Construction of Evaluation System for Earthquake Response of Active Fault by Continuous Measurement of Radon Gas

为了连续、稳定地测量土壤气体中的氡浓度，我们采用电离室法，使用Alpha GUARD （gengenron Inc.）设备进行测量。通过该设备，可以每隔10分钟获得^<222>Rn浓度（Bq/m^3）、温度、大气压、湿度和数据质量等几种数据。选取了位于阿苏山西侧NNW-SSE向断裂上具有活火山孔的温泉区作为研究点。以0.5 l/mm的速率从1 m深的观测孔中吸入土壤气体。我们特别注意从吸入气体中去除水滴，并保持Alpha GUARD周围的温度和湿度几乎恒定。通过对1998年8月11日以来24419份实测资料的分析，发现氡浓度与气温、气压、湿度等天气因素的相关性普遍较低，氡浓度在0.4*10^5 Bq/m^3的日变化范围内变化较大。但氡浓度在一天内的平均值显示了可以用平滑曲线近似的时间变化。这可以看作是趋势成分。利用最大熵法对氡浓度变化进行谱分析，发现氡浓度变化具有24 h的优势期和12 h的弱期。这些周期与潮汐中两个主要元素的周期相对应。此外，发现氡浓度在火山地震活动频繁的时期大大增加。结果表明，地震引起的地下温度和压力扰动影响了氡浓度的变化趋势，而潮汐引起了氡浓度的短周期叠加变化趋势。

In order to measure the radon concentration in soil gas continuously and stably, we have adopted the ionization chamber method and used the equipment, Alpha GUARD (Genitron Inc.), for the measurement By this equipment, the several kinds of data such as ^<222>Rn concentration (Bq/m^3), temperature, atmospheric pressure, humidity, and data quality can be obtained at time interval of 10 minutes. A hot spring area with the active fumaroles, situated in the western side of Mt. Aso and on the NNW-SSE trending fault, was chosen as the study site. The soil gas was inhaled at the rate of 0.5 l/mm from the 1-m depth observation hole. We paid special attention to rem'oving water drops from the inhaled gas and maintaining the temperature and the humidity around the Alpha GUARD almost constant.By analyzing the 24,419 measurement data which have been taken from August 11, 1998, it was clarified that the correlation between the radon concentration and the weather factors such as the temperature, the atmospheric pressure, and the humidity were generally low and the radon concentration changed largely within the range of 0.4*10^5 Bq/m^3 in a day. But the average of radon concentrations in a day shows the temporal change that can be approximated by a smooth curve. This can be considered as trend component.As the result of spectral analysis using the maximum entropy method, the changes of radon concentrations were revealed to contain predominant period of 24 hours and weak period of 12 hours. These periods correspond with those of two major elements in the earth tide. Furthermore, the radon concentrations were found to increase largely on the periods of high volcanic seismicity. It can be concluded that the disturbance of subsurface temperature and pressure resulted from the earthquakes affects the trend of radon concentrations and the earth tide induces the concentration changes with short periods superimposed on the trend.