This paper deals with three aftershock sequences which occurred during IGY 1957/58. The frequency distributions of aftershocks with respect to magnitude and time are studied. These distributions fit eqs. (1) and (4) respectively with constants b, p, and c given in table 1 as shown in figs. 4, 5, 8, 9, 12, and 13. These figures also show the independence of magnitude-frequency relation on time and time-frequency relation on magnitude. The fluctuation of frequency in time-frequency distribution is nearly that which is to be expected in random sampling from a population whose frequency distribution is exactly expressed by equation (4). The geographical variation in the mode of aftershock occurrence in the aftershock area seems to be different from sequence to sequence. For the Aleutian sequence, the central part of the aftershock area is characterized by comparatively small energy release and small value of c in eq. (4), while for the southeastern Alaska sequence the greatest activity is concentrated near the epicenter of the main shock with no indication of geographical variation in magnitude- and time-frequency distributions.

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