Gas geochemical changes at mineral springs associated with the 1995 southern Hyogo earthquake (M = 7.2), Japan

Gas concentrations monitored at a mineral spring in central Japan over the past 15 years showed similar changes before and after two large earthquakes (September 1984, M = 6.9, 50 km distant: and January 1995, M = 7.2, 220 km distant); namely, a concurrent and abrupt drop in all three gas ratios, He/Ar, N sub(2)/Ar, and CH sub(4)/Ar, several months before the events. Improved measurements of gas compositions since 1991 and the addition of gas discharge rate monitoring since 1993 disclosed the following features for anomalies in response to small earthquakes as well as the large 1995 event: (1) the gas ratios increased for both large and small earthquakes, but they did not return to the preseismic values for the large event, as they did within a few hours for the small events; (2) at the 1995 event, the gas discharge rate decreased remarkably 3 h before the event and then increased to higher than preseismic level for several months. In contrast, coseismic rates at small events generally increased for less than 15 min; (3) during a seismically quiet period, from January 1993 to December 1994, the variation in the He/Ar ratio was well correlated with that of gas discharge rate. Such correlation disappeared in August and September, 1994, when the three gas ratios dropped before the large event; (4) gas compositions at an auxiliary monitoring site revealed a coseismic change similar to that described above for the 1995 event. These lines of evidence show that subsurface fluids may habitually respond to changes in physical conditions relating to earthquake occurrences, and thus suggest the usefulness of monitoring subsurface fluids for earthquake prediction.