VOLCANIC GAS CONCENTRATION AND AQUIFER PERMEABILITY ESTIMATED FROM TIDAL FLUCTUATIONS IN GROUNDWATER LEVEL - CASE OF KOSHIMIZU WELL IN IZU-OSHIMA, JAPAN

In order to understand the mechanism of material transport induced by seismic and volcanic activities, we analyzed chemical changes and perm eability variations based on field observations on a recently active v olcano, Izu-Oshima. From March to August 1988, when the volcanic and s eismic activities in Izu-Oshima declined following an episode of those activities that ended in late 1987, the concentrations of CO2 and H(2 )of volcanic origin increased 10-30 times above the previous levels i n the Koshimizu Well located 1 km from the coast. Analysis of tidal-fr equency groundwater level variations in the well shows changes in tida l response of the groundwater level that are inferred to be associated with changes in shallow formation permeability. A simple 1-dimensiona l diffusion model, together with reasonable assumption about the local hydrologic structure, suggest that the horizontal permeability in the upper few hundred meters of the island is high, of the order of 10(-8 ) m(2), and varied by as much as 20 percent during recorded period. We consider that these changes should be the results of vertical develop ment of cracks which was caused by hot water intrusion from magma.