  /*  (,**/)
 /  -3-ῌ.**  ,**.     




  
      ,**/  .  +-  ! ,**/  2  -+  "A Unique Earthquake Activity Preceding the Eruption at Asama Volcano in ,**.
Maki YAMAMOTO, Minoru TAKEO, Takao OHMINATO, Jun OIKAWA, Yosuke AOKI, Hiroko UEDA,Sho NAKAMULA, Hiroshi TSUJI, Etsuro KOYAMA, Noboru OSADA and Taku URABE
On September +, ,**., a middle-scale eruption occurred at Mt. Asama. Before the eruption, long-period
volcanic earthquakes were observed with the broadband seismographs located at the summit of Mt. Asama since
October +1, ,**-. The signals are so feeble that we can hardly recognize them even at the second nearest stationfrom the summit crater. The long-period earthquakes had been occurring at least before September /, ,**,.
The earthquakes have very unique waveforms and can be categorized in - types (type +, type ,, type -). The
sources of these long-period events are located relatively shallow at a depth of a few hundreds meters, just beneaththe crater. The depth of the type + event is just beneath the vent, the types , and - being +**#,** meters deep.
The activity of these long-period events had been synchronized with the activity of volcanic earthquakes until thelast ten days of July ,**.. However, the activity had dropped gradually, and no event has been observed sinceAugust ,., ,**., just 2 days before the eruption. An earthquake swarm started on August -+, lasting just before
the eruption. The hypocenters of swarm, which are determined using the Double Di#erence method, lie justbeneath the vent ranging from -** m to 2** m deep, suggesting that the shallowest part of the vent is broken during
the swarm activity.
Key words : Asama volcano, long-period earthquake, broadband seismogram+  

$%&'()*+,-.! /-! 0-!

r+-+1! ,**- s34t-dc.+uvw
ijx=y6z{|}(~?@! r+ijx=y^U

12-34567-.89! :;,<+12-

5 >W^A\ij(|}?@AF r@4+

& + 1=>7?@A BC! DE! +331"F G

ij>&! {+ij+bῌ\+K&

H +** %+,<=IJ6K! +3** L34-,<56!WN! ( + =k"A5(,MNῌ=OPQRS-(TNUVA(!

V?5<.86=k33'4! 

+30+ +-WXY= ++ %+Z[\=]^A B#34 +* #K${=5^U9! <K&\& ,#-

C! _`! ,**-"F a+Y! +31- K +32, #+32- =

56jG(?@6F A! r@4+5b-(89! Xc+,<KNU&! ,*** =-d\ij& 3  + +-y91=,=,<NU9!

e+fgM(h4@! a+Yij,<klm=,

-=6 ¡W¢£J6¤.¥%5:¦K§'@

MNnn8^A(! ,**.  3  + oY 2 p= ,+ q

6F ¨©.&! r+5\ij+ªW«N!

9+b-(TNAF

a+,<gKj&'W¬­AF ?4=! r+-+ ¶++-ῌ**-, ·¸¹*¸º+T +ῌ+ῌ+·¸ »ij¨©¼
Earthquake Research Institute, University of Tokyo,
Yayoi +ῌ+ῌ+, Bunkyo Tokyo ++-ῌ**-,, Japan.
Corresponding author : Minoru Takeo
e-mail : takeo@eri.u-tokyo.ac.jp

ya ,3 p%134,MNA®ij,<+j&'W¯°=±zN! $%=²6-=6 ¡W¢£³(NAF, ,**.

,**.  3  + +b-=´µ²U! $%)7)MN)$O)P)( )¼Q) )R)D)<Q)S

394



GPS 

 1    !
#$ % &' ( ) ,**/ ; "* )  ,**/+,  -       .   /   (SAN) #0 2  -+  +/ 1234

 3  + 516

0** 78*9: ;<

5 =>$, ;<5? 3  +. 23 +2  2@
A +*** B8* $CDEF5 9:
5G23 +-* km $H IJ %KL,**.+, M?N  OPQ2DE5 R&
3  ,-  ,3  +*  +* E59:$? S!T":$ U V$, :2: W MSX
? ++  +.  #Y$ZF E59:A
O , M? ,**/ % 1  + [\6 5&]3
AOFO,- 

- W@ '^( _ `)ab + *+abcde#AO$ ,**- % +*  +1 5fg, (KAC,)

 -% 1  ,, ., (KAH,)h/ic01# j6+abck23F2>$3lFm4

U F>$, 01#$copqrns CMG-- T -0* *ct

, ;<5t>$,**. % 3  + 1u

J>$ Fig. +!, 5f , v  wO5x
%MAE ; 5f23 +.- km 6++abc01#AO$ ,**. % /  ,0 23h/icy

Fig. +. A seismic network around Mt. Asama, Japan. Stations coded with - characters, KAC,,
and KAH, are operated by ERI (Earthquake
Research Institute) and . characters (except
KAC, and KAH,) by JMA (Japan Meteorological Agency). A seismic array shown in theupper small panel was installed at the east of thesummit on August ,/ and ,0, ,**., just / days
before the eruption on September +, ,**.. Only
four of these stations survived the disaster. The
gray circle in the upper panel indicates the rimof the summit crater of Mt. Asama,

01#AO , #3  7 H$z


)!



