ῒ῔  /+  (,**0) ,  +,/ῌ+-- ΐ ῕,**. 

           . ! ,0 "#$% ,**0

,**/, ! ,1 "#&'

Seismic Activity and Tilt Change Obsereved before
the Middle Scale Eruptions of Asama Volcano in ,**.
Jun FUNASAKI῍, Hirohito NAITO῍῍, Tomoyuki KAN’NO῍῍, Makoto MIYASITA῍῍,Shin TIKAZAWA῍῍, Yoshihiro UEDA῍῍῍ and Sei IIJIMA῍῍῍A medium-sized eruption occurred at Asama Volcano on September +, ,**. after a ,+-year dormancy
following the eruption in April +32-. Before the eruption, the number of volcanic earthquakes had increased
beginning late ,**,. A prominent feature of this seismicity was the appearance of high-frequency BH-type
earthquakes beginning in mid-August, ,**., approximately a half-month before the ﬁrst eruption. Eruptions ofsimilar size occurred on September ,-, September ,3 and November +.. Preceding these eruptions, changes inground tilt were recorded on the EW component of a tiltmeter installed ,./ km NNE of the summit crater. Thiswas the ﬁrst time that JMA (Japan Meteorological Agency) recorded tilt changes at Asama Volcano beforeeruptions. Changes in ground tilt were almost simultaneously accompanied with increases in volcanic earthquake
occurrence. The increase in volcanic earthquake occurrence and change in ground tilt preceding the eruptions
contributed to the precautionary issuance of preparatory volcanic information to the public. In spite of theappearance of preceding phenomena, however, information was insu$cient to predict the exact time and size ofeach eruption. Additional observation and understanding of physical processes may be necessary for the exact
assessment of eruptions.
Key words : Asama Volcano, the eruption on September + of ,**., tilt change, BH-type earthquake

67hiS3*TUVK@Ajkl1UV4mn567+ῌ ῐ ῎ ῑ ῏()*+, ,**.

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 *0*ῌ***, . ,  +2ῌ,Sapporo District Meteorological Observatory, Kita,,
Nishi +2ῌ,, Tyuuou-ku, Sapporo *0*ῌ***,, Japan.
 +**ῌ2+,,  ¡¢N£¤ +ῌ-ῌ.
¥UV3*¦
Seismological and Volcanological Division, Japan
Meteorological Agency, +ῌ-ῌ., Ote-machi, Chiyodaku, Tokyo +**ῌ2+,,, Japan.

 -23ῌ*++/ §¨©ª«¬­®¤¯° ++/+ῌ,¬­®±²
Karuizawa Weather Station, ++/+ῌ,, Oiwake, Karuizawa, Nagano -23ῌ*+//, Japan.
Corresponding author : Jun Funasaki
e-mail : funasaki@met.kishou.go.jp

¡ ¢P£¤¥¦P§¨©ªP«T ¬Pq­ ®PO¯°±P²³ ´126

Table +.

Major eruptions which occurred in ,**..

 + ,**. G[" +*  +*   !" #  $%&'()*", ,**.  3  +  ,ῌ+ 

+,-./0123#4 567
&8! 9:;<&%=>?4 5@ABCDEF&%. +30. GHI-"JGHI.KLMNHOPQR SHTU
 QR%VW 4XPCD YZ[@AB\]^&EF&%" ,**. G 3  _`4abcdef Fig. +g*"+30. GHI, ,QRhXf Fig. ,*"  `2 S+31- G +32,i2- GWghX.

j% k  ^&%[l&'mnoj%2,'" pqr +* G,u+332 G3# +333 G 3v&.

stfFig. +.

hX.w

Location of the observation points.

 + c"[%sx- ! ,**, Gyz3#.hX{.
'j%sx]%&%!"+,A

$%&.|}3#~5

B a ;[<&%! -.

-2W  B .#BH  S2 *.- 5%W  BL ,**, G . HI +, hX $%& S2 *.- 5%W

 4XPCD*5V [ !" '[ .

