Onshore-offshore seismic transect from the eastern Nankai Trough to central Japan crossing a zone of the Tokai slow slip event

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  • Kodaira Shuichi
    Institute for Research on Earth Evolution, Japan Agency Marine-Earth Science and Technology
  • Iidaka Takashi
    Earthquake Research Institute, University of Tokyo
  • Nakanishi Ayako
    Institute for Research on Earth Evolution, Japan Agency Marine-Earth Science and Technology
  • Park Jin-Oh
    Institute for Research on Earth Evolution, Japan Agency Marine-Earth Science and Technology
  • Iwasaki Takaya
    Earthquake Research Institute, University of Tokyo
  • Kaneda Yoshiyuki
    Institute for Research on Earth Evolution, Japan Agency Marine-Earth Science and Technology

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In the summer of 2001, an onshore-offshore integrated seismic survey was carried out along a 485 km long profile crossing the eastern Nankai Trough and central Japan from the western edge of the Izu island arc. Seismic velocity and reflectivity 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 reflective 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 reflective subducted oceanic crust overlaps with the high Poisson's 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 significantly extends the conditionally stable region at the top of the subducted crust.

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