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called the Sanriku Escarpment (Fig. 4 ), forms the boundary between the lowermost
toe and the upper trench wall. This escarpment has been interpreted as the scarp
of a large thrust fault (e.g., von Huene and Culotta, 1989 ). Taira and Ogawa
( 1991 ), however, showed that the upper part of the escarpment is characterized by
large-scale submarine sliding caused by slope instability, which is later to be
known further controlled by thrust faulting (Iwabuchi et al., 1996 ; Sasaki, 2004 )
as shown below.
Following Iwabuchi et al. ( 1996 ) and Ogawa et al. ( 1996 ), Sasaki ( 2004 ) sum-
marized the mechanisms generating the topographical features of the Japan trench
landward slope, attributing them to erosional subduction zone tectonics. According
to Sasaki ( 2004 ), the occurrence of many thrust faults within both the upper and
lower slope has resulted in the formation of a steep, unstable slope prone to land-
sliding. The rocks from the slope are fairly old. The traces of most of these thrust
faults along the slope are characterized by abundant chemosynthetic biocommuni-
ties, dominated by Calyptogena , supported by methane-bearing fluids (Ogawa
et al. 1996 ).
The submersible SHINKAI 6500 (JAMSTEC) made several dives at the foot
of the Sanriku Escarpment and further upslope (Fig. 4 ). The assemblages of
fossil diatoms in rock and sediment samples collected by the submersible from
the landward slope, identified by Itaru Koizumi, were of middle Miocene to
Fig. 4 Multi-narrow-beam swath map of the area around the Sanriku escarpment, off Miyako at
40°N (contour interval 20 m) (Adapted from Ogawa et al. ( 1996 ))
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