On the Time Series of the Seismic Activity

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  • 地震活動の時系列について
  • ジシン カツドウ ノ ジケイレツ ニ ツイテ

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Abstract

Time series of the ordinary seismic activity in Japan and the vicinity is statistically studied. The data are based on the hypocenter cards reported by JMA for the years 1961-1971. The data for the years with a large earthquake and an earthquake swarm were deleted so that the non-stationary process might not be introduced in the data. Under such a condition the monthly number of earthquakes fits the Poisson distribution P(n)=exp(-μ)μn/n! (μ is the mean value. n=0, 1, 2, …) in many regions in Japan and the vicinity. However the Poisson distribution can not be applied to the districts, Kumamoto, near Miyakejima, near Matsushiro where the earthquake swarm is apt to occur, and time series of the number of earthquakes from which aftershocks were not deleted completely do not fit well the Poisson distribution. The distribution of the number of earthquakes during the years 1961-1971 before the occurrence of Tokachioki earthquake (M=7.9, 16 May, 1968) fits the Poisson distribution in the rigion off Sanriku. In the ordinary seismic activity the monthly number of earthquakes is the highest at Boso Peninsula and the lowest at the districts facing the Sea of Japan.

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