関東地方の火山フロント下のモホ面付近に発生する低周波地震

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Activity of low frequency (LF) earthquakes in the depth range from 25 to 50 km was investigated in central Japan, by using data obtained by the seismic network of National Research Institute for Earth Science and Disaster Prevention. A total of 15 LF events were found in five regions, southwest Fukushima Pref., north Tochigi Pref., west Tochigi Pref,. central Gunma Pref., and Izu-Oshima, from 1980 to 1992. All of the LF earthquakes are located along the volcanic front in the Kanto district. Predominant frequencies of the LF earthquakes, magnitudes of which range from 1.4 to 2.7, are 1 to 5 Hz, significantly lower than those of typical microearthquakes of the same magnitude range. The seismograms are dominated by clear P and S phases, S amplitudes being larger than P amplitudes at most of the stations. P waves of the LF earthquakes are often contaminated by high frequency waves at close stations. In most cases, more than two earthquakes successively occurred in a short time. Spectra of the LF earthquakes in southwest Fukushima, west Tochigi and Izu-Oshima exhibit a single strong peak at almost same frequency for P and S waves, while those in the other regions show more complicated structure, presumably reflecting complicated spectra at the sources. Although source process of the LF earthquakes are not modeled in this study, large amplitude ratios of P and S phases, which are estimated to be 5.6 in average, suggest a contribution of shear mode rather than tensile mode at the sources.
1993-12-20

関東地方の火山フロント下のモホ面付近に発生する低周波地震

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