Abstract
On the basis of the results obtained in our preceding papers, further details of spatial distribution of aftershock activities are investigated, and in addition to the principal ones already pointed out, the following characteristics are found:
(1) The spatial distribution of epicenters of aftershocks of the Niigata earthquake of 16 June 1964 which occurred in an intermediately active seismic region shows a rather strong tendency toward clustering within the aftershock area in the initial stage of aftershock sequence. In later stages of the sequence, however, aftershocks occurred spatially rather at random though, of course, they occurred within the concentrated area, that is, the “aftershock area” over a wider range ot regions.
(2) The spatial distribution of epicenters of aftershocks of the earthquake off Ibaragi Pref. of 16 January 1961 which occurred in a region of extremely active seismic region does not show any marked difference between the initial and the later stages of the sequence, and shows slight tendency toward clustering, but this clustering tendency is weaker than that of the spatial distribution of earthquake epicenters in the normal seismic activity in this region.
