著者

三浪 俊夫 久保寺 章 表 俊一郎 木下 保美

出版者

日本地熱学会

雑誌

日本地熱学会誌 (ISSN:03886735)

巻号頁・発行日

vol.3, no.1, pp.43-53, 1981-04-20 (Released:2010-02-05)

参考文献数

20

被引用文献数

1

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The area around the Kujyu volcanic mountains that is locating just in the central part of Kyushu Island is named as the Hohi geothermal area. In this region, the Ohtake and the Hatchobaru geothermal electric power stations are currently in operation. In this region, the micro-earthquake observation system has been set up providing six satelite stations connected to the Hatchobaru central station by public cable lines and giving seismograph records displayed on one sheet of paper in the central station. The operation of this micro-earthquake system has started July 1977 with highly increased time accuracy.Analysis of the seismic records for the last two years has yield following results.i) In these two years, about 200 foci of the earthquakes that occurred in this region have been located. All of these earthquakes gave the magnitudes smaller than M=3. These earthquakes could be classified into two groups. One was a group of earthquakes that might be related to the characteristic tectonics in the geothermal area, and the other were the earthquake that may be related either to the earthquake swarm activity in the northern part of the Aso caldera or the aftershock activity of the Oita earthquake of 1975.ii) The micro-earthquakes that are proper to the geothermal area were seen arranged in a belt zone along fault lines that are developed clearly in this region. In this belt zone swarm type microearthquake activities were observed showing the short duration period of swarm occurrence and shifting the swarm activity from one place to another.iii) For the purpose of obtaining more accurate focal locations, re-determination of the foci of all these swarm earthquakes were carried out by the aid of the improved master event method.As the result of which, it was made clear that foci distribution of each of these earthquake swarm was bounded in a narrow volume having the horizontal and vertial dimensions of 500 m x 2 km, extending almost vertically.iv) With regard to these earthquakes in the swarm, the study was carried out for making clear the earthquake occurrence mechanism of these earthquakes. It was found that earthquakes that occurred in the area near the Aso caldera, showed a mechanism represented by the right-lateral fault movement and for the earthquakes that took place along the geological fault zone, they showed a normal fault type mechanism.v) Within the low gravity area represented by the Kujyu caldera, locating in the north-eastern part of the Kujyu mountain range, almost any micro-earthquake activity is not observed.