Leslie 4 C M Alex 7 A J McCutcheon 1 T Ike 8 H Yagi 9 G Toyama 9 1University of Texas, Institute for Geophysics, 4412 Spicewood Springs Rd. 78759, Austin, TX 78795-8500, United States 2University of Tokyo, Ocean Research Institute, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan 3Geological Survey of Japan, 1-1-3 Higashi , Tsukuba 305-8567, Japan 4University of Hawaii, Dept. Geoplogy & Geophysics 1680 East-West Rd., Honolulu, HI 96822, United States 5Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 6Landmark Graphics Corp, Advance Products Group, 7409 S. Alton Ct., Suite 100, Englewood, CO 80112, United States 7University of California at Santa Cruz, Earth Sciences Department, Santa Cruz, CA 95064, United States 8Chiba University, Yayoi 1-33, Inage-ku, Chiba-shi, Chiba 263-0022, Japan 9University of the Ryukyus, Earth Science Department, Okinawa, Japan In a collaborative effort between the US and Japan, we conducted a three-dimensional seismic reflection survey of the western Nankai Trough accretionary complex. The survey targeted the decollement thrust between the subducting Philippine Sea Plate and the overridding island arc of Japan to examine the physical properties of the thrust from the toe of the wedge into the seismogenic zone. Between June and August of 1999 we spent 59 days on board the R/V Ewing shooting 81 seismic lines spaced at 100 m in a northwest-southeast orientation 100 km south of the Muroto peninsula of Shikoku Island. The total area surveyed covers 80 x 8 km and includes ODP Site 808 and 3 of the 4 planned sites for upcoming ODP Leg 190. The data were acquired using a tuned 14-airgun array with a total volume of 4276 in$^{3}$. Shots were fired at 50 m intervals and 12 s of data were recorded to include reflections from the top of subducting Philippine Sea Plate. The ship was navigated with differential GPS using three or more regional base stations. Preliminary 2-D processing of these data reveals the top of the subducting crust along nearly all of the 80-km-long profile. The decollement reflection can be traced 300-400 m above the subducting crust from the deformation front 40 km landward to a sub-seafloor depth of 5 km, into the updip limit of the seismogenic zone. Overlying the decollement are classic fold-and-thrust structures of the accretionary complex. From an initial examination, these data reveal a seaward decrease in the spacing of thrusts from 3,500 m to 1,500 m implying that either the trench section has become progressively thinner and thus weaker, or coupling has increased along the decollement. |
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