Deep water circulation with reference to geothermal phenomena in Chugoku and Shikoku districts, Western Japan

日本西部三仁和三洋地区的温泉和地震有明显的对比。三隐地区有很多超过50℃（最高可达93℃）的高温温泉流出，而三约地区没有超过50℃的高温温泉。三隐地区地震频发，三约地区地震极少。这些对比可能是由于构造条件造成的。三隐地区的高温温泉可能是由于花岗岩基底断裂体系中大气水的深层循环所致，而三约地区由于花岗岩基底断裂程度较差，可能没有深部水循环。也就是说，在三音地区，可能通过高海拔Chugoku山脉产生的水力位势梯度的裂缝体系进行大规模的深水循环。为明确地温现象与基底条件的关系，对三隐和三约地区的温泉水、冷泉、地下水和河水进行了同位素研究。бD和<80>的大气水，如冷泉水、地下水和河水显示出所谓的“高度效应”。而三音地区高温水的бD和<80>与大气水相比海拔相对较低，与海拔500 m以上的大气水相对应。这表明，三音地区的温泉水是通过大气水的深层循环，从海拔1000米以上的Chugoku山脉中获得的。另一方面，三约地区1000 m以上深孔热水的бD和<80>值明显低于该地区大气水的下限бD和<80>。这表明三约地区深层热水可能不是现在的大气水，而可能是化石水，表明三约地区不可能发生深层水循环。因此，地热现象与基底一定程度的裂缝和水力条件（如高山的存在）密切相关。地热现象与地震学之间的良好关系可以认为是由于基底结构与大型应力场有关。少

There are clear contrasts in hot springs and earthquakes between San'in and San'yo areas, western Japan. Many hot springs of high temperature more than 50℃(up to 93℃) flow out in San'in area while no high temperature springs more than 50 can be seen in San'yo area. Earthquakes occur frequently in San'in area, while very few earthquakes occur in San'yo area. These contrasts may be owing to tectonic conditions. High temperature springs in San'in area may be due to deep circulations of meteoric water through fractured system in granite basement, while no deep water circulation may occur in San'yo area because the granite basement may be poorly fractured. In San'in area, in other words, deep water circulation in a large scale may occur through fractured system with hydraulic 'potential gradient produced by Chugoku Mountains of high altitude. To clarify the relationship between the geothermal phenomena and the basement condition, isotopic study was made for hot spring waters, cold spring wa … More ters, groundwaters and river waters collected in San'in and San'yo areas.бD and б^<80> of meteoric waters such as cold spring waters, groundwaters and river waters show a so-called "altitude effect". бD and б^<80> in high temperature water from San'in area, on the other hand, show relatively low as their altitude compared with meteoric waters, corresponding to those of meteoric waters at elevations higher than about 500 m. This suggests that hot spring waters in the San'in area are derived from Chugoku Mountains having altitudes of more then 1,000 m above sea level through deep circulations of meteoric water.On the other hand, бD and б^<80> values of hot waters from deep bore holes more than 1,000 m deep in San'yo area are clearly lower than the lower limits of бD and б^<80> of meteoric water in the areas. This indicates that the deep thermal waters from San'yo area may not be the present meteoric water' but might be fossil water, suggesting that no deep water circulation may occur in San'yo area.Consequently, geothermal phenomena are related closely to a degree of fractures in the basement and to a hydraulic condition such as the existence of high mountains. A good relationship between geothermal phenomena and seismology can be considered to be owing to the basement structure related to a stress field of a large scale. Less