ANOMALOUS HOT-SPRING WATER CHANGES - POSSIBLE PRECURSORS OF THE 1989 VOLCANIC-ERUPTION OFF THE EAST-COAST OF THE IZU PENINSULA

Geochemical and hydrological observations have been carried out at two hot spring wells in Ito City since July 1989. Anomalous changes in water level, temperature and chemical composition were observed during the 1989 seismic and volcanic events off the east coast of the Izu Peninsula: a water level rise of more than 2 m, a temperature rise of about 1-degrees-C and an increase of the SO42-/Cl- ratio by about 5%. Coseismic changes in these parameters were observed for two nearby earthquakes that occurred during the observation period, on February 20, 1990 (M = 6.5), and on August 5, 1990 (M = 5.1), supporting that these wells are very sensitive to earthquake occurrence. Relation between amplitude of coseismic change, magnitude and hypocentral distance indicates that the anomalous changes observed during the 1989 events were too large to be caused by the seismic events only. It is likely that magma movements associated with the seismic and volcanic events had a close connection with the geochemical and hydrological anomalies observed during the 1989 events. Major part of the significant anomalies including self-spouting observed in Ito City are interpreted to be possible precursors of the 1989 submarine volcanic eruption.