ῒΐ  /*  (,**/)
 /  -*-ῌ-+- ῕ ῔ ,**. 





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

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

,**/  .  ,1  ,**/  1  +/ ῎Comparative Study of the ,**. Eruption with Old Eruptions at Asama Volcano
and the Activity Evaluation
Setsuya NAKADA῍, Mitsuhiro YOSHIMOTO῍, Etsuro KOYAMA῍,
Hiroshi TSUJI῍ and Taku URABE῍
The eruptive activity at Asama that started on + September ,**. was characterized by several vulcanian small
explosions, including a continuous stromblian-explosion period. About ,+*0 m- of magma (DRE) was issued
in this eruption, most of which ﬁlled the summit crater ﬂoor. The sequence of eruptive activity was similar to thatof the +31- eruption among previous eruptions at Asama Volcano. However, the two eruptions are di#erent in
the following points. +) Both magnitude of seismic activity and scale of eruption products are small in the ,**.
eruption ; about half or less than in the +31- eruption. ,) A-type earthquakes occurred in the later stage of the,**. eruption, while in the early stage in +31-. -) Partial melted country rock (rhyolite tu#) is found among the,**. eruption products but not in the +31- ones. As the swarm of A-type earthquakes at Asama occurred in the
period of inﬂation detected by GPS in this decade, intrusion of magma in the deep-seated magma reservoir (ordike) was associated with A-type earthquakes. It is likely that the di#erence of occurrence of the A-type
earthquakes in the both eruptions reﬂect di#erent magma supply rates ; larger in the +31- eruption and smaller but
long-lasting in the ,**. eruption. Continuous inﬂation and occurrence of A-type earthquakes even in ,**/
suggest not declining of the eruptive activity but renewal near future.
Key words : Asama volcano, ,**. and +31- eruptions, A-type earthquakes, sequence of eruptive activity, magma
eruption

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6hstutvwxR*y-῍Cz=g\-1{1{(+3-/) -§¨1W =H ©ª FG (,**-) # Minakami@AB (Minakami, +3.,)H g<(R ,* $%45R+,

R¦ῒ6«¬­®c7-(K)* )* )* 

6|}W +30* jXY~i"#|a7>?-)*§¨1W =H ῐ¦ῒ#¯°6#±²#1W 7³++-ῌ**-, IJ´µJ¥¶· +ῌ+ῌ+IJKLMNOPQ*)*¸¹OPºb»¼ut
Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo, Tokyo ++-ῌ**-,, Japan.

Corresponding author : Setsuya Nakada
e-mail : nakada@eri.u-tokyo.ac.jp

I¸JP ¹ºP!»KPL

304¼PM N

Fig. +. Index map of Asama Volcano. GPS baseline between Tsumagoi (3/*,,+) and Tobu
(3/*,02) of Geographical Survey Institute isalso shown.

  +31-  -  +*  B+32-  .  2     C











Fig. ,. Yearly numbers of erupting days and large/
medium explosions at Asama Volcano. Largeand medium explosions were counted, referring
to Miyazaki (,**-).

,**.        ,**.  3 

!

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"#$% &'( )(* +,

Y/0 -./V/./ km 106H

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N+!!OPQR%ST ,**. a6KU

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85;   ef67,ῌ+ 

G,**. @AB ,*** CD,1

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 L MN+!!OPQR%ST' 6 ,**. ,1K +,  3 ,**.U ,**-  ,V. 1WXKa =X' "9,**. )(+,YZ[1\1   Pw (,**/) ' 60 +0 : t ;/]^1 ( 3  + _(`+X<0=!

,1Ya '8)b_K)( Fig. - 

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(Table +)

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Yr%+ A s+ L Minakami et al., +31*U& ªH

LFig. 0a+, «¬ a,U 3  + 60  23! +31- @t0 A s+'(­6H®!'D,¯

1IJ; b_1u Lv&Pw +31/U 1 =X

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T ,**.U ²C0 ³

 L|)Pw ,**/ ; }n ,**/U (Fig. .)

(Fig. 0a-) 3  +  µ¶µ )§ D

3  +  ,* : *, ~1*0 *

GY°´6)(

EF; 9G1*H1 ®H·¶·M ,**. *¸¹%%+º.7»,-¼

305

Fig. -. Temporal variations in monthly number of volcanic earthquakes during these +* years (top) and the daily
number at Asama Volcano (bottom). Earthquakes triggered in automatic processing system (WIN system) of
the Asama Volcano Observatory were counted.

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2* m 1nR $% h ª«%P!!

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306  

Fig. .. Temporal variations in monthly number of the A-type earthquakes and in length of the baseline between
Tsumagoi and Tobu in the GPS network by the Geographical Survey Institute, Japan (Fig. +). The latterdata were from Geographical Survey Institute (,**/). The former is the same as in Fig. -.

Fig. /. Distribution of tephra from major explosive events in the ,**. eruption at Asama Volcano. Curves for the
distribution limit and contours of tephra weight (in unit area) are shown ; see Yoshimoto et al. (,**/) for the detail.

