1990〜1995年雲仙岳噴火活動の予知と危機管理支援 : 第3回火山噴火予知シンポジウム

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The eruption of Unzen volcano began on November 17, 1990 and extrusion of dacite lava lasted from May 20, 1991, until February 1995. Pyroclastic flows occurred frequently from partial collapses of the growing lava dome and had caused severe damage to the local residents. The total volume of the erupted lava is estimated to be about 2×10^8 m^3. Although it was very difficult to predict the beginning day of the eruption, some precursory phenomena of each eruption stage could be recognized. From November 1989 on wards, active seismic area migrated from the western side of the volcano with a depth of about 15 km, where a magma reservoir had been proposed, to the summit area. Anomalous attenuation of seismic waves passing through the area beneath the summit was also observed. From July 1990, about four months before the first steam eruption, volcanic trernor began to be observed. Just before the initiation of the lava extrusion, bulging of the summit area was detected by tilt and EDM (electro-optical distance measuring insturument) observations. Also large demagnetizations were observed by proton magnetometers. Daily magma supply rate had been monitored based on the amplitude of long-period oscillations recorded by tiltmeter, which was very useful for evaluating the ongoing eruptive activity. Variation of SO_2 emission rate measured using COSPEC and the deformation of volcanic edifice measured using leveling, GPS and EDM were rather useful for the relatively long-term prediction of the activity. Twenty-four hours watching was carried out by GSDF (Ground Self Defence Forces: Japan Army) and SEVO (Shimabara Earthquake and Volcano Observatory). The information was immediately released to the local society. This system was practical for supporting the risk management conducted by the local governments and mitigating the volcanic disaster.
1997-03-07

1990〜1995年雲仙岳噴火活動の予知と危機管理支援 : 第3回火山噴火予知シンポジウム

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