Earth Planets Space, Vol. 57 (No. 10), pp. 943-959, 2005
Shuichi Kodaira1, Takashi Iidaka2, Ayako Nakanishi1, Jin-Oh Park1, Takaya Iwasaki2, and Yoshiyuki Kaneda1
1Institute for Research on Earth Evolution, Japan Agency Marine-Earth Science and Technology, Showa-mach 3173-25, Kanazawa-ku, Yokohama 236-0001, Japan
2Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan
(Received June 9, 2004; Revised February 19, 2005; Accepted May 17, 2005)
In the summer of 2001, an onshore-offshore integrated seismic survey was carried out along a 485 km long pro¥le crossing the eastern Nankai Trough and central Japan from the western edge of the Izu island arc. Seismic velocity and re¥ectivity images showongoing subduction of repeated ridges. Two ridges are clearly resolved, the Paleo Zenisu-south ridge and -north ridge located between the Nankai Trough and central Japan. The two subducting ridges are 12 km thick by 40 km wide, and 20 km thick by 60 km wide, respectively. We propose that the north ridge may have prevented the eastward propagation of co-seismic slip during the 1944 earthquake due to strong coupling. Highly re¥ective subducting oceanic crust is imaged at 25-45 km depth beneath central Japan, and has a 2 km high dome structure at 30-40 km depth, indicating an even deeper ridge structure. This highly re¥ective subducted oceanic crust overlaps with the high Poissonfs ratio zone imaged in a seismic tomography study. From these relations, we propose that the Tokai silent slip can be attributed to a high pore pressure zone, which signi¥cantly extends the conditionally stable region at the top of the subducted crust.
Key words: Nankai Trough, seismic imaging, subduction zone, slowslip, earthquake.