NEW SCOPE OF JAPAN TRENCH AND NANKAI TROUGH: MIOCENE TO RECENT TRENCH WEDGE HISTORY AND 2011 TOHOKU-OKI EARTHQUAKE

The Japan trench and Nankai trough have been thought to be the best examples of subduction erosion type and an accretionary prism type trenches, respectively. More importantly, they were thought to be type examples of the Mariana type and Chilean type (without and with a large earthquake) margins, respectively. However, the 2011 Tohoku-oki Earthquake (Mw 9.0) along the Japan trench clearly shows that the previous classification was not the case but a new scope is needed for general understanding. The Tohoku-oki Earthquake recovered a back slip of 25 m at the hypocentral area (25 km depth) and much longer slip (50 m) at the trench area after 500 to 1000 years absence of the same scale subduction earthquake. A small, but typical, frontal prism of stacked thrusts is developed in front of a large listric normal fault submarine sliding body on the occasion of the great thrusting, as a result of the total trench slope stretching. By means of our submersible study, it has been known that in the upper part of this very frontal prism the lower trench slope is composed of Miocene diatomaceous, calcite cemented breccia, probably the exhumed Miocene accretionary prism. The mechanism of this exhumation is considered by surface gravitational collapse by frontal wasting as graben fill for tectonic (subduction) erosion. Also according to our submersible mapping on two submarine canyons which cut the total accretionary prism of the Nankai trough, the frontal part of the splay fault is composed of Quaternary beds, the present prism being as young as only 3 Ma. These two lines of recent researches indicate that the two trenches, Japan and Nankai, must be defined by a different way as repeated development of prism formation and erode type, in the latter case with large-scale sliding and collapsing of the toe area. The Miocene prism formation might be caused by trench outward proceed with massive sand supply to the trench, and the Quaternary collapse might be cause by volcanic ridge subduction in the Nankai, by large-scale thrusting which triggers a trench toe collapse. Thus the trench wedge is controlled by the total balance in island forearc, topographic and stress regime conditions, and this concept is in accordance with the wedge stability discussion in thrust belts.