Formation Pattern of Seismic Gaps before and after Large Earthquakes

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  • TANAKA Kenji
    Department of Earth Sciences, Faculty of Science, Kobe University

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  • 大地震発生前後における地震活動空白域の形成パターン
  • ダイジシン ハッセイ ゼンゴ ニ オケル ジシン カツドウ クウハクイキ ノ

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In this paper, study is made on the spatio-temporal variation of the seismic gaps associated with 15 major earthquakes (M≥7.0, depth≤80km) in and near Japan. As a result, we classify the occurrence of major earthquakes into the following three types according to the process of the formation of quiescence in both time and space.<br>(I) Trench type; Smaller gaps, whose radii are about tens of kilometers, appear around the epicenters of major earthquakes before the occurrences of main shocks and the final radii expand to some hundreds of kilometers around the epicenters. A expanding of quiescence seems to appear after the occurrence of the main shock in both time and space around the epicenter of the main shock.<br>(II) Hyuganada type; Earthquakes occur at random in both time and space before and after the main shock. Seismic gaps cannot be found.<br>(III) Inland and Japan Sea type; Seismicity around the large event is always quite so low that quiescence looks like to exist always before and after the main shock.<br>For the case of Trench type, we found that the seismic gaps rapidly expand to the larger area than the epicentral area just before the main shock. No systematic relationship is found between the precursory time of the rapid expansion of the seismic gaps and the magnitude of the main shock, however, the following relationship is found between the maximum radius of the quiescence and the magnitude of the main shock.<br>logR=0.33M-0.77 (M≥7.0;R[km]).

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