地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
1953年房総沖地震及び1972年八丈島東方沖地震前後の広域地震活動の変化
吉田 明夫高山 博之
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1994 年 103 巻 6 号 p. 696-705

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We show that seismic activity decreased clearly in a wide area before the 1953 Boso-oki earthquake and the 1972 Hechijojima-toho-oki earthquake. In both cases the seismic quiescence extended to the Tokyo Metropolitan area. Marking precursory activity appeared several years before the Boso-oki earthquake in the Chiba prefecture and in the sea region off-Ibaraki prefecture. It is also noted that the seismicity in the Kanto region as well as in the sea region south off-Boso Peninsula decreased conspicuously after the Boso-oki earthquake. On the contrary an increase of the seismicity was observed after the Hachijojima-toho-oki earthquake, although deep seismic activities decreased remarkably. We show that seismic quiescences preceding the Boso-oki earthquake and the Hachijojima-toho-oki earthquake were detectable before their occurrences, and no other statistically significant quiescence has appeared in the south off-Kanto region since 1926. It is proposed that the occurrence of a large earthquake may be predicted by monitoring changes of the seismicity in a wide area.

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