῍῏  //  (,*+*)
 -  +.1῍+/. ῎ ῐ ,**3  ,  , cd

ebf g h ibj 4 k lbm 0 n obp q r sbt u v wb
x y z {,b| } ~ l,b+ u  ,
,*+* ^ - _ +1 =`a ,*+* ^ / _ ,0 =`?

Course and Ejecta of the Eruption of Asama Volcano on , February ,**3
Fukashi 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 glass
composition. 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.
 : -*/῍**/, %&'()*+, +῍+ !"#$-."#$ !/0"#1
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.
 : -*/῍***0 %&'()*OPQ -῍+
KLMN
RSETU"#$/0RS"#1
Institute of Seismology and Volcanology, Graduate
Volcano Research Department, National Research InSchool of Science, Hokkaido University, North +*West 2, Kita-ku, Sapporo *0*῍*2+*, Japan.
stitute for Earthquake Science and Disaster Preven -*/῍2/01 ()* +῍+῍+tion, -῍+, Tennodai, Tsukuba, Ibaraki -*/῍***0, Japan.
VWTUXY"#$ Z[\"#1]
Institute of Geology and Geoinformation, National
Corresponding author : Fukashi Maeno
Institute of Advanced Industrial Science and Technole-mail : fmaeno@eri.u-tokyo.ac.jp   148

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

 ,**3  ,$", !"#

" % &'() *+,!"  - #$% & !.'(

, ,**3  ,  ,,+ / , )01*++,-23 4.

5/ 67%#8023 $91 $"1:2;#13 1 )01 *3<'4=9*+*+5>6&7)6?@A89D" :4#;13+,!A

BC E<=" 7%>1?FG'@HI ABC

4" D ,**3 +- )"J9EKF#G23

H*+IL#J6 ,**.  3  ++

"KNOPQ# , 1 - RSMT13

 3+0 +1 "MKL
7%#

U I

,  + ) /+ C"#E<=4"

0W13 XY " %Z9[.\OVPQ !

,**3 "

JT^U_`a" Ib+ c  ,**/ '(57% U3,+9 deeaefg#8023+ , ) ++- C

Xh9 +32,  "@2i+j+ 

", "

N"-V&,**3 

# 26I?(SVW

9ER]SH13D ,*+* U3,+9 $ @k?lmnXY@] 2 C0MK#opq'(57%#rs$9ZC[\23+# tu]`Mw,J6!6 a ,**- ; bc*+9bxt' .AUv^MK#_<  +- C0Zi$MKdb "J

 ,**/ 7%ypz{Ye b " f|}g+B + km @hiXj#!"*+99 ~k1 $?o"J1 ,*** m p]

7%tl'S+hHm8n*+

X23

"

q#j+# tu7%ypzULr!+% sEt-FG'6@HI 1

u9 IhH!6 \n'v*+Twy(23 xy1/HIz  -

Q  {|  ,**3a ; ,**3b }~:'*( #$%  &+ W9 Ij+ $,!

" Ib6  (MTSAT) ;13 

1 ,**3  ,~"3v1(),U +-/ kmῌ

, ",+7% ' I6$9 V"#

9S"F

G'*+Mypz9 I{#j+   ,**3 ","h #] / / ) -* C&


" %#

]

 ,*+*

#26I 3&bI{
6+$9 D6Ug" MK'*+Mj+   ,**/

7%#8023

 C" ,* 0@M!"MK#]23 )-   ,, /

E<= 4,+zpM}n#3 "v "Jr U9E¡#123(Fig. +a) $

D9 I j!"¡Vg

9S13 ¢£9¤69 "Jj] @, BG#¦§3+Z.7%#8

 +31* X7MK¥D6!AIb+ a ,**- +32* "#¨SIb023+# !bt:"jA 9ª#] M %¡DC9 I0W13

13  +31* '« MK¢£;>>B ,**-** m ¤¬ Vmn#6  ,**/  ,**.  3++

®Q_¯¨"'d©"#]23 +31*
Xv A3<#²³23Ib+9ª17%g HI(©6

­6¥¦

 "JhiXj §"J+>BCH

I] D° '±IU>B. 'ªK´«#M0W13

hiXj´«¬!6

3Ib+   ,**/ ; ­  ,**/ Uv9 + km XM80*+$9#S $3",**2  2';©"'xI ,**3  ,"^U_`aµ1U!"¶# +./ km X®+# $,!BC6M!MK·¸'ªM9¦13Ib+$99°*+M jm±¹6 '² 61