R& KL (ASMB,

ASMD, ASME, ASMG) {8y|} ~9:$,5fw:

01:$h/ic ,**- % +*{8 aF0;G :A#Y:$¡¢ .3F<ab {8#AO j &Z$, #3
 KAH,,**. % 1 

01#$h/ic018 ;F<ab {8:AO$, j>Ay.3F£&Z$ %Fig. - ¤¥+, j33 =5fw: (KAC,, KAH,) 8

¦§ {8#$;F<ab c¨o{8#FOy It , 2  ,0 2©pHyFP ª t j :

3g>> c01:A

AO , j3UyE$ZFy:$ 5>A?P@#

5f23 +.- km $ MAE yF£&Z 

 AB$y . kt>$, jU

?P«j<abeI5fw: , 8 ,  (E*1, E*3)  5C5

{8#AW3¬ 5f­Pw:t

N ' 9O$,

# ,

®

*¯°$;F<ab _ J±F - Q.Fig. ,KAC,²K IK# ,01#$h/ic ,**. %0  ++ 5{8!, $U ;F<ab, jc .

&!

D:
j

 + 
³p´±F£µQJ±F³p´¶1-E -ῌ. *t , #3  ·aFF¸¹:AO y

01#$+abc?O, ºBL°$55b- %,**, % 3 »,**.

,**. ª ±R²³)Cc´#JEK+,µ>

Fig. ,. The monitoring record at KAC, over half aday (++ June, ,**.). We can recognize clearlysome unique long-period events pointed by arrows.

 2  

 - !" #$%&' Fig.

- () 0  ++ *+,-. /012 + Hz 

304567810'9:;$% <=+>?@A

395

Fig. -. Waveform examples of type + event occurred on June ++, ,**.. The vertical ground velocity recorded at KAC, is shown in the top
panel. The second trace is a low-pass ﬁltered
ground velocity with a cut-o# frequency of +.*Hz. The third trace is a ground displacement.
In the lower panel, we compare the long-period
waveforms of type + event recorded at KAC,
and KAH, ; a low-pass ﬁlter with a cut-o# frequency of +.* Hz is worked on these records.

;$%'(BC DE$FG'HI2 485"
JEKL>MNOPQC 2  / *-. +Hz 304567810'9:; KAC, 2 KAH, )Q#a ,u2R

RSTUV-.'(BC WVXY#(Z[C+,$%-.Z[C KAC, L\ KAH,

 , 

]^_ , `2#aC b+,JEKFGL2EKI#Co#JEK$ E\cd'e^C;f ,**.  2 ;b1gh

K]j rQ#gC

+, KAC, 2 KAH, -.Z[;$% +10-. +3 ¡ ++ G<= +3 ¡ /, Gw¢s[

Fig. , Hz 304567810'i]j; klm

CrQ#EK+,'£a;JEKL> !" #$KAC, n KAH, opq

rQI + `9s -%BC JEKFG'¢|)I)C;f L\'¤r`L\'tb2Ms9#a; b[,uo

;$%&' Fig. . () b1gh+,¥UvVw2bx@yCz{d|,**.  0 9s 1 9:Z[ ¦§,u+,$}~n BCb2z{d¨©¡ª -*«+** ¬BC'()C n >n  , V­2EKg'£a , ®(KAC,, KAH,) MAE 2RSw'vg¯ª°ok .*«+** ¬BC ¯;gZs

¦<·=>·¸?·¹·º@·»A·4¼·B·r¦·A·C1

396

Fig. .. Macrograph of seismograms of type ,events. We can conﬁrm long-period waveforms
in these seismograms clearly.

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

Fig. /. Waveform examples of type - event occurred on June ,-, ,**.. A vertical ground velocity recorded at KAC, is shown in the top
panel. The second trace is a high-pass ﬁltered
ground velocity with a cut-o# frequency of +Hz. A low-pass ﬁlter with a cut-o# frequencyof +.* Hz is worked on the third trace.

$

 %&'(!)*+!,-

],> 234 + 234 , gh

./01 234 , 56"G9E + Hz I9

3789:3789;<=>?

E *.- Hz Lq2LBW?J - 

 / @A! +** B, -**  CD Fig. 0b  234 - ghG+EFG



H!#?

I

LMq

L8 Fig. 0a q

2LU, +* BW?LMq

L( J KLMN"J !OP8 Fig. 0c RS

> >¡  ¢-I£Q   Fig. / RS T234¤.LMq

L8/Q I

 U ,**.  - VB, 1 VBW;<=>9E + Hz Lq2LBW?

