近地地震のマグニチュード

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タイトル別名
  • Determination of Earthquake Magnitude at Regional Distance in and near Japan
  • キン チ ジシン ノ マグニチュード

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Several formulas to determine the magnitudes of earthquakes with shallow foci from amplitude and focal distance in the regional range up to about 1, 000km, were newly derived by using the data observed at the Abuyama Seismological Observatory and its array stations, and compared in the cases from displacement and velocity seismograms.<br>The patterns of maximum amplitudes versus focal distances show that the manner of decay of the displacement amplitude is practically similar to that of velocity amplitude in the distance range up to 200km, but markedly different in the range over 200km, because the phase of the seismic wave corresponding to the maximum amplitude varies from the body wave to the surface wave at the focal distance around 200km and the long period surface wave is sharply cut off with the steep slope of the frequency response curve of the velocity seismograph.<br>The amplitude-distance curves at the close distance range up to about 40km, however, are folded in the case of displacement amplitude. This phenomenon may arise from the effect of exitation of refracted or reflected, or both phases having longer periods, resulting in an increase of about 0.3 in the magnitude value. Taking no account of these slight folds, the decay of the maximum amplitude is supposed to be nearly uniform throughout the distance range concerned. Thus, the decay factor, including the geometrical spreading, is estimated as r-1.73, which is just the same as in the Tsuboi's (1954) formula.<br>The periods corresponding to the maximum amplitude were found to increase according to the earthquake sizes alone, when the same type of phase was traced. On the basis of this finding, an attempt was made to infer the relation between the source factor of displacement spectral density and the magnitude.

収録刊行物

  • 地震　第２輯

    地震　第２輯 24 (3), 189-200, 1971

    公益社団法人 日本地震学会

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