Abstract
An ocean-bottom seismographic network observation was performed around the northern part of the Japan trench, including the aftershock area of the 1968 Tokachi-oki earthquake (M=7.9). The method of observation is an anchored buoy system.
It is found that a high seismic activity of micro-earthquakes exists in the oceanic lithosphere of the east side of the axis of the Japan trench. This suggests that the oceanic lithosphere is capable of generating earthquakes. The seismicity in the oceanic lithosphere was about five times greater than in the island arc side in the summer of 1970. This will be interpreted that the post-seismic activity of the 1968 Tokachi-oki earthquake has migrated into the oceanic lithosphere crossing the Japan trench.
It is also found that a high micro-earthquake activity exists along the inner wall region of the Japan trench, implying the presence of a tectonic activity there. The depth distribution of these earthquake foci shows that the deep seismic zone extends within the upper part of the lithosphere.