"A  SP  S -82W B  S~

#~f Fig. -&%g*" ,**. {2[)*?)*jkoR(O ¡127

Fig. ,. Monthly numbers of earthquakes (upper) and tremors (bottom) from January +30. to December ,**..
Long and short arrows indicate middle and small eruptions. Monthly numbers of earthquakes in February+31-, September ,**. and October ,**. are /0,+, -2++ and ,12/, respectively. Monthly numbers of tremorsin February +31-, October ,**. and November ,**. are +22, +01 and +22, respectively.
`d ?`d `+30.  + ,**.  +, d #)* :# , )* +31-  ,  ,**.  3 ? +*  RR /0,+  -2++ ? ,12/  +31 ,  ,**.  +* ? ++  RR +22  +01 ? +22 

Fig. . BH  ,**-  0  ,**.  0  -*/* !!"#$%&'#(,**.  3  + )*+ 2  -+ ,- 3  + ,.

/ BH &" 0 ,. 1234

 ,**5670( 89:12 BH &" 0( ,**,  3 ; < , =>8?@A(BL  ,**-  0 !!"#$%&'#(& ,**.  . B1C
A ?HI +  *&DE0(FG BH  BL ῌGJ6 ,**.  0  KL,MNOP&Q-RST UVW" XW,A(0,0 ,**.  2  +/ !!YZW A &0 [\ ]*[^_ `[abcb< , kmd eR&fg-R( `hijklmnoR(8pqrstuvwxy +.1 z{2[[abcb< +./ km, B|}[~Sf*[^G ,FGd pqr

S&B

CpqroR P0( +331 +*  {2[5hijklmnoR/#S2+*Tstuvwxy +./ T&FG@> YZ#0/#S ,**.

 2  +/ stuvwxy +.1  +331  +*Fig. -. Examples of volcanic earthquake waveforms
in Mount Asama. From the top to the bottom,type A-earthquake, type BH-earthquake andtype BL-earthquake.

 -  ,- A BH  BL ST UVWXYZV[\]^V_` aVbc dVefgVhi j128

Fig. .. Daily numbers of earthquakes from June ,**, to January ,**/.
A-earthquakes, type BH-earthquakes, type BL-earthquakes and tremors.

M .

 NOPQ )*PH N,**, )*% +30.
 +33.
 + 

0 ,,**/

 +++  ,.  +   + !

"#$%
Fig. , &'()* +31-

- ./0123456./'789:+33,+ Q% L#R A O  BH O  BL O,,. )- ./ABC$:% ,**-./ῌ;<6<= >?@ ,**-,,. )- D )*./ῌEF:% ,**.

2

G )*H +* ++56<9: I
D ,**.

+ +32,,+32-

From the top to the bottom, type

3  + -B;<$?@J9:%,ῌ,  BH,**.

3  + - A ,**.

2  +* 'KL

,**. zv{}~X ¹a?}~XVWºFig. /. Temporal change in dominant periods of
BH-type earthquakes observed on NS component at station B in the period from June toSeptember, ,**.. The dominant periods started
to decrease around August ,**., before theeruption on September +, ,**..

¸ / BH  )*5v +B $ NS'(K ,**.  0  + Y ,**.  3  +  ,* 52 BH 129

Fig. 0. Running average of the dominant periods oftype BH-earthquakes for ,* consecutive eventsfrom June ,**, to January ,**/. Arrow denotes the eruption of September + ,**.. The
dominant periods decreased in August ,**.,
before the eruption.

¸ 0 BH  )* +,*  4`abcK B $ NS '(K ,**,  0  + Y,**/  + +/ 2Fig. /  ,**.  0  +  3  .  BH <k? W  +JK +31/ ;  !"# B $%&'()* +,-./0

-  zv~{K +32-a, b2 !"#12 3456789:;<=> BH  ?@ABCDEFG)**.- HIJ>

K Fig. /  3t?K ,**.  3 }~X?K +32,  +* | +32-  . }~X

DE,-0W

1>LK DEFGM *.- HIJ N?O(Figs. 1a, b) +32, K +32-  DEqrq*.- H3QRST=>LKr>LK )*!"# A $DE=P=,-01
Fig. / *.- HIU==89:;VW> kvn(qrK A K B F ,**.  1 IX BH )*KVWy"?  k +31- K +32, K +32- }~5?

*.,Y*.- HZ/> K ,**.  2 [\J\ DEK ./K  (] K *.+ HZ/=

PqrG  > K }~XhWIX

<,**,  0  ,**.  +, K BH  ^ ,* _)*`abc3 Fig.

