Changes in Crustal Strain and Hot Spring Temperature as Related to Volcanic Eruptions.

Abstract

Variations in the crustal strain at two stations and water temperatures at four hot springs have been continuously monitored in the Izu Peninsula. For the 1986 Izu-Oshima eruption, the temporal . variations of a strain meter at Yugashima showed a good correlation with the cumulative discharge of magma from the summit crater of the volcano. The strain meters of all components at two stations showed the extension during the fissure eruption of November 21. The temperature variations at Shimosagano hot spring had a fairly good correlation with the cumulative discharge of the eruption. The hot spring temperature at Yugano abruptly dropped 25 days before the fissure eruption. For the 1989 Ito-oki swarm earthquake, the strain meters at two stations showed an extension or contraction. The hot spring temperature at Shimosagano increased in conjunction with this earthquake sequence. The hot spring temperature at Yugano abruptly dropped 2.3 days before the 1989 Ito-oki eruption. On the other hand, neither strain nor hot spring temperature changed in the case of the 1989 Ito-oki eruption. The extensions observed in the Izu Peninsula at the time of this period of the 1986 Izu-Oshima eruption can be explained by a decrease in the magma pressure. Therefore, no strain change during the 1989 Ito-old eruptions suggests that the amount of discharge might have been so little that the magma pressure was scarcely reduced at the source. Although the abrupt changes in hot spring temperature were not directly correlated with the eruptions, they may rapidly reflect movements of the magma.