,*+* "J9E³hI6q#j+
U$! 

¯

!b 



}nº´D1µT

hiXj#23 # U31bk3 " ,**3  ,, "dJ¶'»· Ib 9¸3+2¹"mjA

jCH 7% " %»·jbI §@  23+¥¦jCHgVº­ µ AC»ypz'#bI1*+ @  

1¼R13"½&bI¼  tu"9½@@j] " % hiXj9Z9[

9L6j+  "JB



,**3

,

,

 149

Fig. +. (a) Aerial photograph of the summit crater (Kamayama-crater) of Asama volcano taken on /February ,**3. The rim of the crater is heavily covered by ash and lapilli. (b) A hole (about -* mwidth) inside the crater produced by the eruption on , February ,**3. (c) A layer of eruptive deposit
from the same eruption near the rim of the crater (Point A in ﬁgure a). The deposit has a thickness of /cm in maximum and covers brown-colored pre-,**3 deposits. (d) Ash fall deposit overlying snow at thesouthern foot about / km from the summit crater.


,**3

1



.ῌ ῏ῒ῎ῑ῔῍ΐῐ!" Fig. + # Point A $%&'()*+ ,**3





/

,-./012345  67 ,$89:!;<=>?@ -* m ABC!DEFG=HIJKLMNO / cm 1PQ (Fig. +c) R

STJUVDPQ:I (Fig. +b) WC!

,X$ YZ[\]^IJ77_X`1ab

c ,**21d efcghijk !l"$MNDKL

$c mnopqrstu457 vwxDJyz{Dk<=>-.J (Fig. +a) i

?%|}~G$PQ:I

/0EF JW WEc3 DD c ,



,

,

9:IIJ  ,3iC:k ,-.
 3,D ,cST ,**3,,=<= DIJ WW
 C!c ,**3,,IJ ,**3/9:I
 ,**//

>¡#JW

!/01¢J !;STJ£¤¥7 ,MN¦i§!¨©ª-«¬cE­®DQ'IJ¯°cLkIJ D ,**3c!;±C!1



150

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

Fig. ,. Isopleth maps of the ash-fall deposit at a proximal area (about .* km from the volcano) and a distal
area (bottom left ﬁgure) for the eruption of Asama volcano on , February ,**3. The unit of numbers is
g/m,. Upper right ﬁgures show the grain size distributions at the summit (topmost) and at 1ῌ2./ kmfrom the summit that are arranged with di#erences of weight per unit area (g/m,) of the deposits indescending order.

  !"#$%&'()*+,-)-.
0;<=+.5>?":4/01)2345678*9":.
@. 06AB!0CD<EF.GH0I)JK. Fig. , LMN O%P QRS/  

CTUVW0 Sympatec XYZ[\[]^_`ab0cHde)"!%34.9% Gfg

hij HELOS & RODOS klmn)6. Fig. , %0 , o , 5 - %pqrstu0v<cHJwP x0.9% Gyb Fig. + 0 Point A %sz){+. |JwCD%s6,GH0}~=.4I*. 0CDP Gy!" 12./ km0

0"#%= }~$.0(g/m,) )`. s cH )!+,6

4%P5EF (Fig. +d) %g,)`.

34t7% 3!"!4t7%& 0.
9

:"0CD<0IP 0 !"