}!^ ,**.  ~ BCK 
Table +.

307

Whole-rock compositions of products of the +31- and ,**. eruptions4

Eruption+31-

Smpl ID

+

,**.
,

-

.

/

0

1

Major element (wt. ῌ : recalculated into +**ῌ)
SiO,
TiO,
Al, OFeO*
MnO
MgO
CaO
Na, O
K, O
P, O/

0*4.,
*41,
+04/2
04.,
*4+,
.4*3
14+/
-4,+
+4+1
*4+,

0*4-/
*41+/402
043+*4+.4014+2
-4++
+4+0
*4++

0*4-+
*41+04/1
04/1
*4+,
.4+.
14*3
-4,*
+4+0
*4+,

0+4.1
*41,
+/430
04.+
*4++
.4*1
0403
-4+0
+4,3
*4+,

0+4/*
*41+
+04,*
04-*
*4++
-42/
0403
-4,+
+4-+
*4+,

0+4/2
*41+
+04,,
04,1
*4++
-42040.
-4,*
+4-+
*4+,

104+/
*4/,
+-4**
-42/
*4*+4.1*420+422,4+*4+*

Original
sum

334/*

334/3

334/*

334-+

334/.

3342-

334,2

,/.
,1
01
/3
,/
+3/
.,
1,2
3,
-*1
,+

,20
,0
0*
02
+3
+21
-3
03
,2
3,
--*
+3

,02
,,
0,
.0
.
,*
+12
-1
1+
-,
33
-*+
,+

,12
,,
/0
.1
.
,*
+1*
-.
02
-,
+*+
-*0
,*

,3.
,+
//
.1
.
,,
+10
-/
03
-+**
-*0
,+

-+.
+*
/.
,0
+*
+*
+**
,3
1.
2,
++++/
-+

Trace element (ppm)
Ba
Co
Cr
Cu
Nb
Sc
V
Ni
Zn
Rb
Zr
Sr
Y

,/3
,0+
02
,+
+2.
.*
02
,3
3,
-,2
+3

* total iron as FeO+ : Dense lapilli erupted on + February +31-, Manzanbo (ASM1-*,*+), : Dense lappili erupted on +* March +31-, Manzanbo (ASM1-*-+*-*+)- : Pumiceous lapilli on +* March +31-, collected in AVO (ASM1-++W). : Pumiceous bomb erupted on + September, Maekakeyama (AS*.*3+--,)/ : Glassy dense bomb erupted on + September, Maekakeyama (AS*.+*,3-/)
0 : Dense lapilli erupted on +. November, near AVO (ASO.+++.--.+-B)
1 : White xenolith in glassy dense bomb, Maekakeyama (AS*.+*,3--.)- 
-ῌ+/*+3/*  +3.3 DEF%GH 3

,**.  ,*   $%&'()*+,I ,- >JK#0LM#$6 N 0 km MO




P$6 QRST 3 km UV$W>XY 0 cm Z/* !"#[#$6 \]^_Q`ab#0cJde-./

fghi,+d ; jk-,*** tl mno&0123 456 Fig. 2  78 /*pV$G6 qr s +, IMF$6

!9 )*+,:; +3/*  +3/2  +31-

+3/.  +3//  +3/2  +3/3  +30+ 

tF(=uIFE v,w+xyzA{|,**. <=>?@ABC$6

308

 

Fig. 0. Photographs and infrared camera images of the summit crater at Asama on +- September ,**. (a+ῌ-) and,3 October ,**. (b+ῌ,). a) Southeastern part of crater wall and bottom from the western crater rim.
Dash-line rims the crater of the + September eruption in the close-up view (a,). Color scale of thermal image
a- covers *ῌ2**. b) Northern to northwestern part of the crater wall and bottom from the southwestern
crater rim. Dash lines show the margin of lava cake. Color scale of thermal image b, covers ,*ῌ-**
Temperature image was taken with an infrared camera (AVIO Neothermo TVS-0,*).

Fig. 1. Air views of the summit crater on + October (upper left) and 3 November (upper right), and infrared
camera (stereographic) images on 3 November (lower). North is lower left. The crater ﬂoor is convex
downward with the vent (hottest spots) in the center and fumaroles (bright spotted line) along the peripheryof the crater ﬂoor (lava cake). Taken from helicopters of Tokyo Shimbun and Asahi Air Survey Co. Ltd.

<p ,**. 1º»1j LRC¼

309

Fig. 2. Comparison of daily number of explosions in recent four major eruptions. Arrows show dates ofintermediate explosions. Data except for the ,**. eruption were after Miyazaki (,**-).



