It has been pointed out by many seismologists that the space, time and magnitude distributions of earthquakes should be combined with each other and be discussed from the unified and systematic viewpoint. Because of lack of good data, however, only a few studies on seismicity have dealt with these distributions this way.This author wants to study these distributions of earthquakes from the above-mentioned standpoint. This is the first step to this purpose. Aftershocks of great earthquakes give a considerable amount of good data.The followings are the main results of the present study:(1) In the initial stage of aftershock activity where most of the largest aftershocks occur successively on a line including the epicenter of a main shock, the distribution of shocks has the strong tendency of clustering in space and time.(2) In the later stage of aftershock activity, the distribution of shocks has the tendency of randomness in space and time. In addition, some related problems such as randomness and non-randomness of events in space and time is briefly mentioned.
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Norio Yamakawa (1968) studied this question.