日本列島弧の地熱場について

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There are many geothermal fields and active volcanoes in Japan. Many of the active volcanoes have, on the slope or at the foot, the so-called geothermal areas where superficial manifestations such as fumaroles, boiling springs and thermal springs are included. Ten of active volcanoes in Japan have summit fumaroles but no geothermal areas and six of them have neither geothermal area nor summit fumarole. Most of these volcanoes are simply stratified volcanoes with beautiful scenery. Such morphological characteristics suggest that volcanoes are young and have never suffered from strong erosions nor crustal movements which may destroy the volcanic edifice. Therefore, it is supposed that the stratified structures in these volcanic edifices were left intact. Conversely, it is supposed that stratified structures of volcanoes with geothermal areas were destroyed by strong erosions or crustal movements. The meteoric water infiltrating into the underground from such a destroyed surface is heated by the deep heat source and rises up to the surface and makes thermal manifestations. Total heat discharge from geothermal areas in Japan is estimated to be 3.5×10^8 cal/sec. Those from active volcanoes and hot springs are also estimated to be 10.1×10^8 cal/sec and 12.8×10^8 cal/sec, respectively. Total heat discharge from the Japan arc amounts to 1.2×10^<25> ergs/year, about 60% of which by the so-called terrestrial heat flow and about 40% by mass transfer such as volcanic eruption, steam and thermal water. Representative underground temperatures below geothermal areas in Japan are 150-175℃ at 500 meters deep, 200-225℃ at 1000 meters deep and 225-250℃ at 1500 meters deep, from many bottom hole temperature data. These temperatures are much higher than those calculated from the background heat flow values in geothermal areas. The high underground temperatures suggest the existence of the upward flow of thermal fluid. Based on a simple model, the upward velocity of thermal fluid is estimated to be about 1.0×10^<-7> cm/sec and the heat transferred by the flow about 25 to 30 HFU. As a reaction of the concentration of heat in a geothermal area, sometimes the heat fiow around the geothermal area would fall to less than the normal value. Therefore, if we estimate a thermal pattern of the earth on the supposition that a hydrothermal system exists, we have to take into account the heat transfer by a mass flow in addition to the conductive heat flow. A revised heat flow map was proposed, based on the above considerations. Thermal structures of the crust and upper mantle are discussed based on the heat flow and seismic wave attenuation data in the central Kyushu geothermal area, as an example. Seismic waves through the upper crust of central Kyushu attenuate more rapidly than those outside it. This fact may correspond to the existence of many water-filled fractures in the upper crust of central Kyushu. Perhaps the hot water bringing surface geothermal manifestations must circulate through such fractures. Uppermost mantle beneath central Kyushu has a very low Q value. Such a low Q zone agrees well with the high temperatures deduced from high heat flow values in central Kyushu.
特定非営利活動法人日本火山学会の論文
1981-10-01

日本列島弧の地熱場について

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