Deep-focus Earthquakes of Small Magnitude (II)

Results of Continuous Tripartite Observation at Matsushiro

Abstract

Selection of deep shocks is made by examining the seismogram form and by measuring the apparent velocity according to the method proposed in Chapter II. The number of deep shocks thus found, in and near Japan, the magnitude of which is less than 5, is 69 in July, October, November, December,1959 and January,1960. By comparing this number with the frequency of larger deep shocks, the following conclusion has been drawn: the rate of increase in frequency of small deep shocks with decreasing magnitude is significantly smaller than that of large deep shocks or shallow shocks. Aftershocks of deep shocks also seem to be lower in frequency than those of shallow shocks. In addition to the above facts, the following has also been concluded: (1) the direct waves from the origin are preserved in the part which is close to the onset, but the wave conversion takes place even just after the onset, and (2) the seismogram form is one of the most valuable seismometrical data. It should be noted that the present investigation has been made possible by the highly “ zweckmdssig ” observation. Chapter IV. Tripartite observation at Matsushiro