ῒΐ  /*  (,**/)
 /  -11ῌ-20 ῕ ῔ ,**.  3  +    




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

               ,**/  .  ++!"# ,**/  2  -*

!$Gravity Change Preceding the ,**. Eruption of Asama Volcano, Central Japan
Sadato UEKI῍, Shuhei OKUBO῍῍, Hiromitsu OSHIMA῍῍῍, Tokumitsu MAEKAWA῍῍῍,
Wenke SUN῍῍, Shigeo MATSUMOTO῍῍ and Etsurou KOYAMA῍῍Asama volcano, one of the most active andesitic volcanoes in central Japan, started a series of eruptions on
September +, ,**. and the eruptive activity lasted about three months. We have carried out microgravity
measurements at the volcano three times ; one year before, immediately after the eruption and one and a halfmonths later. “Hybrid measurement”, combining relative measurements over an area and an absolute measurement at a reference station, was employed to observe temporal changes in absolute gravity value. The dataobtained before and after the eruption shows that the gravity changes preceding the eruption are from %0 to &3
microgals, which are of the nearly same value as that of the accuracy of observations. The gravity changes
predicted from the tensile fault models and Mogi models proposed for the ground deformations are always lessthan + microgal at any gravity station. The observational fact that gravity changes did not exceed +* microgals
gives some constraints on the magma accumulation in the conduit. A numerical examination suggests that thevolume of the magma accumulated in one year preceding the eruption may be less than ,'+*1 m-.
Key words : microgravity, hybrid measurement, Asama volcano, volcanic eruption, magma accumulation

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ABGH=`*+qF YB# ¡?r+¢QX£¤= ¥32*ῌ2/12 ¦§¨©ª«¬© ¥*/,ῌ*+*0 l¹ºῌ»¼ +.,­ ® ®u®¯°t >*±¯°²³´µ
Research Center for Prediction of Earthquakes andVolcanic Eruptions, Graduate School of Science,
Tohoku University, Sendai 32*ῌ2/12, Japan.
 ¥++-ῌ**-, ­¶·¶ª@ +ῌ+ῌ+­¶ ®t¯°¸Earthquake Research Institute, University of Tokyo,
Tokyo ++-ῌ**-,, Japan.

l¹ ® ®u®¯°t*¯°²³´µ
Institute of Seismology and Volcanology, Graduate
School of Science, Hokkaido University, Sobetu */,ῌ*+*0, Japan.
Corresponding author : Sadato Uekie-mail : ueki@aob.geophys.tohoku.ac.jp

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ef&g  +h$ i

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 b-. /op&78h['&',**-

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3 E 3 ' q ++ ' ',**.

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1G js# %/&hG2(
'35%45&6thG7(&1
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

( >T. ?h5
#3D 7(%js#&@8 . ABC Fig. +  AVO-FG/#$ 3
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65b

G

%/&<G

Fig. +. Map showing the distribution of gravity
stations on and around Asama volcano, central
Japan. Solid circles denote the gravity stations.

»z­¼ ,**.  3 x + .­_gw
   

   !"#379,**/D
,**-  3 x ,/  y,1  ,**.  3 x 3  y++  *' %+

+* x ,, y,. ! =z ),-./ ' 01 2&3

4 56&  lm (AVO-FG/)$%&'()' 78 9: ; $%&<='>?

@ABC +321 ; Valliant, +33+D EFGHI  ,'JKL/ + MN

Table ,ayc-./

 i 

O&

PQ  Goi + RGi , JKL/ Di , 0

2&3

4 qi

ST' ei 

]& !  v2 &^s& b ] ? + &r b ¡> A +,-./ 'Z Bc
456#Z Tj +<='<= Cj

Sj +['Z>?  (  ' '+ *.**0 mgal

Table ,

]



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+ _ABC (+330) `aBC (,**.)bcd

^^ w$ Table -g  w$¤z') Fig. ,a, b f'(bc

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^ZTable +

(0mgal) (>?]&^ e  

u bd &m'+ ^ 2& 'ῌ

Z\ [

PQv2dm' 78 ]V῍WCjcos@,pDiῌTjDVSjsin@,pDiῌTjDXVei

Bs +3

bc [^^ =z>+ 313,***.*** mgal   +** mgal f

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YQ

n2

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7

d

>  £   []&  ]

 7P BMh( "ijk( @k  dU+.,D )/.3 \   +¨©, ^ +

EFGH !PQlm 2&n2o !

PQpq7n2+ &r bc^ n2stu bcd ,ῌ, ῒ῎ῑ῏ῌῐ῔ΐ῍

BM+*.+0 + ¥"$¦§*}

ª *^ [ >f«-2¬ ,  +&.­ + _ ,**-  3 x.­/g ,**.  3 x
£>=z ^ w0

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v2 d lm (AVO-FG/)w+ ,**-  3 x ,/y,1 4 + z>+{*.**0 mgal (U+*{/ m/s,) ,**.  3

x 3y++   '²2³7w+^+* x ,,y,.  .- z>+V*.*+0 mgal>? 7P g|w + ,**.  +* x !}"~# ,, @+* x -  y ,+   # ,- 

7

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Table +.

