地震に伴う温泉ガスの挙動

Abstract

Mineral spring gas compositions at several sites in central Japan have been continuously monitored by our laboratory for the earthquake prediction study. In connection with it, a new system for the measurement of mineral spring gas flow rate has been installed at Byakko mineral spring, Miaunami, Gifu Prefecture, a momitoring site. The continuous observation with this system has been conducted since January, 1993. The main body of the gas flow rate meter is a vertically submerged plastic pipe of 4.1cm in diameter attached by pressure transducer. The mineral spring gases issuing from the bottom of the spring pool are collected into the pipe, the upper end of which is closed by an electric valve. As the sample gas is accumulated, the water level in the pipe becomes lower and the inner gas pressure becomes higher. After a fixed period, the inner pressure is measured with the pressure transducer and subsequently the accumulated gas is librated by opening the valve and thus the operation proceeds repeatedly for the continuous monitoring. The gas flow rate is calculated from the measured pressure. These operations including gas analyses are controlled by two personal computers. The obtained data are sent to our laboratory through the public communication line. Although observed gas flow rates considerably fluctuate, the flow rate on moving averages is positively correlated with He/Ar ratio in the gas. Thirteen earthquakes felt around the monitoring site occurred during this one year, and, at the events of the largest three seismic intensity, coseismic anomalies emerged in the gas flow rate. The feature described above suggests that the coseismic anormalies in the rate are caused by the bubbling at the earthquake tremor. On the other hand, it seems that the coseismic change of He/Ar ratio is not always caused by the bubbling. Although the cause of this change still remains unsolved, it is likely that the behavior of He/Ar ratio is controlled by the changes of the compressive stress in the crust.