HR: 10:35h
AN: S42B-02 INVITED     [Abstracts]
TI: Causes of co-seismic rupture segmentation and inter-seismic slow slip in the Nankai seismogenic zone revealed from active source seismic studies
AU: * Kodaira, S
EM: kodaira@jamstec.go.jp
AF: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3175-25, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Park, J
EM: jopark@jamstec.go.jp
AF: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3175-25, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Nakanishi, A
EM: ann@jamstec.go.jp
AF: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3175-25, Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3175-25, Kanazawa-ku, Yokohama, 236-0001 Japan
AB: In order to obtain structures affected on co-seismic and inter-seismic processes in the Nankai seismogenic zone, we have carried out a series of active source seismic surveys since 1997. The most important finding from the surveys is a ridge and seamount subduction preventing the rupture propagation of the last 1944 Tonankai and the 1946 Nankai mega-thrust earthquakes. Remaining issues to be solved by seismic surveys are causes of a slow slip at a deeper part of the seismogenic zone and a structural factor controlling the segmentation boundary between the Tonankai and Nankai earthquakes. In this paper we present results of recent active source seismic studies to address those questions. Data from an onshore-offshore integrated seismic survey was acquired along a 485 km long profile crossing the eastern Nankai Trough and central Japan from the western edge of the Izu island arc. The observed inter-seismic slow slip event is situated at the middle of the profile. Highly reflective subducting oceanic crust is imaged from those data at 25 - 45 km depth beneath central Japan, and shows a 2 km high doming structure at 30 - 40 km depth, indicating a subducted ridge structure. This highly reflective subducted crust overlaps with the high Poisson_fs ratio zone imaged in a seismic tomography study. We propose, from these structures, that the slow slip can be attributed to a high pore pressure zone, which significantly extends the conditionally stable region in the subducted crust. The most recent wide-angle seismic data for imaging the segmentation boundary between the 1944 Tonankai and 1946 Nankai earthquake were acquired in January to February, 2004 along three margin parallel profiles (150 _E190 km long). We found that a low velocity uppermost mantle underlying a thin subducted oceanic crust at the segmentation boundary. This structure is interpreted as a subducted fracture zone. The low velocity uppermost mantle may be attributed by serpentinized mantle, which is created by a hydration reaction along the fracture zone.
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting