This paper consists of Part I and Part II. In part I, the measurements of the depth and the surface-and vertical-water temperature distributions which were carried out by means of the boat-borne infrared radiation thermometer, echo-sounding type fish detector and the thermistor thermometer, respectively, are described of crater lake Yugama, Volcano Kusatsu-Shirane, on the three days in the summer of 1978. The results show (1) the conical shape with 14.6 deg of the slope and 34.4±1.4m of the maximum depth, (2) 21.1°C of the mean surface water temperature with one high and one low regions which are considered in relation with upwelling of hot spring water and the slope of the lake bottom and (3) 19.6 t of the mean water body temperature with 20.8 to 23.4°C of the lake bottom temperature which suggests heat supply from the bottom to the overlaid water body. In Part II, to be published in not-so-distant issue, water and heat budgets of the lake will be discussed historically, based on the data presented here in connection with many morphometrical and thermometrical data in the previous papers.

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Comparison of the experimental and the two dimensional numerical results are presented for steady state natural convection in an opened model reservoir. In the region of the Rayleigh number of 0∼240, the water discharge from the top surface of the reservoir increases monotonically with the Rayleigh number but the pattern of convection current does not changed. The numerical results agree well qualitativelly with the experimental results.

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It is very important to grasp the flow conditions of geothermal fluid at the well inlet in order to investigate physical properties of the reservoir and consequently establish a technique for forecasting changes of the well output. However, at present, the data of temperature and pressure logs in discharging wells of the Otake and the Hatchobaru geothermal areas are rather rare. Almost all of the data are restricted to the ground surface measurements. In this paper, the relation between the fluid pressure, P₁, and quality, x₁, at the well inlet, and the saturated pressure at the flashing inception level, P'₀, and variable well head pressures are calculated by means of the flow rate of steam-water mixture and the pressure measured at the well head, and also by the momentum equation. A plot of P₁, and P₀ for x₁>∼0.1 (mean values of P₁, P₀ and x₁, respectively) against the well inlet depth defines a region which, to some extent, can be used for the availability of the producttion well. The rise and fall of the flow characteristics of the production well is forecast by taking into account the change of the flow conditions at the well inlet with time.

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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.

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