Geomagnetic variations observed after the 1986 eruption of Izu-Oshima Volcano.

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Abstract
After the 1986 eruption of Izu-Oshima Volcano, proton magnetometers were installed at various sites in the island. Most of the sites started measurements by March, 1987. During the period from March to November 15, the day before the 1987 gas explosion of the volcano, large magnetic variations in total intensity were observed at the sites located on the caldera floor. The variations can be divided into three stages, March-June, July-August, and September-November. The first stage of the variations can be attributed to after-effects of the 1986 eruption, and the last stage could be a precursory change to the 1987 gas explosion. The location of the source region for each stage of the variation was estimated based on the spatial distribution of the rate of the variation. The center of the source region for the first stage was estimated to be located about 400m below the caldera floor under the pit crater. The source for the last stage is under A crater at a depth of 200m below the caldera floor. The direction of the magnetic moments responsible for the variations is anti-parallel to the geomagnetic field. Hence, the thermal demagnetization of the remanence of the rocks is a possible cause of the variation.

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