Change of Seismic Activity in an Extensive Area before and afterthe 1953 Boso-oki Earthquake and the 1972 Hachijojima-toho-oki Earthquake.

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  • YOSHIDA Akio
    Seismology and Volcanology Research Department, Meteorological Research Institute
  • TAKAYAMA Hiroyuki
    Seismology and Volcanology Research Department, Meteorological Research Institute

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Other Title
  • 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
  • 1953ネン ボウソウオキ ジシン オヨビ 1972ネン ハチジョウジマ トウ

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Abstract

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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