GHPpqr'b<P6C% ,$%'!7I0(¡),64 5%4,s ) ¢PcH0#$<*%4567

Fig. , LMN%3t7% Gyb<P £A+8)*B (Fig. ,) Gy!", / km pqr¤¥¦C§ ¨6  <P+§ -A 0©6ª[«)¬ 'b<P GHP)u+!+,64de­Bt7%4 "% %b6CD (,** P®R< /** g ῌ m,-** g/m,) <P£<+§ -A¯°±[²³<=4 

0C<P ´µ cm /0G.

!"/:.CD<P¶a·%%+§P< (Fig. + d) }~=.0/<=+.

*9":. W%t+,PGH ¸,µ mm¹º¨A4

:P /f »¼0%ª[« ½

,**3  ,

, 151
!"#$%&

' ( )* +, -./0-1 !"  2-3456-789 :;) Fig. , <=& ?@ ?@ ABC
D0EF)GH!"K LM> ,**3!"DIJ)
NO!"-PQ

1RS (Fig. +a) DTU"&NVSWXJ%$YZJ1&[VS\ !

]^D_`

"VSHabc#4)d4 e [ f$ %g h-ijZ
1&A'!k D(Ul%m&PnVS)*UbZo+4 pq,-Wr &f.J% . km /0

1Ds#Ht-;)Tu

S MTSAT v2 34)

f '3

wx !" 4y5 zg6 ,*+* &{J)l%%g |7-&}1]^J%$TuS0 

!"+

&



'

(

 

)*

!"+

weight [g/m,]) D!" ( (A ; log area [m,])(W ; log(U*~ }  8 &9Q?@D:(J$)R  -$/0 Z;<0
1!" Dd= }TuS >u ?@6,**+ ,**.  k  d=ZA$S) 

B6 ,**/ B  Z  ;U)R 8DAJ) Fig. -a & ,**3  u% +32,  .

,0   ,**. +  ++

3!"?@ C +32, ; B6 ,**/

+. ZIJ) ,**3 

N>?@& KLMZ

19Q -/ cm (Fig. +c)' ( )* + &D 12 kgῌm=E& VSe,: )J/ [ Z:)R F( G<7 ]^&.&H= }$ e 

N>?@#$%& KLMDY&EJ)/0 Fig. -a  I

Fig. -. (a) Relationship between log area (m,) [A]and log weight per unit area (g/m,) [W] of ash
fall deposits from the eruptions of Asama volcano.
A thick solid line with solid circles is from theeruption on , Feb ,**3. Lines (a)-(d) mean
possible ones based on observation and extrapolation from the proximal data. Solid lines with solid
squares and diamonds are from the eruptions on,**. (Yoshimoto et al., ,**/). A solid line with
open circles is from the eruption on ,0 April +32,
(Yoshikawa, +32,). (b) Distribution of ash fall
deposits from four eruptions of Asama volcano
mentioned above.

a  w ?@ JEDYEJ)/0 Fig. a  I b DTJ) H)  K=@-V + DR:(-: r LM

SNO #$%&!" -1w E¡¢£¤2Z1 PQR ¥¦&SJ$)R >?@DJJ)/0 Fig. -a 

+  +./ km TP(& +  ,  +*0 m, d=4 H

I c  UVWX !" D7§ * gῌm ¨=J))© ª« TP¬­ Y® +* cm ¯=,/0 Fig. -a  I d DTJ)  { N°
]


!"

