Abstract
We have monitored water level of a hot spring in Ito City, radon concentration of hot spring water in Naka-Izu and temperature of fumarolic gas from Izu-Oshima Volcano. These parameters have changed prior to and during the seismic and volcanic activities starting on June 30, 1989, off the eastern coast of the Izu Peninsula. We summarized our observations together with other reported hydrological changes. The observed changes are classified into two types. One type is a co-seismic change which occurred at the times of bigger earthquakes among swarm activities. These changes were observed at several hot springs in the area 20-30km away from the Teisi erupting center. The other type is precursory changes observed in the area close to the erupting center and at the Izu-Oshima Volcano 40km away. The upward movement of magma due to possibly a dyke intrusion may disturb violently the hydrological systems in Ito City and Usami Town. Precursory changes in temperature of a fumarolic gas emitted from a well in the Izu-Oshima Volcano imply a possible linkage between Izu-Oshima and Teisi magma systems.
