B Brief Note on the Eruption of the Tokachi-dake Volcano of June 29 and 30, 1962

The activity of the volcano seems to have increased in intensity since 1952, when the fumarolic activity on the southern slope of the Suribachi crater recurred. In 1962, the fumarolic activity in the Taisho crater increased remarkably, and since late April preliminary earthquakes have occurred frequently in the area around the volcano. The first outburst of June 29, 1962, is considered to have consisted of steam explosion, which ejected rock debris from the southern rim of the Taisho crater. During the period prior to the outburst, the amount of ground water beneath the volcano had increased by the melting of snow due to the warm climate of May and June. This was suggested by the gush of muddy water at the foot of the eastern wall of the Taisho crater, on the southern slope of the Suribachi crater and on the western side of the central cone. It may be possible to consider that such filling of water in the ground might induce the steam explosion. After the first out- bursts of steam, a rapid rising of magma column followed resulting in the second eruption of June 30 and several days following, which consisted of intense discharge of ash and bombs with many accessory and accidental ejecta.十勝岳火山は大正15年(1926年)の大爆発以降しばらくの間噴火を続け,昭和3年(1928年)12月の小噴火を最後にして活動を休止していた.それ以後は新たに生じた爆裂火口(大正火口または新噴火口と名づけられた)内に多数の噴気孔が生じ,噴気活動を続けていた.昭和27年(1952年)に摺鉢山南側斜面上の古い噴気孔が活動を再び回始していらい(昭和火口または新々噴火口と呼ばれた),大正火口の噴気活動はしだいに活発化する傾向を示してきた.昭和37年(1962年)に入つて,大正火口の噴気は強まり,特に5月23日の広尾沖地震以来,その温度は上昇し,ガス含有量も増加してきた.また火口附近や山麓一帯で地震を頻繁に感じるようになつた.大正火口の南壁上に昭和30年(1955)年頃より亀裂を生じていたが,今回の噴火の直前にはその中から86℃の噴気が認められ,またその附近に新たにいくつもの亀裂が生じた,大正火口の噴気孔の噴気量は噴火直前にこれまでの3倍以上になつたといわれている