Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
Fig.17 Subducted ridge imaged from multi-channel seismic data at the Kumano-nada.
mated to be a maximum of ~.km high, ~-km
wide, and ~km long. Spatial mapping of the ridge
shows that it is located roughly at the seaward edge of
the coseismic rupture zone of the Tonankai
earthquake (M = .). This ridge appears to be in close
contact with the seaward end of the rigid backstop of
the Tonankai segment, and is located exactly beneath
the backstop of the Tokai segment. These spatial correlations
and the ridge-backstop collision geometry
suggest that the subducted ridge might be strongly
mechanically coupled and may thus play a significant
role as a seaward barrier inhibiting the earthquake
rupture from propagating farther seaward. We
propose that possible differential mechanical coupling
caused by the heterogeneous ridge-backstop collision
might help to create the rupture segmentation of the
eastern Nankai subduction zone.
We also successfully imaged subducted oceanic
crust beneath central Japan down to km depth by
processing onshore-offshore wide-angle seismic data,
which were acquired from the western edge of the Izu
island arc to the coast line of the Japan Sea (Fig.).
This image shows a –km high undulation at the top
of the subducted oceanic crust beneath central Japan.
This might indicate that the ridge subduction, mentioned
above, extends even deeper. The subducted top
of oceanic crust is clearly recognized as a high reflective
layer. This part of the subducted crust is imaged at
the same location where a high Poisson's ratio layer
was obtained by a previous tomographic study. We
propose that the highly reflective/high Poisson's ratio
structure might be an evidence of dehydration from
the subducted ridge system.
(ii) Micro-seismicity and aseismic-seismogenic transition
process at the subduction seismogenic zone
Off cape Muroto, fault area of the Nankai
earthquake, several micro earthquake clusters locate
Zenisu Ridge
Depth (km)
0
10
20
30
40
50
6
7
4
5
Island arc upper crust
Island arc lowe
6
4
6
3
2
7
Paleo Zenisu - north ridge
-
Zenisu - south ridge e-
Subducted ridge
Subducted
beneath central Japan ?
oceanic crust
1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5
0 50 100 150 200 250 300 350 400 450
Distance (km)
Fig.18 Seismic velocity image crossing the central Japan obtained from onshore-offshore seismic integrated seismic
survey. Lighter colored region indicates where no seismic ray sampled.
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