0 d ,**.  1 eK )*`abc
*., HIUf`<

K ,**.  2  +* g

L

? ,**. ?" ¡¢ ?£RW
+31-  ,  + }~X

5!"#qrG

<  K ¤5¥L + 5v

K *., HIJ? hi ,**.  3 jkl

4,- t013¦t?k??)*`abc ,**.  1 IX *.,Y*.- H§K _¨  VW m<<K

nopqr> ,**,  . Ist?K u5M)* BH   < <F3©ª«¬ (+31-) ? ,  + }~X

< K +30+  2  +2 }~X=

5 > +,**,  0  ,, eK ,**,  2  0Y3 

eK ,**,  3  +2Y,* eP2 ,**.  2 [\K, K +32,  +* K +32-  . }~K ,**.  3 ¯°}~K }~X+* vNPwx<y< +­®K +31.2 k +31- <

<K hWIX ,**,  0Y3 tWK  ~{± 3=J²³´BH   p<wx<µ¶ W}~X=· 

zv{ +31- | +32, }~XK ?£RW¬­ ®¯°±²¯³´¯µr ¶¯c ·¯C¸¹º¯» ¼130Fig. 1. Temporal change in dominant periods ofseismic events observed at station A. (a) Theeruption of October ,, +32,. (b) The eruptionof April 2, +32-. Running average of +* consecutive earthquakes. Periods declined before
both the eruptions.

¤ 1

z| ¦§¨T 8 [A NS © +*ªn vw«]! (a) +32,  +* 0 , z! (b) +32-  . 0 2  z!

P   Fig. +  0,ῌ-Fig. 2. Di#erence in P-phase arrival times at Station B and Station G. Arrow denotes theeruption. (a) The period from June ,**, toJanuary ,**/. (b) The period from January,**. to August ,**..

¤ 2 B ¥G  P efSg! (a) ,**,
 0 0U,**/  + 0! (b) ,**.  + 0U2 0!

=&! \ ,**.  2 0 P   ! "#$%&'() *+$,-&ef

Sg$ *., .r M $W !
#GBPefSg$8s)$@tuvw&#xQ9: ! L:&,**, ./ ,**.  2 01 234567 ypqbz{cD) y|}c [b8'9:5!

z{&; ":9 <=>

&? P @.CD EF 'GHIJ$KL:&A'B)$ /2** m, befSg$

Ph ,,.3- m] D #~=#~
*.- \9#G*., "# P=$ -./ kmῌs,**.  RST + D&) 1*U+** *V=W

$1** m 5&#aL: P

=$ ,./

EF

0** m D $ " XYL: + D&km ῌ s)+ZV= R#% +* ZV=D[,**.  5&#aL: !

EF! B

M! *+G+NOPQi

 1& Ph +,-** m Dh +,3** m D $*+FG ,*,*/. *D $ " XY$KL:&  .-2 Z\5]!

-  L:5^5_ WE`&? bc28aBPefSg 8' ! Fig. 2  [b[b i, d`+.2 km,h +,20. m]  G

! "#,./ km G7F! Fig. + ]! G [Pinnacle Tech-[PefSgnologies  //** ]  \9 +, mSj klmn&M D ! PefSgLL: !   + ,**- ./ ,**.  0 0o1 pq *.,U*.. VD $   5¡)¢£+., km,h ,,+2* m]-ῌ+  F z w b 

,**. 5123>?R{|«m>?RdEe,131 ! *.*- "#$%&'()*+,-./0-ῌ, 123 4 ,**. 5 3 6 ++ 6784 9:;<=>? @A+, 4 BCDEFG ,* Pa HI()JKLMN / O>? 4 >?@APQ2:-*Q2HIRSTU F VWXYZ[Y)\ ]F V 3^__X`abc]Table + fgc0 F VXYZ[ 4 i_Z[dEe,>?Rhjk 0 >?Rl4 >?@AmnQW 4 opqrs
tue,0 / O>?v84 >?@A! ,wR

L Fig. 3 xb0y; 4 F VWXYZ[4 i_Z[4z + Q2/,\{|OP ]A }4 BH }4 BL }
.Wc L~%x+80 3 6 ,- w4 ++ 6 +.
w>?R 

\mnQ 4 {|OPLx+80 >?mnQW
opqrs

4 3 6 + w4 ,- w4 ++ 6 +.

