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 500 m 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 -5.3 nT/year (-0.44 nT/month), amounting to a 27 nT 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 -2.2 nT/ month. At the time of the eruption, a very fast and significant variation was observed. The total intensity decreased 4 nT 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 7 nT was also observed at this time at a site 2 km southeast of the central cone.