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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 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+
MINOPQRST
Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
G-*/ῌ2/01 UVWXM +ῌ+ῌ+
YZ[\]^RSTP_`aRSbc
Institute of Geology and Geoinformation, National
Institute of Advanced Industrial Science and Technology, +ῌ+ῌ+, Tsukuba, Ibaraki -*/ῌ2/0+, Japan.
G-11ῌ+1+ defghiNjghjklm 0.+ῌ-0
MInZNOopqRSrstu
Volcanic Fluid Research Center, Tokyo Institute ofTechnology, Kusatsu 0.+ῌ-0, Agatsuma, Gunma -1+ῌ
+1++, Japan.
G,0-ῌ2/,, vwXxyJKLi +ῌ-vwNOEObPz{O{
Department of Earth Sciences, Faculty of Science,
Chiba University, +ῌ--, Yayoi, Inage-ku, Chiba ,0-ῌ
2/,,, Japan.
G21.ῌ*3*- N|}~X m -*22ῌ+10
INONOEORS{Pz ORS

Institute for Geothermal Sciences, Kyoto University, -*22ῌ+10, Noguchibaru, Beppu, Oita 21.ῌ
*3*-, Japan.
G+/0ῌ2//* J/ -ῌ,/ῌ.*ANOHEObPz{O{
Department of Geosystem Science, College of
Humanities and Science, Nihon University, -ῌ,/ῌ.*, Sakurajyousui, Setagaya-ku, Tokyo +/0ῌ2//*,
Japan.
 : G32*ῌ2/10 XwJ .+MNOMRSrstu
Center for Northeast Asian Studies, Tohoku
Univer-sity, Kawauchi .+, Aoba-ku, Sendai 32*ῌ
2/10, Japan.
 : G+*,ῌ2/-3 MIvJi /ῌ.
An
Nippon Koei Co., Ltd., /ῌ. Kojimachi, Chiyodaku, Tokyo *,ῌ2/-3, Japan.
Corresponding author : Mitsuhiro Yoshimotoe-mail : myoshi@eri.u-tokyo.ac.jp

520 ²5WYZWW[WW\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 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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Chronology of the ,**. eruption of Asama volcano.

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Locality map of Asama volcano.

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523

Fig. ,. Isopleth maps of the volcanic ash fall deposits for each explosion. Numbers beside the closed circles
indicate ash sample weight per unit area (g/m,). Closed circle : ash fall observed ; tr : small amount of ash fall
observed ; x : not observed.

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526     

Fig. -. Photographs of the products of the ,**. eruption. (a) Ash fall deposit erupted on + September at ..+ km
NE of Asama summit. (b) ,* g ash of +/ῌ+2 September explosions deposited in the ash sampler (-+ cm indiameter), at ..0 km N+.*E from Asama summit. (c) Lapilli erupted on + September. (d) Juvenile pumicewith bread crust erupted on + September. (e) Lapilli erupted on ,- September. (f) Ash erupted on +September. (g) Ash erupted on +/ῌ+2 September. (h) Ash erupted on ,- September.

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5 6@A [\]^Bg !h,' +H+*3 m, IJ/Ki  @A N

Fig. .. Photographs of the proximal deposit of the ,**. eruption. (a) Bread crust bomb, (b) pumiceous block/** m from the crater, and (c) ash fall deposit on the northern rim of the crater were taken on +- September.
(d) Dense block with xenolith of volcanic sediment, and (e) ash fall deposit of the +.ῌ+2 September explosions
on the southern rim of the crater were taken on ,3 October. (f) Ash sampled from on southern rim of thecrater.

 ,**. 



529

Fig. /. Relationship between area (m,) and weight (g/m,) of ash fall deposits for the ,**. eruption. Weights
were calculated by integration of the regression lines in each segment. Thicknesses of ash fall deposits near
the source area were estimated as /, /*, ,./, *./, ,./, *./ and ,./ mm on +, +/ῌ+2, ,-, ,/, and ,3 September, +*October, and +. November, respectively. Amounts of ashes were calculated in the range of +.0+*/ m, to +
+*3 m, for avoiding overestimation due to the uncertainties at both of proximal and distal areas.

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3  +  +/ῌ

V~  {|BC `<

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$% &' ++*3 m,  3 +( ..3+*1 kg, 3  +/ῌ+2 +* kg, 3  ,- ( /.3( 2./+* kg, ,/  ,-( ,./+*1 kg )*+* *!@y@*V (Suzuki,,**.

((./0*(++*3 m, 

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Fig. 0 4

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(Fig. 0b) RS !TUVHWX5(.Y!UZ"0 DA. ,**.

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Fig. 1. Plots of log weight per unit area against the
distance from the vent along the principal axisof the ash fall deposit for each Asama ,**.
eruption.

Fig. 0. Relationships between mass of ash fall
deposit and (a) repose intervals of the ,**.
explosions, (b) maximum grain size at c.a. . km
from the vent, and (c) amplitude of an
infrasonic wave at Asama Volcano Observatory(AVO, Ohminato et al., ,**/) and Karuizawa
Weather Station (KWS, JMA, ,**.a, b, c),respectively.

Fig. 2. Diagram showing the cumulative mass ofash fall during the period of Asama ,**.
eruption.

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Minakami, T. (+3-/) The explosive activities of volcano
Asama in +3-/ (Part +.). Bull. Earthq. Res. Inst., +-, 0,3ῌ

533

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Minakami, T. (+3.,) On the distribution of volcanic ejecta(Part +.). The distribution of volcanic bombs ejected by
the recent explosions of Asama. Bull. Earthq. Res. Inst.,,*, 0,ῌ3,.
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Suzuki, T. (+32-) A theoretical model for dispersion of tephra.
In Arc volcanism : Physics and Tectonics (Shimozuru, D.
and Yokoyama, I. eds), 3/ῌ++-. TERRAPUB, Tokyo.
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