Abstract
Relations between activity of volcanic earthquakes in Izu-Oshima volcano and stress and/or strain state around it are investigated. In this paper three kinds of relations are examined during and after the 1986 eruption of Izu-Oshima volcano : (1) How did big earthquakes affect on the volcanic activity of Izu-Oshima? (2) How did the pressure decrease of magma reservoir induce earthquakes around it. (3) How did the dike intrusion induce big earthquakes. The results are as follows : (1) Nine earthquakes of M＞5.0 occurred from 1987 through 1990 within the epicentral distance of 100 km from Izu-Oshima volcano. They gave influences on the activity of volcanic earthquakes and/or volcanic tremor when they exerted normal strain of over 10-8 on the vertical plane trending N30W. (2) Swarm-like seismic activity in north and western part of Izu-Oshima volcano during the summit eruption is interpreted as an induced swarm due to pressure decrease of magma reservoir. The earthquake swarms occurred both sides of the magma reservoir whose location is estimated from the data of crustal movement. The location of two-lobed earthquake swarm is well explained by a simple model ; infinite two-dimensional elastic medium under differential stress with a circular hole. (3) Large earthquakes accompanied the dike intrusion of 1986 eruption of Izu-Oshima. Two biggest earthquakes occurred at both ends of the dike. A M 5.1 event occurred at the northwestern end and M 6.0 occurred at the southeastern end. Both events may played a role to stop the further propagation of dike intrusion.
