Changes with Time in HO Gas Discharge Rate from the Fumarolic Area at Kuju Volcano

The discharge rate of HC1 gas from the fumarolic area at Kuju volcano had been estimated on the basis of distributions of Cl^- concentration in rainwater and precipitation. The rain collectors of open type were set at eleven locations. within 7 km from the main fumarolic area. The rainwater was collected and then analyzed for chemical, compositions at intervals of two weeks from March to December during 1993-1997. The HC1 discharge rate in 1995 was stable at about 2.0 t/d until, early in July, and then increased abruptly to 6.7 t/d late in July. This increase of the HC1 discharge rate can be a precursor of the phreatic eruption on 11 October 1995. The maximum discharge rates were observed from April to May in 1996. In this period, both seismic activities below the fumarolic area and the energy discharge from the new craters increased. The discharge rate of HC1 decreased from July. 1996 with some fluctuations.