Volcanomagnetic effect observed during the 1986 eruption of Izu- Oshima Volcano

Abstract

The remarkable changes in the total geomagnetic intensity associated with the 1986 eruption can be summarized as follows: 1) long-term precursor: decrease in the total intensity at one station on the southern slope of the central cone, Miharayama, continued since 1981; 2) middle-term precursor: the decrease there accelerated beginning in April, 1986; 3) short-term precursor: in October, 1986, an increase in the total intensity was observed at two stations on the north side of Miharayama, while readings at two southern stations decreased; 4) at the first stage of the Phase I eruption, step-like changes were observed at two southern stations; 5) a decrease in the total intensity was observed at two other stations outside the caldera at the time of the fissure eruption on November 21, and a few survey points and one station showed significant changes. These observations are interpreted and discussed. -from Authors