Abstract
Mineral spring gas compositions at several sites in central Japan have been continuously monitored in our laboratory for the earthquake prediction study since 1979. The mineral spring gas flow rate has been also monitored since 1993. Southern Hyogo prefecture earthquake occurred on January 17, 1995. At this event, we observed preseismic and coseismic anomalous behavior of mineral spring gases at Byakko spa, a monitoring site of ours, 220km away from the epicenter. The gas flow rate decreased remarkably and rapidly 3 hours before the event. This decrease may be attributed to a decline of water flow rate due to a preseismic crustal deformation at the site. Three gas ratios, He/Ar, N2/Ar and CH4/Ar, conspicuously increased after the event. The ratios of N2/Ar and He/Ar began to increase one or two days before the event, while CH4/Ar did at the occurrence time of the event. These changes can be interpreted as the result of changes of a mixing process of the gases dissolved in ground water. The present observation indicates the efficacy of monitoring of subsurface fluids for earthquake prediction.