 ,**- 

+3/2  -  +* 



!"#

 $%&#'()* +,+,-./#
2

 0 ++  +* 1 +,  /  ++  +* - 34) 0ῌ2 km

# 56 0 t 78)9:* -.2 km  ,--ῌ,+31- ῐῑ῎ῌ῏῍+30+ # ++ )2( +31-  ,  + :*

 +31,  +, :*3)$%

# u) + $:*v6S> +31. ;+%A +31/  - 9&#7)KL1)'6S>L) S,-<p#  +31-  +30+ ;:* ++ R- ( ) 97)<=>  (!"#  ?@ ,  

*Zu# 1!>+-KL-  , < / A BC-  . D&u-K<./#  +31-  EF ,**.  p) +31, $#GHI>J-KL (Fig. 2) +31- M

 +31/  Fig. . +31-  + $&#>+%A(Fig. 3) ,  A  +31,  ++ :*>$6*N#'OPLQ: R-KL) +32,  .
 ,0 1 +*  ,  ?STS")4> +31-  , ",L1 )2

K+%A +31/  Fig. . ,  + ,-!"# R

](.#)* . 

( . -U)V#  +32-  . 

,0 ,-/>  0lH( ,  + Q: 0 

2 J )2(W-XY#  -  +* V# >SJ(/ p01Z[\- +, ] +/  t 1^_J*S>L `2,-U)+#Aab +32- c_defghij L1-93 3 / km KLD -¡¢£w) ,*+32-  .  2  ,**.  3  + 

4J¤SLr0l¥¦)§56S  ,  ,  +0(Jk)06:* +2 7,-l>
 /* < mnopqpY

2 +31. ,  + )8<¨-/ S9©:ª«¬:*;->1)v +31- 1) <­q?V<¨:* ,**. 1=>wx,**. Q: +3/.]0+ )KL +3/.]0+

yz{|}®9¯L1*SL -  +* (-9-7v wxyz{|}

)° -±J0l' ²-J³ +./ cm ?

rstso)Zu#

V gzp0Zr:* sts))X

~W # 1*SL #:# ?S

 B )@# A )´#> 1 6! +32,  +32-  2l sts)
~W #>:)Z#:

 KL> W #Jsts

")4> R- +%A (+31/)  *  R-K

B )@# 1µA# +31- R)¶·@pB9

7D¸¹)Zu#>L)v6S

 +31.310

: :::@ 

Fig. 3. Temporal variation in daily number of earthquakes around the +31- and ,**. eruptions. Shaded zones
represent eruptive periods in which dates for explosions of intermediate scale are shown by dashed lines.
Earthquakes during the ,**. eruption were counted in the automatic processing system, WIN-system. The
data in the Sannotorii station for the +31- eruption (counted on the drum recorder) were from Shimozuru
et al. (+31/). See Fig. A+ for comparison of these two recording systems.

 ,**.  

   !"#$ +/* m % +32,  . & ,0 '()* +,- ! . /0 12345
6+ 789:; +32,<=+32-  

>3456/?@A+,2 /?B8 0
BC  DEFGH I564J !KL-ῌ-+31-  ,**. 

+31- M ,**. NONPQ=

RS

>MT UVWXYZ[\KL ,**. ]^_`ab 6c_`Fig. +*. Weight comparison of tephra from the+31- and ,**. eruptions, which were depositedalong the national road about . km
east-northeast of the crater. The data for the+31- eruption were from Shimozuru et al.
(+31/).

WXY de f2 gPhijklLm=!
: Fig. 3

no,**. M +31- _`pqM6c_`pqrj ojst +31- 

fLK A I6cu_vwxyK

L 789:; +31/< .! zK ,**. 
A I6c 1 & _`KLk xy{_`Kf=b xy= +&|}~!K A

QbT ,**.
  ,**.

H»%@¼  

  Fig. 3  ! , "#$ %'( ,**.

& +31-)*+,

-./0 1WIN -./02 345*(67 +31-

 ,**.

89

311

b~C!ijῌ#kd 3$^] 1l m* ,**/2 G<C! Gv
$&noG&p !'!& A "#$pq]C raG
8&he!?97

#kῌsd !w+ $:  , ;(;!&  Fig. A+ <Gp !&zGtG u ,-./0= & -   v e"#&QbTJcd

& +31-

 > ,**.

&9 A7
 : +31-

?!@cwx

>7'*y9

 +31-

 A 

;!& B(!z{& z{Jcdc

 C9C D9" ../ km #$ %E&FG|`7 !zuC #bXcdc'H()*+,  - -./ 0I 

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Minakami, T. (+3-/) The explosion activities of Volcano
Asama in +3-/ (Part +). Bull. Earthq. Res. Inst., +-, 0,3ῌ
0...
Minakami, T. (+3.,) On the distribution of volcanic ejecta(Part +). The distributions of volcanic bombs ejected by
the recent explosions of Asama. Bull. Earthq. Res. Inst.,,*, 0/ῌ3,.
Minakami, T., Utibori, S., Hiraga, S., Miyazaki, T., Gyoda,
N. and Utsunomiya, T. (+31*) Seismometrical studies ofVolcano Asama Part +. Bull. Earthq. Res. Inst., .2, ,-/ῌ-*+.
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Fig. A+. Comparison of volcanic earthquakes on adrum recorder with those triggered in theautomatic processing system, WIN system, at
the Asama Volcano Observatory. The drum
records are at the Sannotorii station. WIN
system is more sensitive than the drum
recording.