1973年の浅間火山の噴火活動と小規模火砕流

Asama Volcano began an eruptive activity with an intense explosion on Feb. 2, 1973, after a duration of quiescence of more than 11 years from Nov. 1961. In Nov. 1973, red-hot lava was observed in the center of the crater-bottom. The amount of vapour and gas issuing from the crater-pit began to increase by degrees from the early days of Dec. 1972. The number of minor volcanic earthquakes felt by the seismographs set up on the mountainsides began to increase simultaneously. On Jan. 30 and 31, distinct glowing above the crater-pit was witnessed from the northern mountain slope, which showed that the new molten lava ascended filling over the crater-bottom. In the morning of Feb. 1, the number of minor volcanic earthquakes began to increase suddenly and the first outburst occurred at ni9hton Feb. 1. Since that the eruptive activity continued with some surges until Apr. 26. Maior outbursts occurred on Feb. 1, 6, 11, 14 and 15, March 10, Apr. 18 and 26. There was a remarkale surge of eruptive activity from Feb. 16 to 18, when minor ash eruptions took place successively with vigorous rumbling and roaring. The outburst on Apr. 26 was the last one which occurred during the period of euptive activity in 1973