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Course and Ejecta of the Eruption of Asama Volcano on , February ,**3Fukashi MAENOῌ, Yuki SUZUKIῌ, Setsuya NAKADAῌ, Etsuro KOYAMAῌ,ῌῌ,Takayuki KANEKOῌ, Toshitsugu FUJIIῌ, Jun’ichi MIYAMURAῌῌ,ῌῌῌῌῌ and Masashi NAGAIῌῌῌῌῌῌ,ῌῌῌῌῌῌῌ
Shin’ya ONIZAWAῌῌῌῌ,ῌAsama volcano erupted in the midnight of , February ,**3 with the ejection of ash and ballistics. The ash
was dispersed toward the southeast, and observed in areas up to the southeast of the Kanto plain in the nextmorning. The ash fall deposits at the southeastern foot of the volcano were surveyed in order to determine adispersal axis and detail isopleth contours. Isopleth contours of the ash fall deposit stretch out long from northwest to southeast, and they are denser in the western side of the dispersal axis than in the east. In the summit
crater area, the ash is not recognized in the northern side. These indicate that the distribution of ash fall wasstrongly a#ected by a wind from the northwest. Based on the isopleth contour map, the total weight of ash fallis estimated to be ,1,***῍-+,*** ton, using a log area (m,) - log weight (g/m,) plot. The weight is approximately
three ﬁfth of the eruption on + September ,**. and the same order as ones on +- November ,**. and ,0 April+32,. Major components of the ash sampled at about 2 km southeast from the source are non-altered and alteredlava, individual crystals, and ceramisite, but minor glass particles (less than + wtῌ) are also included in ﬁne grains.
The glass particles can be identiﬁed as juveniles and divided into two groups based on their shape and glasscomposition. One is ‘dense-type’ with rhyolitic composition which is the same as juveniles in the ,**. eruption,and the other is ‘vesicular-type’ with dacitic composition which is di#erent from any juveniles in the recent
eruptions including the +12- Tenmei eruption. These chemical characteristics of juvenile particles indicate that
two-types of magma have recently coexisted beneath Asama volcano and were erupted on , February ,**3.
Key words : ash fall, Asama volcano, isopleth, components, juvenile ++-῍**-,  +῍+῍+ogy, +῍+῍+, Higashi, Tsukuba, Ibaraki, -*/῍2/01, Japan.
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Earthquake Research Institute, University of Tokyo,+῍+῍+, Yayoi, Bunkyo-ku, Tokyo, ++-῍**-,, Japan.
Seismology and Volcanology Research Department,+**῍2++, 23456 +῍-῍.
Meteorological Research Institute, +῍+, Nagamine,-.7 !/01
Tsukuba, Ibaraki -*/῍**/,, Japan.
Seismological and Volcanological Department, Japan  +/0῍2//* 849:;< -῍,/῍.*=>?1 @ABCDEE
Meteorological Agency, +῍-῍., Otemachi, Chiyodaku,
Tokyo +**῍2+,,, Japan.
Department of Geosystem Sciences, College of Hu : *0*῍*2+*
  +* 2 manities and Sciences, Nihon University, -῍,/῍.*,F?"#FGH !/0"#IJ
Sakurajosui, Setagayaku, Tokyo +/0῍2//*, Japan.
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Institute of Seismology and Volcanology, Graduate
Volcano Research Department, National Research InSchool of Science, Hokkaido University, North +*West 2, Kita-ku, Sapporo *0*῍*2+*, Japan.
stitute for Earthquake Science and Disaster Preven -*/῍2/01 ()* +῍+῍+tion, -῍+, Tennodai, Tsukuba, Ibaraki -*/῍***0, Japan.
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Institute of Geology and Geoinformation, NationalCorresponding author : Fukashi Maeno
Institute of Advanced Industrial Science and Technole-mail : fmaeno@eri.u-tokyo.ac.jp