High Accelerations in the Epicentral Area of the Western Nagano Prefecture Earthquake, 1984

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  • UMEDA Yasuhiro
    Abuyama Seismological Observatory, Faculty of Science, Kyoto University
  • KUROISO Akio
    Regional Center for Earthquake Prediction, Faculty of Science, Kyoto University
  • ITO Kiyoshi
    Regional Center for Earthquake Prediction, Faculty of Science, Kyoto University
  • IIO Yoshihisa
    Regional Center for Earthquake Prediction, Faculty of Science, Kyoto University
  • SAEKI Tatsuo
    Regional Center for Earthquake Prediction, Faculty of Science, Kyoto University

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Other Title
  • 1984年長野県西部地震による震央付近の大加速度
  • 1984ネン ナガノケン セイブ ジシン ニ ヨル シンオウ フキン ノ ダイ

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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.<br>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.<br>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.

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