Erosional Subduction Zone in the Northern Japan Trench: Review of Submersible Dive Reports - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 42

144 00 E
39 30 N
Crack site
Epicenter of 1933
Sanriku-oki Earthquake
20 km
Fig. 2 Multi-narrow-beam swath map around the dive sites at 39°30¢N off Miyako ( top ; contour
interval, 50 m). Topographic section made by a 3.5-kHz subbottom profiler along 39°30¢N ( bottom ).
Vertical exaggeration ×60. Notice that the slope angle differs depending on the order. The ocean-
ward slope toe gradients are 2.6°, 5.1°, and around 20°, increasing as the order decreases (Revised
from Ogawa and Kobayashi ( 1994 ))
Many open cracks could be seen from the submersible windows (Fig. 3 ) (Ogawa
et al. 1997 ). Ogawa et al. ( 1997 ) reported similar cracks in the Mariana trench, and
they concluded that the open cracks resulted from shaking caused by large earth-
quakes occurring beneath the oceanward slope of the trench. This area of the Japan
trench includes the epicenter of the 1933 Sanriku-oki earthquake (M = 8.1;