Seismic noise measurements in the Takinoue (Kakkonda) geothermal area. 4 Relation between amplitude, distribution of dominant frequency and geothermal activity.:Relation between amplitude, distribution of dominant frequency and geothermal activity
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A seismic noise survey was carried out in the Takinoue (Kakkonda) geothermal area, Iwate prefecture, in 1985. 80 survey points are included in an area 6 Km wide along the EW direction and 3 Km long along the NS direction, centered in the Takinoue hot spring. The relation between noise amplitude, distribution of dominant frequency and geothermal activity was investigated.<BR> The instruments used for in-situ measurements consisted of three-component seismometers, amplifier and tape recorder. The system has flat response to ground velocity in the frequency range of 1 to 40Hz. The data were analyzed in the laboratory with a microcomputer. The original analog data were digitized at 100/sec. Fourier spectra of 2048 selected samples were computed using the fast Fourier transform. The results are summarized as follows.<BR> 1) The amplitude of the horizontal component is two or three times that of the vertical component, but the spectral patterns are similar to each other.<BR> 2) Large amplitudes of seismic noise are observed in an area centered in the recent development zone and surrounded by Shironuma, the point half-way of the mountaineering path of Mitsuishiyama, the entrance to Kakkonda forest road, and south Shirasawa. The frequency of 5-15 Hz is dominant in this area. On the other hand, very small amplitudes of seismic noise are distributed to the south-east side of the Takinoue tunnel, the entrance to mountaineering path of Hiragakuranuma, and the downstream of Kakkonda river. The seismic noise in the frequency band 0.8-0.5 Hz, which is dominant in this area, is considered to constitute the microseisms.<BR> 3) Activities of fumaroles and hot springs are recognized at the area showing large amplitudes. It seems that the zone of generation of seismic noise is shallower than the geothermal reservoir.<BR> 4) The seismic noise in the frequency band 2-15Hz is generated by the stream of steam and hot water going upwards from deeper reservoir to the surface.
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