浅間山2004年噴火の噴出物の特徴と降灰量の見積もり

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(5) / 0 1 2 3 4 . 5 6 7 8 , 9 0 : ; < = >,**/ ? / @ +* ABCD ,**/ ? ++ @ ++ ABEF. 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 collected about . 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 G++-ῌ**-, HIJKL +ῌ+ῌ+ Institute for Geothermal Sciences, Kyoto UniverMINOPQRST sity, -*22ῌ+10, Noguchibaru, Beppu, Oita 21.ῌ Earthquake Research Institute, University of Tokyo, *3*-, Japan. G+/0ῌ2//* J/ -ῌ,/ῌ.* +ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan. G-*/ῌ2/01 UVWXM +ῌ+ῌ+ ANOHEObPz{O{ YZ[\]^RSTP_àRSbc Department of Geosystem Science, College of Institute of Geology and Geoinformation, National Humanities and Science, Nihon University, -ῌ,/ῌ Institute of Advanced Industrial Science and Tech.*, Sakurajyousui, Setagaya-ku, Tokyo +/0ῌ2//*, nology, +ῌ+ῌ+, Tsukuba, Ibaraki -*/ῌ2/0+, Japan. Japan. G-11ῌ+1+ defghiNjghjklm 0.+ῌ-0 : G32*ῌ2/10 XwJ .+ MInZNOopqRSrstu MNOMRSrstu Volcanic Fluid Research Center, Tokyo Institute of Center for Northeast Asian Studies, Tohoku Technology, Kusatsu 0.+ῌ-0, Agatsuma, Gunma -1+ῌ Univer-sity, Kawauchi .+, Aoba-ku, Sendai 32*ῌ +1++, Japan. 2/10, Japan. G,0-ῌ2/,, vwXxyJKLi +ῌ- : G+*,ῌ2/-3 MIvJi /ῌ. vwNOEObPz{O{ An Department of Earth Sciences, Faculty of Science, Nippon Koei Co., Ltd., /ῌ. Kojimachi, ChiyodaChiba University, +ῌ--, Yayoi, Inage-ku, Chiba ,0-ῌ ku, Tokyo *,ῌ2/-3, Japan. 2/,,, Japan. G21.ῌ*3*- N|}~X

(6) m -*22ῌ+10 Corresponding author : Mitsuhiro Yoshimoto INONOEORS{Pz ORS e-mail : [email protected].

(7) 520 ²5WYZWW[WW\W][W³&W´WZWWµ¶W·¸W3^W¹&WZjWº_W°`W»5Wa¼ 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 of proximal 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 is more 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 of ejecta. + . /,/** m D1jz ? ,**.a 5

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