長野県西部地震に伴う温泉ガス, 火山ガスの前兆的組成変動

―地殻歪および岩石破壊と地下ガス組成変動―

抄録

Remarkable anomalies in gas compositions were observed at a fumarole and three mineral springs before the Western Nagano Prefecture Earthquake, 1984. The epicentral distance are 9 km for the fumarole and 50 km, 71 km and 95 km for the springs, respectively. Deep seated gas emission induced by the compressive stress of the earth tide had been observed previously at the two mineral springs as the monitoring stations and at a lava lake in Hawaii. It is analogically inferred that the gas anomaly before the earthquake can be attributed to the variation in the emission rate of deep seated gas resulting from changes in pore pressure and magma-reservior volume, which are subjected to the tectonic stress that caused the earthquake. Another aspect is emissions of H₂ gas at three mineral springs and gases from different sources at one spring. These indicate the formation of new cracks near monitoring stations and the permeation of groundwater into the cracks. The mechanical processes inferred from the present geochemical observation are compatible with those described by geophysical models based on fracturing experiments.