Changes in the geomagnetic total intensity observed before the eruption of Oshima Volcano in 1986.

Abstract

Before the eruption of Oshima Volcano on November 15, 1986, 5 proton magnetometers were operating on the volcano. One of them, installed about 500m south from the center of the central crater, recorded anomalous changes in the total intensity preceding the eruption. The anomalous variation began around 1981. The total intensity decreased gradually at a rate of -5.3nT/year (-0.44nT/month), amounting to a 27nT decrease in 5 years. The decrease of the total intensity accelerated about six months before the eruption. The rate of decrease was estimated to be about -2.2nT/month. At the time of the eruption, a very fast and significant variation was observed. The total intensity decreased 4nT in one minute. Following the eruption from the central cone, fissure eruptions took place on November 21. A decrease in the total intensity of about 7nT was also observed at this time at a site 2km southeast of the central cone.The observed magnetic changes preceding the eruption are supposed to have been caused by the volcanic body being heated during the intrusion of magma, at the initial stage through the main vent, and at the later stage through a new path toward the newly formed crater, called A crater. The variation at the time of eruption is considered due to the disorientation of magnetic materials by the catastrophic outflow of gases and liquids through pores and cracks inside the volcano.