雲仙火山噴火にともなう地磁気変化(1991-1993)

Abstract

The summit eruptions of Unzen volcano started at J and K craters on November 17, 1990, after 198 years quiescence. The volcanic activity intensified at B crater, about 150m west of J, on February 12, 1991, and a new dome was extruded at J on May 20, 1991. Twelve domes were extruded until March 1994. Preceding the major volcanic activity, a geomagnetic change of about 20 nT was detected by repeated surveys at the summit during the period from 1986 to the end of 1990. To elucidate the cause of the geomagnetic change, closely placed and continuous geomagnetic observations with proton magnetometers have been conducted in the vicinity of the summit of the volcano since January 1991. In association with the volcanic activity at B, geomagnetic changes amounting to about 18 and -5 nT were observed at close points in the northern and southern directions of B, respectively, from February to March 1991.During the early stages of the new dome extrusion, large geomagnetic changes of more than 80 nT were observed at several points near J from May 12 to June 3, 1991. The changing rate of about 5 nT/day at mid-May 1991 decreased exponentially to about 0.1 nT/day by the end of 1991. The total change reached more than 160 nT at three points closer than 400m from J from early March 1991 to the end of June 1992. The relations between the geomagnetic changes and seismic activities or ejection rates of lava dome, resulted that thermal magnetic effect dominated on the Unzen volcano.