Accurate Determination of Source Depths and Focal Mechanisms of Shallow Earthquakes Occurring at the Junction between the

We study twenty shallow earthquakes (mb~6) occurring at the junction of the Kurile and the Japan trenches for the period from 1964 to 1983 in order to understand the behavior of the oceanic and continental plate beneath the trench-arc system. Improved focal mechanisms and source depths of these events are simultaneously determined by the least square waveform inversion technique. The results show that the thrust faulting type is predominant throughout the region studied. These thrust events can be considered to be the interplate underthrust earthquakes between the downgoing slab and the overriding plate according to the following characteristics: their mechanisms are the same as that of the 1968 Tokachi·Oki earthquake; their slip vectors coincide with the motion direction of the Pacific p.\ate; the depths of these events increase systematically with the distance between the epicenter of event and the trench axis; and the spatial distribution of these events from the trench axis continues smoothly to the upper envelope of the intermediate seismic zone in this region. The location of the thrust events represents the top of the downgoing slab