1991年ニアムラギラ火山の側噴火

ミコンベ (Mikombe)

抄録

An eruption began on September 20, 1991, at 1:30 a.m. (LT) from a fissure with the strike of N30°E at about 15km NE from the summit caldera of Nyamuragira after two years and a month's dormancy. A swarm of earthquakes for three days preceded the eruption by one week. The swarm activity gradually declined up to the eruption with changing the waveform of seismogram as lower frequency content. The beginning time of the eruption was clearly recognized by the appearence of continuous tremor on seismograph at BLG station located at 40km south from the eruption site. Lava fountain activity was continuing at the northern end of fissure and produced new cone and two lava flows. The new cone grew up 60-70m high with basal dimension of 400m×300m during a week. The lava flow extended about 6-7km in a NE direction along and over the old 1958, 1967 and 1980 lava flows. This activity was still vigorous on September 30 when we were compeled to leave from this region by a riot in Zaire. The new cone was called Mikombe (meant many bats). Total volume ejected by this eruption will reach the same order of previous ones judging from the out put rate for the first week. We can, therefore, conclude that this eruption lies on the continuation of the recent high active stage of this volcano which has begun from 1976 eruption. After that seven flank eruptions including new one took place on this volcano during 15 years. This occurrence frequency is 2-3 times higher than the previous period from 1900 to 1976.