1983 年 10 月三宅島噴火に関連した地磁気変化

概要

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After the eruption of Miyake-jima Volcano in Oct. 1983, magnetic observations were carried out by using proton magnetometers. At a reference point on the northern part of the island, about 3 nT increase of total intensity was observed during 3 months' period after the eruption. This implies expansion of the demagnetized area which is located at the central part of the volcano. Magnetic surveys conducted in Oct. 1983 and Feb. 1984 revealed a total field change well represented by a change in a dipole below the central cone, Mt. Oyama. By use of least squares method, this variation is explained by a change in the moment of a single magnetic dipole located below the north-western flank of Mt. Oyama at a depth of about 2.5 km below sea level. Since the variation occurred only in 2 months' period, we can hardly ascribe the cause of demagnetization to ordinary thermal conductance. A large amount of hydrothermal circulation may be its principal cause. At the northeastern tip of the fissure, geomagnetic measurements were repeated. About 10 nT decrease of total intensity was observed for one month. This decrease is explainable by cooling of surface ejecta as well as that of subsurface intrusion. Some portion of the observed change may be piezomagnetic origin due to closure of the fissure at depth which is associated with the drain back of magma at the post-eruption stage.
特定非営利活動法人日本火山学会の論文
1984-12-28