秋田市茨島工業地帯周辺における地盤の振動特性

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Observations of short-period seismic noise were made togather information in ground vibration characteristics in Akita city, Akita prefecture, from 1986 to 1988. A three-component seismograph was used at 270 sites shown in Fig.5, and noise levels were observed to cover a large area. Contour maps of the survey area showing amplitudes in mkine indicate that a zone of anomalously high sesmic noise corresponds closely to the Barajima industrial area. The changes in amplitude and power spectrum were investigated along a survey line in the west side of Brajima. The dominant and stable frequencies of 4.00, 4.88 and 5.66 Hz arerecorded along the survey line. However, power levels of these frequencies decreased with increasing distances from Brajima. Array of vertical seismometers with 20 m spacing were also installed at four sites near BaraJlma. The results by the f-k analysis show that the directions of arrival of such frequencies corespond to Barajima. Therefore, it may be said that the seismic noise of these predominant frequencies are continuosly generated by factories on operation and are propagated to the sites of comparatively long distance. The f-k method also gives phase velocities which correspond to the theoretical dispersion curve of a model subsurface based on boring data. Since the observed values are consistent with the theoretical cureve, it follows that factory noise sources can effectivelly be used for estimating shallow velocity structures. However, it is pointed out that careful attention must be paid to source spectra of such noises in seismic zonation.
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秋田市茨島工業地帯周辺における地盤の振動特性

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