WHAT CONTROLS THE LATERAL VARIATION OF LARGE EARTHQUAKE OCCURRENCE ALONG THE JAPAN TRENCH

The lateral (along trench axis) variation in the mode of large earthqu ake occurrence near the northern Japan Trench is explained by the vari ation in surface roughness of the subducting plate. The surface roughn ess of the ocean bottom near the trench is well correlated with the la rge-earthquake occurrence. The region where the ocean bottom is smooth is correlated with 'typical' large underthrust earthquakes (e.g. the 1968 Tokachi-oki event) in the deeper part of the seismogenic plate in terface, and there are no earthquakes in the shallow part (aseismic zo ne). The region where the ocean bottom is rough (well-developed horst and graben structure) is correlated with large normal faulting earthqu akes (e.g. the 1933 Sanriku event) in the outer-rise region, and large tsunami earthquakes (e.g. the 1896 Sanriku event) in the shallow regi on of the plate interface zone. In the smooth surface region, the cohe rent metamorphosed sediments form a homogeneous, large and strong cont act zone between the plates. The rupture of this large strong contact causes great underthrust earthquakes. In the rough surface region, lar ge outer-rise earthquakes enhance the well-developed horst and grabens . As these structure are subducted with sediments in the graben part, the horsts create enough contact with the overriding block to cause an earthquake in the shallow part of the interface zone, and this earthq uake is likely to be a tsunami earthquake. When the horst and graben s tructure is further subducted, many small strong contacts between the plates are formed, and they can cause only small underthrust earthquak es.