著者

伊藤 潔 梅田 康弘 黒磯 章夫 村上 寛史 飯尾 能久

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.39, no.2, pp.301-311, 1986-06-25 (Released:2010-03-11)

参考文献数

16

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The source characteristics of the Japan Sea earthquake, May 26, 1983 (M=7.7) is inferred from the seismic observation system with wide-frequency and large-dynamic range at the Abuyama Seismological Observatory. The duration of oscillation of the long-period low-gain seismogram (T0=25s) is much longer than those of other earthquake with nearly the same magnitude and nearly the same epicentral distance, which implies that the earthquake is a multiple shock. The relationship between a multiple shock and duration of oscillation is more clearly indicated in the figure of double amplitude envelope to eliminate the difference in amplitude by magnitude and focal mechanism. This simple method is applicable to detect multiple shocks in seismograms at one station, especially in historical seismograms with a few instrumental records.Seismograms of the main shock of the Japan Sea earthquake recorded by Wiechert seismographs and those of middle-period (T0=10s) low-gain velocity seismographs show a clear onset of the second event at about 22 seconds after the first arrival. Since no such second arrival is seen on the seismograms of the aftershocks at the same station, the phase is not a crustal phase but a P-wave arrival of the second event of the main shock.The main shock recorded by the middle-period low-gain velocity seismograph contains more complicated high frequency waves than the largest aftershock. This indicates that the rupture process of the main shock is much complicated compared with that of the aftershock. Further, comparing the spectrum of the first event of the main shock with that of the second event, the average amplitude at a low frequency (5-10 s) of the first event is smaller than the second event, while that at high frequency (1-2s) is larger than the second event. This suggests that the main shock is composed of double events of different rupture type; the rupture of the first event is smaller and radiated much high frequency waves than the second event.

著者

梅田 康弘 黒磯 章夫 伊藤 潔 飯尾 能久 佐伯 龍男

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.39, no.2, pp.217-228, 1986-06-25 (Released:2010-03-11)

参考文献数

27

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Many boulders were thrown off out of their former sockets by the Western Nagano Prefecture Earthquake of September 14, 1984. Anomalous high accelerations of 5-30g, in the frequency range of 5-10Hz, are estimated from the displacement of these thrown out boulders.Almost all of the thrown out boulders were found on the tops, ridges and suddles of the mountains covered with the volcanic ash. The amplification effects by topography and soil sediment of the surface are estimated from the observation of the aftershocks recorded on the mountain-top, -foot and rock. The spectral ratios of seismic waves with the frequencies of 5-10Hz, namely mountain-top/-foot and soil/rock, are 2-7 and 2-10, respectively.The accelerations on the basement rock are obtained dividing the accelerations estimated on the mountain-tops. The high accelerations exceeding 1g distribute within a small area with a length of 3km and a width of 1km. This small area corresponds to the large dislocation portion of the assumed fault and the low active region of aftershocks.