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.
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.
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.
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.
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.
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.

View full abstract

An anion index, AI, is proposed as an indicator to estimate the proximity to the center of geothermal activity. AI, defined as following, is calculated from major anion composition of a hot spring water.AI=0.5×(SO₄/Cl+SO₄+Cl+SO₄/Cl+SO₄+HCO₃) (unit: equivalents) AI is unity at the steaming ground where geothermal activity is highest. It gradually decreases with increasing a distance from the center of activity, with the appearance of Cl type thermal water, and with mixing of HC0₃ type water. AI is recommended as a practical qualitative indicator presenting degree of closeness to the center of geothermal activity, but should not be applied to thermal water from deeper wells, to volcanic fumaroles emitting HCl, or to springs originated from sea water.

View full abstract