^w$+± ] ' 1(>?

7´  .­µ¶g ,**.  3 x·+* x·z + +) +/ mgal f  '¸
²¹º>^

Coe$cients of various kinds of error for the employed G-type gravity meters.

(AVO-FG/380  

Table ,a. Gravity values obtained by the measurements in the period of ,/ῌ,1 September ,**-.

AVO-FG/ is the station where an absolute measurement was carried out and the reference
station for relative measurements. To obtain absolute gravity values, 313***.*** should be
added to the numerals. Unit is mgal (+*/ m/s,) for all the numerals.

Table ,b. Gravity values obtained in *3ῌ++ September ,**..

yV ,**.  3 - + ' V/x i
Table ,c.

381

Gravity values obtained in ,,ῌ,. October ,**..

Pt**- BM1*0* BM1+** BM0*- BM0*1) ~<+,34> +$+ i

Pt**- BM0*1 BM1*0*  BM1+**  ,* mgal  +* mgal R ,) +/ mgal v   0 mgal dn..tl  '~   ! +  V  "!#"!$%"!&'()*+,

<w+, i zy$  +* - . / 01234 567 8,9:; -** mm <=>?@ A BCDEF38,,

$ G HIJ ,**/K<=>L.MN +/ mgal OP Q ?@3'

R@   STUVW XY<l+ &8,
R

V/x' p'W $.,abaMNaba c9
j, i XY<kl-ῌ+ 



V0+'$yVR aba j, +, GPS p

 Z[  U3 >' , j$- ¡¢£, GTable .

R /\]+.^_ 4`
aba

c97d<

e+,fV/g ,**-

 3 -h,**.  3 - ij,kl
DE. mnoE.2,L ¤ Fig. -K ¥I J (,**/)  ,**.  1 - h ,**/  - -, V  0 kmkm (<§+,S¨©ª

GPS ¦@R -«¬­ G®9 0.2¯+*0m-K °!++, ±" ²#'$³ (,**.)

p BM1+** MNqF3.r s ²#'$³

tuv<wl Fig. , (a) ]´$' µ¶·¸  1 -R¹º , V/x + yMNi zy  { |}u 'GPS , ,**.  . -&8+ V  2 km¦@R -./km ( %"!+»¼ &¶ G®9 ,..¯382  

Table -. Summary of the gravity measurements in September, ,**- (a), September, ,**.
(b) and October, ,**. (c), and the di#erences between the observations in September, ,**- and September, ,**. (d), and in September and October, ,**. (e).
Unit is mgal for all the numerals.

Fig. ,. Distribution of gravity changes (a) obtained from the observations on ,/ῌ,1 September ,**- and *3ῌ++
September ,**., and (b) those on *3ῌ++ September and ,,ῌ,. October ,**.. Unit is microgal (+*2 m/s,).

¦§ ,**. ¨ 3 © + ª«¬­YZ[383

Table .. Various source models proposed for geodetic observations (top) and gravity
values calculated from the models (bottom : dGcala). For the source models,
locations measured from the crater, depths below sea level of the centers andvolume changes (dV) are presented. Gravity values are in microgal.

+*0 m-   / km ,./ km  *..+*0 m-  , 
 !"#$ %&' ()*+,-. (,**.) / 012" GPS 345678349:;<=>#?'
@AB ,** m  ,.*+*/ m- C,,,** m  3+*. m- DEFGHI JK

LM#$
NOPQ'R%S#TUTV&WX%S>
YZ[\]'^3%S#YZ[_`'ab
cd=>$ efg'hGYZ['ab/ Okubo (+33,) i'j1 DEkl JK
Fig. -. Source models for ground deformations andthe assumed conduit (shaded area) projected
onto a E-W vertical cross section running
through the summit. Gravity changes caused
by Dike + (Aoki et al., ,**/), Dike , and Sill
(GSI, ,**.), Mogi sources + and , (MRI et al.,,**.) are evaluated. The distribution of magma accumulated in the conduit without signiﬁcant gravity changes are also examined.
Mogi sources - and . are after Miyazaki(+33*).

'j>YZ['ab/mn (+311) i'
jobpq"#$ #r" st#?' uvwx'ab lym zw" {|/}7~h TU
T hD" ,,.** kgῌm- "#$  #i/ Sh ~'1^3 '>'"b&S#h>$  %3wV&p3=>'job 3w
L"#$ #r"C ,,,** m DE

kl'1 3w V&%"#$O V&pq%S#¡YZ'¢£>YZ[4 Table .

