噴煙映像を用いた火山灰噴出量の推定−浅間火山2003年2月6日噴火の噴煙解析−

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(3) . . . . . ,**. / -+ ,**/ - ++ . Evaluation of Total Ejected Ash in Volcanic Clouds Using Video Records : Application to the Eruption of Asama Volcano, Japan, on February 0, ,**Akihiko TERADA῍, Taketo SHIMANO῍῍, Sei IIJIMA῍῍῍ and Jun OIKAWA῍῍ From February to April ,**-, four small eruptions occurred in Asama volcano, central Japan. During these eruptions, movements of volcanic clouds were automatically recorded by video cameras. Based on these camera records, we studied the features of ascent processes of volcanic clouds. The volcanic cloud on 0 February ,**had mostly isolated symmetric shape and expanded linearly with height as it ascended. The squares of heights from virtual origin are proportional to time. These features are consistent with the characters of a thermal, which is well known from dimension analyses and experiments. To evaluate the masses of ejected ash we developed a simple model for volcanic clouds based on thermal assumption, and described the ascent velocity as a function of mean temperature and mass of ash in a volcanic cloud. In this model, a volcanic cloud is composed of ash and ideal gas, driven only by buoyancy. Any fall-out of particles was neglected. We compared the model with the video records of the volcanic cloud on 0 February ,**-, and deduced that the volcanic cloud had less than about .** ton of ash. We found new vents at the bottom of the main crater. Volume of the most prominent vent is comparable with the ash volume derived from our model. Chemical compositions of the products resemble those of Maekake stage, the youngest stage of Asama volcano, suggesting that the ash were derived from the shallow part of the volcanic edifice. It is interpreted that the small explosion occurred at the shallow part beneath the main crater. We propose that high temperature of ejecta was responsible for the formation of unusual volcanic cloud. Key words : Asama volcano, thermal, volcanic cloud, camera recordings, mass of ash. +ῌ ῐ ῎ ῑ ῏. J-!.7/12h#WHIîjklm-!./01. !"#$ ,**- , %& . '%()* . +,. 2nopqr,῍ ῍?@ s C ,**+ %&F. -!./0123 45&,-!$ 6!"#78'9:9. t&%^Du' v+,J-!,wxyzῌ{'|}. -!./01);2 +32* <=> ῍?@ AB C. 1 /0~yt&%'4$ vh, !",. +31/ DEFG%'HI.JK%L2. M῎,-!'. y e\##3. NOP QR,S/T-Uῌ,VW !XYZ[\]^. -!yzῌ{'^l,. _à^bc.deK5);2 ῍?@ Ufg ,**- a3. 1)!"-ῌ.&5 M'Q'(. *0*ῌ*2+* nopnn +* 2 ¡ nopW¢W¢£¢¤¥¦§¨QR!"¤¥de ©ªl Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, North +* West 2, Kita-ku, Sapporo *0*ῌ*2+*, Japan.. ++-ῌ**-, «¬®¬¯0 +ῌ+ῌ+ «¬W¢QR¤¥° Earthquake Research Institute, University of Tokyo,. +ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.. -23ῌ*++/ ± ²n³´µ¶·¸¹º» ++/+ῌ, Ufg¶·¸e¼° Karuizawa Weather Station, Japan Meteorological Agency, ++/+ῌ, Oiwake, Karuizawa, Kitasaku, Nagano -23ῌ*++/, Japan. Correspnding author : Akihiko Terada e-mail : [email protected].

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