³´DR%µ(Z;) ,**3  ,+,& ,.1 `·¸ I b-d DA$)/

0-.+ `·¸ I a-c DA$)/0  V
+ #$%& »N \ b±J)PQR

N\OZ-±² [ -±\<RS
^gVS

NO

_¶< ' ( )* + &F$¹$ +*+** kgῌm, /0TuS ' ( )* +NO

aºE - ¥¦H

I¼<½U¨=J% e (EVSPQR152

   

  ! *.+*.. "#$% &'()! * ,.2-./ "#$% +' ,!-./0"12#$34%&56'7()8( +*,* *$%)'9:; +<,* ,0+*. kgῌs #$34 ,**3 -
=>?@A +32, -B1C ,**. -D 3 E + .
++ E +. . /FG *0H(1IJKL2

M!NL3C4*O5G (Fig. -b) => (A)=>6 (W) PQRS78G (Fig. -a) 9
T:U*V594;D=>W*;L<!X'YZ;4 =>S[X!* +32, -(="# ,**. - ++ E +. . ( ,./  "#$ ,**3 -  >$%( ,**. - 3 E + .*? / "#$ ,**3 -
, @A\B*C]X +32, -D$*DE^_`$=>(aF34'( bcd&!K1S1
D >GHI5e'fJK(gL34!X h
MNO +32,
+32, - . E ,0 .D* =>(="#$%)'

4 iP5QRjSk5X lT +32,  Smnm Uopq+5X hMNO +32, X
r4$ ,**3 - , E , .D1L<!X'sZ$%tr &:& +32, -D$*? , )8D6'(7(&' lT +32, Xr4$ ,**3 -DV51 



Du:;? 2 km V4'W4$DXv ,**3 - , E

Fig. .. Back scattered electron image of fresh glassy
particles from the eruption of Asama volcano on ,February ,**3. The particles are divided into two
types ; (a) dense-type and (b) vesicular type.

, .w - ) .* * SxY&'D >yZ [\i]zS12>$&' b{| %&yZ * +}5^_6$~+4'`>a:;>abcUd

BMeafX:; b4;*gh h? 2* wtῌ >iS`$j\%X*k*ISlmA#  n_34 94;D /

5X&oapGcUd ? +0 wtῌ qV{

rZ EPMA S12*s&' *sS* tCW

u ? . wtῌ vYZ;4lawcZx  y  .z{

/" (*.. wtῌ) $% cUd* d  ~{  +/ kV, { +, nA, @ +*| JXA-22** R }* T EPMA]bcUd12*X & cU

mm $X' +' /F' S ,**. -6(&!X9(% ,/* mm¡N' 3 E +0+1 .D $&'D /S[X!

S* ¢yZ SZ!] + wtῌ¡ jL£k¤! *s &' 5B { u  rZ S[X!*cU5X¥aD /(+4 9D /

EPMA $*sGS*5C]3{u(*d U(¦8$§+)'1r5}56

\[:¨(©$%' ª«*¬&!X$ j­®]¯°34'±[ 91r5* D

5X D/ rZ * dense-type $*   U

S²S§{&!X' U(^_&!$]'r*

SiO, ? 1- wt ῌ vesicular-type $*?";:SV52 D /(.³"e$&!$]U SiO,  0001 wtῌ $%)' (Fig. /, Table +)

'5$%9´G D /* R (dense-type) R (vesicular-

: bcUd( ,**. -µSDuS+)'d$%: ª«D&'mnmk$type) S*M9($] (Fig. .) ¡¢SiP5af

%:*n_$]5X ,**. - 3 E + .DSBX!

X*F ;45X /£kS*m¶·/"* ¤34'¸hhUd(¥<rZ[9(+4!X( ¦¹i]zº»B1C§{ "S

:; v¢(DS¨¼&!½©§{&'zU U

234 ,**3  , . ,!8



567

153

9:; ,**/ $% <=

4>? @ /#A&,B C
D ! +E+ + !FG&A D' 
HIJK/LM N0 ,**3 
4>OPQR 4STUV- - 
STU WXY E+WX


Z[\]WX&^, _`a`b+

Y  - 4 ,**.

 c ,**. &V (Fig. /)   ,**3  d' !)*)e !f g@ )*) ,**. h ij
bWX&kYZ' l m +**  7ijbWX )*)n o
,**. 