¤ (dGcalc) '¥=$ S

384²$G³4 ´HµI4 ¶·49¸J·4K L¹4º »M4!P¼N

    X(q $$ 4   CCe0

*.2 mgal  !"# $ $%%S 8# C+)* + ",-&'(" .$%%

fg( / km i0kl
121,

56%8#  C5612
1 <, ? -  +*/"#-ῌ, 

mgal,  / mgal(%"0121 345678

 121 TU%56 5(~ 8# 9:;<0&'< + =>?!

*@)$%,A .#  +320 B 
CD ,*** EC"(%"AFC121,Gῌ  121 H
<.CI,JK$%, : NOP ,**. CQR

(%"A"121 56,C.
%S  121 TUDV

W+"%0C56121(%67h

 M 8( -* m O *`+/* m, *+ 9( /**m O/.* km`,., km, 7*.*`1., km( ,** m O:+  ;, ,,.,* m (+"%8 1217

L  (Furuya et al., ,**- ; Watanabe et al.,+332)# M$&'(<<

(# 7< =   Singh (+311)¡(># 121 %6$$

,,.** kgῌm- %S #



WX

 ¢

, +*  / mgal

[#8

%0 1217 8 *( 9 

NOCP C \$4 (,**/)  P 4} ¬­ £¤¥¦§¦ 5 S : 8®

X?

<0"Y!,Z"- £¤¥¦§¦ O i¨i"?©ªA«,1# (,**/) <.]%^+_

 ,**.  3 _`,**/

ab&'cd) Ce

fg( +,*** m)hii.8jkl M *+m n

 5(%@$%,A .#   1217
8%?©ªA« (AB8?

©ªA« # Fig. .  1217 *+ op",.%8q+(,r#  s(Zb) ( t (+33*)  +3-* -AB88%

+32+ iuv gw  x.(y/ Ce0 fgw)h% M
m! , kmfg( / km )h, z{|b(Zt) (=+¯ª  (?©ªA«:(L8# $$ °, +* mgal  C± / mgal ?©ªA«(D8# Fig. .a *((¢fg(1}(~ # $ 2 |b 1213

/ km %¯ª Fig. .b 1217<.W%q+ #km

(,fE ,..

Ce*0F¯ª# F ¯Fig. .. Pairs of radius and height of magma columns which cause gravity changes less than *.*+* mgal (dot) or*.**/ mgal (open circle) at all the gravity stations. The magna column is assumed to represent the magma
accumulated in the conduit within the one-year period preceding the eruption. Zb and Zt are the elevationsof the bottom and top of a magma column, respectively. (a) Zb is assumed to be /.* km, (b) Zt to be ,..
km and (c) Zt to be *.0 km.

V7N ,**.  3 = + Nt !"

  -* m

385

# 1r  D3D zAx% d   -* m    .

!

km  

%D / mgal JK$ |}~opo FG2Hx%

&

'

!"

/ mgal #$%

*.0 km(,.. km  ) *Fig.

.b  Height++.2 km

,  +/* m {]  Fig. /  zA 8" D  
!" +* mgal JK / mgal JK- ./# !"0-$%12

I / mgal JK

3 +*0 m- JKd

Okubo et al. (,**/)

89 5:;<%

45!

67" ,**.  3 = +* 1> ?' 0** m @A%(Zt) '0** m DE BC



FG2

H Fig. .c I 1 ) 

-* m JK LM& (,**/)NMOPQRSTU (+33*)

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Furuya, M., Okubo, S., Sun, W., Tanaka, Y., Oikawa, J.,
Watanabe, H. and Maekawa, T. (,**-) Spatiotempral
gravity changes at Miyakejima Volcano, Japan : caldera
collapse, explosive eruptions and magma movement. J.
Geophys. Res., +*2, +*.+*,3/,**,JB**+323.
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(+330) wxy e. ]+33,
 3 s ,2 z+* s - z_ i - xwxy {/a| ./ῌ/,.
Minakami, T. (+30*) Fundamental research for predicting
volcanic eruptions, Part +. Bull. Earthq. Res. Inst., -2,.31ῌ/...
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Okubo, S. (+33,) Potential and gravity changes due toshear and tensile faults in a half-space. J. Geophys. Res.,31, 1+-1ῌ1+...
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 lXZ­eU  U®¯ °q -+ .1ῌ/2.
Okubo, S., Ohminato, T., Hirabayashi, J., Kazahaya, K.,
Mori, T., Matsumoto, S., Koyama, E. and Sun, W.
(,**/) Gravity changes as a predictor of volcanic eruption. Science (Submitted).
,vde 0Ye4±e  (,**/) jk/&'&V
U ,**/ U² ¡³kDy¢,yM V*//ῌ**0.
Singh, S. K. (+311) Gravitational atraction of a verticalright circular cylinder. Geophys. J. R. astr. Soc., /*, ,.-ῌ,.0.
GZ´X e £,¤ e ,ZH e µ¥ (+312) ewxy .m[  Z`a /- /*3ῌ/,,.
¦v§ (+313)  i , ¨ ¶©ª| +.** ¶©|« ,1. P.
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xwxy{/a| ]de +/  1s++ s_ -1ῌ.1.
Valliant, H. D. (+33+) Gravity meter calibration at LaCoste and Romberg. Geophysics, /0, 1*/ῌ1++.
Watanabe, H., Okubo, S., Sakashita, S. and Maekawa, T.
(+332) Drain-back process of basaltic magma in thesummit conduit detected by microgravity observation
at Izu-Oshima volcano, Japan. Geophys. Res. Lett., ,/,,20/ῌ,202.
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