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タイトル: 11. The Eruptive Activity of Mt. Asama from 1958 to 1961 and the Associated Minor Pyroclastic Flows.
その他のタイトル: 11. 昭和33年より昭和36年に至る間の浅間山の噴火活動と小規模火砕流
著者: MURAI, Isamu
HOSOYA, Yoshichi
著者(別言語): 村井, 勇
細谷, 与七
発行日: 1964年5月30日
出版者: 東京大学地震研究所
掲載誌情報: 東京大学地震研究所彙報. 第42冊第1号, 1964.5.30, pp. 203-236
抄録: Mt. Asama, one of the most frequently active volcanoes in Japan, began an eruptive activity in Oct., 1958, after a duration of quiescence of 40 months from July, 1955. The eruptive activity continued for the remainder of the year and the first half of the next year with an intermediate halt of two months. From the latter days of July, 1958, the fumarolic activity in the crater became gradually active, increasing in intensity through Aug. and Sept. The amount of vapor and gas issuing from the fumaroles at the bottom of the crater and the lower end of the crater-wall began to increase by degrees, being accompanied by a gradual increase of roaring and rumbling. The number of minor volcanic tremors which were felt by the seismographs set up around the volcano began to increase simultaneously. The ash-fall which might have been brought from the first eruption was observed on Oct. 3, 1958, on the eastern slope of the volcano. After that, the eruptive activity gradually increased in intensity with frequent occurrence of minor ash eruptions and puffings of smoke, being associated with a great increase of minor volcanic tremors. The activity culminated in an intense outburst on Nov. 11, which was considered to be one of the strongest explosions that had occurred for some decades. Moreover, the eruptive activity continued with some surges until the end of the year. There was a remarkable surge of explosive activity in,the early days of Dec., when several intensive outbursts and major ash eruptions took place successively. From the latter days of Dec., 1958, to the end of Feb., 1959, no eruption was observed and the volcano seemed to rest in a fairly calm state. On March 10,1959, however, the eruptive activity recurred with an intensive outburst. Afterwards, the eruptive activity continued with intermittent outbursts and minor ash eruptions. On April 14, there was a major outburst, the shooting out of a large amount of ejecta consisting almost entirety of bread-crust bombs. About the end of June and the middle of July, there were noteworthy surges of explosive activity, when several outbursts took place with the ejection of pumiceous materials. The ejection of such porous materials at these outbursts showed a striking contrast with the mode of each explosion occurring in 1958 which ejected dense materials of consolidated rock fragments and subordinate bread-crust bombs. The eruptive activity lasted until the latter days of Aug., 1959, when it died down to the ordinary fumarolic activity. From Sept., 1959, to the middle of Aug., 1961, the volcano had been in quiescence, only emitting white vapors from the crater. On Aug. 18, 1961, the volcano erupted suddenly with an intense explosion, and the eruptive activity continued until about the middle of Nov., 1961. Following this the volcano has rested in a calm state without any eruption up to the present(Dec., 1963). The explosion on Nov. 10, 1958, was the biggest among all which occurred in the period of eruptive activity from 1958 to 1961. As to the volume of solid materials ejected by the eruption on Nov. 10, 1958, it is considered to be the largest among all the explosions covering recent years. The total volume was calculated to be 3.6x 105 m3, which puts the explosion in the grade III of Tsuya's intensity scale of volcanic activity. The explosion on Aug. 18, 1961, ranks next to the explosion on Nov. 10, 1958, among the explosions from 1958 to 1961 on the intensity of explosion. The volume of the ejected materials was estimated to be 7×104m3, which puts the explosion in the grade II of Tsuya's intensity scale. These two explosions were the principal ones in the period of eruptive activity from 1958 to 1961. The other explosions were by comparison unimportant, being in the grade I of Tsuya's intensity scale, about one hundredth of the explosion of Nov. 10, 1958. Moreover, the two major explosions are noteworthy in that they were accompanied by the discharge of minor pyroclastic flows. The discharge of pyroclastic flow on this volcano was eyewitnessed for the first time in the eruption of Nov. 10, 1958, although the occurrence of much larger pyroclastic flows in the 1783 eruption of the volcano had been geologically proved. About 2.7×105m3 of incandescent fragmental materials were deposited inside the crater-wall of Maekake-yama as well as on the upper slope of it, occupying the larger part of the bulk of ejected materials from the eruption of Nov. 10, 1958.
URI: http://hdl.handle.net/2261/12154
ISSN: 00408972


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