 +31- p)*)7!

o'g 234!( fO )*)
qr stg u"; ,**/ ; #$
; ,**/ ,**3  @ )*) h%7

!"# h&  ,**. v_`a`b+i
jbWX )*) w7 xyY )*)nz {X' c|}E~ (\ FGstg OP 4STU!A&#o )
/f)D $! 7 & 0*o 234 +&Do)*@ qrUYDf,&t0-.!"# 234  07\  /07 )*)7 ijb

4STUd'7  1FG! 2t +32,  $% 

 + wtῌ &'(%)*) +07 o   `

!"#

Fig. /. Glass compositions of juvenile particles from
eruptions of Asama volcano. Solid squares andopen ones are ‘dense-type’ and ‘vesicular-type’ glass
particles from the ,**3 eruption, respectively. Opencircles are from the September +0῍+1, ,**. eruption. Each plot represents the average of up to +*points in each particle with the error of +s. An
area surrounded by a broken line is for juvenile
glasses from the +12- eruption (unpublished archivesof microprobe analyses in Earthquake ResearchInstitute).

2ῌ ῎ ῍ ῏234
7,**3  , . ,4 ¡> 





Y

4> ¢X  )*) ¥ e 56Y* 

Y

4>



3 OPQRSTU

V- £¤'gijb¤ 7 stg
QR 4STU ,**.  7 E+ 8

  ¦9 §:¨'g/07 &;

3<(dense-type)  © !'g= ª«&¬0­-d>3<(vesicular-type)>?&¯°±  ©@% ? ²³´a`b+

4STU  -  c  0µX¶·'gA±,/#¸B   234! ( l

© C¹º°  ¡>  ,.1-.+ Db» ,**. 

,**.  3 . +

-ῌ/ E<  ++ . +.

, -  +32,  . . ,0

 /01 F&Z' _`a`b+ O ®Y'  _`)*) ; @vWX )*) }¼f ,**3  stg½G)*)e

154





  !"
Table +.

῏

Matrix glass compositions of juvenile particles.

῎

+

,-. /01 23

8@A'/0( B)C*DE%FGH I

LMH789,1. %NCO2P9Q-,34RYZEῌ ῑ ῐ ῍r;s<=% t> (+32,) +32,  . S ,0 $TUy   ,1 ,*-ῌ,+/.
 b F G  (,*+*) MTSAT -KETU
,**3  , S , $75V_  ++3ῌ+,2
/0  1 (,**3) TU  P  Q
0 ,+  , S /0PQ 0 ,+ , S +ῌ++.
{WbA XY;.¡Zza]¦-§¨ (,**/) TU ,**.  3 S%« 7©Dª  /* ---ῌ-.0.
 c (,**-) TUP­d ®, 1eS 12 ,2-ῌ.0..
  i%jk    gkl(,**/) TU ,**.  ¶· o¸78
9Pp¼  /* -*-ῌ-+-.
f ¯n  h°  i%jk   G  (,**/) TU ,**.   ½?!:q /* -+/ῌ--,.
#$%&'  (,**3a)  ()*456789!
"# :; $%<&=>?
+& ,**3 ,J V+/3ῌ*,-.
#$-KA%.G/ 0 (,**3b) ,**3
 , S , $TU75V!
"#/0WX[X78\#]^E_`aX*b6 78 $%?Jcd,Jef"Ng 13.
9hMi%jk:%0lmnopq
%?@u  b A v  =wx>  IL
MzB{|,}C~ D EH Ih#'Ji   LM  %Uk  MN  xbA O (,**+) 4 ,*** R  S /, +01ῌ+13.
i TU  (+32,)  V /1  . S ,0 $ TU   /0 2,ῌ0 -*, +-ῌ+..
i%¢£ [¤¥\  U  ]^ _% `(+32,)+32,  . STU
7©Dª $%
b¬t?Jef"Ng +, 32ῌ33.
i%jkf ¯n gh°  ±% ²  ³i j´  
=wx>Jiµ%mn Q ML!" (,**/) TU
#¹º»%,**.      /* /+3ῌ/--.
rgst 5 !<