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Mass Estimation and Characteristics of Ejecta from the ,**. Eruption of Asama Volcano
Mitsuhiro YOSHIMOTOῌ, Taketo SHIMANOῌ,ῌῌῌῌῌῌῌ, Setsuya NAKADAῌ, Etsuro KOYAMAῌ,Hiroshi TSUJIῌ, Akiko IIDAῌ, Masaru KUROKAWAῌ, Yuko OKAYAMAῌ,ῌῌῌῌῌῌῌῌ, Miyuki NONAKAῌ,Takayuki KANEKOῌ, Hideo HOSHIZUMIῌῌ, Yoshihiro ISHIZUKAῌῌ, Ryuta FURUKAWAῌῌ,Kenji NOGAMIῌῌῌ, Shinya ONIZAWAῌῌῌ, Kenji NIIHORIῌῌῌῌ,ῌ,Takeshi SUGIMOTOῌῌῌῌῌ and Masashi NAGAIῌῌῌῌῌῌ
After -+ years of dormancy, magmatic eruption started at Asama volcano on + September ,**.. Five major
vulcanian explosions, intermittent strombolian explosions and many small-scale explosions were observed from
September to December. The initial explosion on + September was the largest explosion and the volcanic ash
a#ected areas up to ,/* km northeast of the volcano. The maximum grain size and the weight of ash collectedabout . km NE of the vent were 30 mm and +,*** g/m,, respectively. Ash from the strombolian explosions during+0ῌ+1 September covered most of Karuizawa Town, including areas SE and E of the volcano. Ash fall also
a#ected the Tokyo District on the night of +0 September. The main axes of ash fall on ,-, ,/, and ,3 September,+* October, and +. November were dispersed toward NNE to NE, NE, N to NNE, NE, and E, respectively. The
distribution of ash fall deposit was thought to be inﬂuenced by wind direction and velocity. Ballistics from these
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Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
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Institute of Geology and Geoinformation, National
Institute of Advanced Industrial Science and Technology, +ῌ+ῌ+, Tsukuba, Ibaraki -*/ῌ2/0+, Japan.
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Volcanic Fluid Research Center, Tokyo Institute ofTechnology, Kusatsu 0.+ῌ-0, Agatsuma, Gunma -1+ῌ+1++, Japan.
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Department of Earth Sciences, Faculty of Science,
Chiba University, +ῌ--, Yayoi, Inage-ku, Chiba ,0-ῌ2/,,, Japan.
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Institute for Geothermal Sciences, Kyoto University, -*22ῌ+10, Noguchibaru, Beppu, Oita 21.ῌ
*3*-, Japan.
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Department of Geosystem Science, College of
Humanities and Science, Nihon University, -ῌ,/ῌ.*, Sakurajyousui, Setagaya-ku, Tokyo +/0ῌ2//*,Japan.
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Center for Northeast Asian Studies, Tohoku
Univer-sity, Kawauchi .+, Aoba-ku, Sendai 32*ῌ
2/10, Japan.
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Nippon Koei Co., Ltd., /ῌ. Kojimachi, Chiyodaku, Tokyo *,ῌ2/-3, Japan.
Corresponding author : Mitsuhiro Yoshimotoe-mail : myoshi@eri.u-tokyo.ac.jp

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explosions were thrown at least , km away from the vent, and the nature and proportion of rock fragments
deposited on the volcano ﬂank di#er, depending on the size and type of explosions. The total thickness ofproximal ash fall deposits from the + September and +.ῌ+2 September explosions varies from less than / mm at
the northern rim of the crater to /* mm at the southern rim. Using isopleth maps and excluding ballistic
ejecta, the calculated amounts of ash deposited within the area of +±+*3 m, are as follow ; ..3±+*1 kg, /.3±+*1 kg, 2./±+*0 kg, -±+*/ kg, +.-±+*1 kg, ,.2±+*0 kg, and ,./±+*1 kg on +, +/ῌ+2, ,-, ,/, and ,3 September,+* October, and +. November, respectively. The total amount of ash generated during the ,**. eruption ismore than +.0±+*2 kg, but still small compared to the lava that formed within the crater.
Key words : Asama volcano, ,**. eruption, distribution of ash fall deposit, mass estimation, characteristics ofejecta+    /,/** m D1jz ? ,**.a 5 1@ +12-   -  Ay8BCZbC6'8 NHK 7G   

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