w>? XYX) \0 ++ 3 6 ,3 w>? Y) \m80i_Z[ >?Q

4 XYm9e4 i) \/0 N>?Q@AbWopqrs

4 >?8 8\4 >?RW
\4 >?Wm 80 >?Q {|L+703 6 + w>?R 2 6 -* w -+ w784¡ +0 ¢w£¤L_)+80¡w£¤L_)

Fig. 3. Changes in ground tilt before the eruptions
observed at station F. (a) September +, ,**.,(b) September ,-, ,**., (c) September ,3, ,**.,(d) October +*, ,**., (e) November +., ,**..
Thick arrows denote the eruptions, thin arrows
denote the beginning of tilt change and bargraphs are hourly numbers of earthquakes (totalof types A-earthquakes, BH-earthquakes, BLearthquakes).
0 >?Rl - w2STy 3 >?RW0 ¥¦§ >?4 ¨¦U F V,m0§  EW Z[y©~% Q2ª{|OP ]A }4
BH }4 BL }b8{|OPc0

132  ¡¢£¤ ¥¦§¨ © ª J« ¬ !­®¯ °± ² F   ,**.  0  ,+,,    !"#$% &'&&() 3 &*+,-().
% (Fig. +*b)  & 3 & /0,-&.1- (Fig. +*a) &2345678
9:.;<%=>?@AB.<C ,* DEFG

H@"#5 &IJ
Fig. +*a &MN4&a .

4K;<&L&*+LO45 P- Fig. +*QR"#5S=>?&TU VW

41-SXYZ[\O MTU&

M% ]^#5_`O

3  + &abc&&deMabc]^

f41#5L4" 0 &23&g_ M

45 3  + &abchijki! %&\4%lm4

1-no (Fig. +*a)
3  + \O ++  +. P.& / p&abq]cH@- F  hijk&
P%\OabtcP.&uCC r

&svw7x ab&yzz{&|} Fig. ++ ~ ab&yzz{Clm4||}LO45

ab4Nyzz{ab&deno  p& / p&abq]&=>?@AB.& h\Oh  *.0+.3 mῌs .

'44KMyzz{ &deK4
5no ab&yzz{ a&XY
"#5_`  -abc&C

ab&a&7lm4|}4N..ῌ ῍ ῐ ῏ ῎,**.  3 sP1-678&ab &Fig. +*.
(a) Ground tilt records. Upper : fromAugust -* to September ,, , days before theeruption, tilt change includes the e#ect of atmospheric pressure caused by a typhoon.
Thick line indicates the ground tilt to be caused
by a typhoon. Lower : ground tilt records fromJune ,+ to June ,-, no eruption occurred in thisperiod, but tilt change includes the e#ect of
atmospheric pressure caused by another typhoon
which had almost the same track as theSeptember typhoon. (b)Typhoon tracks of Jun.
,+ ,**. (left) and from Aug. -* to Aug. -+,**. (right). These typhoons may have hadalmost the same e#ect on the ground tilt at Mt.
Asama.

 +* (a) abc& ,**.  2  -* 3  , &  r!x ab 4K&de&L&  r,**.  0  ,+ ,-  x2  -+ 3  + & &de no 0  ,+ &23&"-&de. r!x (b),**.  0  ,+  rx  ,**. 2  -*-+  rx &() , &
678& ^MN4de`-_`O

,**.  g@> '7^_(1339:  ; $ ^_( ` 5; ;"=@ ?  a  b3"25 cbdZ2efa,**.  g@> '  ^_("h6  =@i-* =@% ;X" / 7 & 5"j'7k - Xlm^_( ;"72 ";G2KL")3 dZk k>'D(G2n   $()
KLDo 2  ^_("h6 ;6pP=@ =@i-* =@%*"  qr 3 ;=sb t""3  6 ;G2 u+b  ^_(v" U3  ]/  X  ^_( ;w 3x";" ^_("y#FBz {";3"3|3 }  ! U2  KL

 |b,-D~ 2
Fig. ++. Tilt changes vs. amplitude of air shockwaves. Upper : total amount of tilt change andlower : duration time of tilt change vs. amplitudeof air-shocks.

¹ ++  ^_( ^_(L ^_(y#=@¡ = º»»* M¼ ,+      ,    !
"#   + $%&' '( )*+,-./ 0"1234)6"

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