On the Secular Variation of Magnitude-Frequency Relation of Earthquakes (II)

The writer tried to find out whether any secular variations occur in the value of β in the expression of relation between the number N of earthquakes and their magnitude logΔ, log N=α-β logΔ, using, in place of magnitude M, the value logΔ, a common logarithm of the maximum felt distance of the earthquake.In the previous paper Regions A and B in Fig. 1 were treated, and in the present paper Regions C and D in same figure are treated.The maximum felt distances, Δ, of the earthquakes occurred in these regions were sorted out for the years 1925-1966 and were divided into four groups: Δ=150km (100km≤Δ<200km), 250km (200km≤Δ<300km), 350km (300km≤Δ<400km) and 450km (400km≤Δ<500km). With regard to the number N of earthquakes during the period of every seven years, the coefficients α and β in each region were determined by the method of least squares.The obtained values of α and β are shown in Table 3, together with their probable errors, and the variations of the value of β are shown in Figs. 3a (Region C) and 3b (Region D), respectively. From these diagrams it is certain that the value of β in each region shows secular variation as the previous results in regions A and B. On the abscissae of these diagrams the arrows show the years for which the annual seismic energy exceeded about 1022 ergs, and it is noticeable that release of great seismic energy took place when the value of β was becoming minimum.