XYZ

$3789[\ ]

LMqL809E^ Fig. / R?  _ `Fig. /  +.2* *.- Hz Lq2LBW?LMq L

a  . !XY! bc !

8¥ RQ U,1

( +** N `Fig. /  +/2* a  +* S s]]XY#

XYS!

T>,gh¦§B,23>?¨©4hw!ª
5 . B ¦6;< T>,ghtdef? T>,ghijk

=>I,v ¦§B,23>?¨hw!Q, ;<lm n3op7q(r=s t]

u .v hwjk[ Qx<=>h«!=,!? I + Hz Lq2LBW?

S yz{|T} Q(~hw!"# hwT$TTT LMq L8B,

hw

TghU,1  P wx"J !¬#?LMq L
8!7>I( $"x!­®8¯°!¬x!{ T>,gh + Hz lOP! c + Hz l

LMq L8B, $"x!

J %&

$"B,ghwx!{? TT %&S'(E)* QB,

E km B, +* km )*Q( jkg WTT

 *.- Hz Lq2LBW?

±²234 + gh jk9x!=>?

³£-. ¦: , I

MAE ´µj;B,¶S"

Tu . (Fig. 0a) 234 + ghMAE ,**. hT~ °C±²O ¯yª
(a)

³A

397

(c)

(b)

Fig. 0. Particle motions of some unique events.
Horizontal particle motions are shown in theupper panels. Vertical-EW plane projection of
the vertical-transverse particle motions areshown in the lower panels. Solid and brokenlines indicate the particle motions inferred using
low-pass ﬁltered ground velocities with the
cut-o# frequencies of *.- Hz and of +.* Hz,
respectively. (a) type + ; June ,1, ,**., +1 : -2.
(b) type , ; June +,, ,**., +1 : .+. (c) type - ;June ,-, ,**., . : ,-.

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

! , "#$%&'()$*+,-./01234567 8/9: ;- <=>?@A ,  Z[  + P\BBZXY
45[L (Fig. 0c)8 y^ A

KAH, 

Y[L no S

KAH, #$%&'()$*+,



-

BBCDE(FGHI233JK.L  M./S 8 KAH, #$%&'

Fig. 0a OPQRST233J8()$*+,N KAC, /L

N>?

 ,  I UVCWXY Z[

 + P\BBZCD]-.^P/ +**_,** m῔ῐ῏ΐῑῌ῎῕ῒ῍

XY45[L (Fig. 0b)8 `A3a CDbcd" CW-.efO23g8  - ,**. h 0 i ,- j . k ,- lmnop,qP 67 8

KAC, g ,**- h +* i +1 j KAC, MNl) $-.jxo¡¡ b¢9:O8 /rst\uvC 

wxayS z{A|l +* }~@A23-

1 8 ª. XY9:

I UVCWXY

+*

­0

£

n

SAN [L¤¥no¡¦§¨©q9: Fig.
¡

KAC, @« /¬mῌs t¡t® 8 ¯yª

"jDklDgmD.nD&'DopD;qDrDS"D pDs#

398

> (KAC,, KAH,)

 P 2 S 2yD

F2G#$ A w&9A

E!"2G&

 gI Gg &{Q lI6 +AzP 2  ,- Ss&
;&}g $| p   J$p &~I Double Difference L

HI9JK

CDD LE (Waldhauser and Ellsworth, ,***) MNI DD L #$>:2O
mn& 39P C [U_E  QqOQq CR : Double DifferenceE SS&@ 39 L  N

ITUV >:V&3 
& WU I9JN@

Fig. 1. Daily frequency of the unique events shownwith that of normal volcanic earthquakes observed at SAN from October +1, ,**- to September +, ,**. (From May ,0 to June +-, nodata).

J l    [U_&9A   >:[_[Dhj^o X  >g =&XQ 9N&S  [U_Vb (MAXSEP) -N@ YZ
X  dJ  JJ  MAXSEP  ~#%L 
 1 


  2  ,.     !"
' !&()! #$%& *"+$P 2 S 2E_q£BC>¡¢V> 6!

"7&8

p [\S X I "+]y
> #^ TT[  JJ
WU3 I9J@J& l !x,-./012&3

GPS 45
,**.

$NI^ #~¤`a B, G, E > KUR

T P 2E_q£^ T // ¥&¦B§¨ 

39&

1 & "!"#:;<;=$>?@A% B C&'D( ,**/E ;<;NI P9J  © /¥7ª STy "+>==$F$#G 6)&H*IJb 0** m #$ + km cU X  gI P«KL$ 

mn!x¬^­ d&XB "®e +/**¯,.** m"+&M,NO-.@  +P&9J@  MAXSEP  2* m S°  N

/Q I& 0+POR1#$O+234I ST3 UVW + +P" /** m C®e ,*** mE &±;@  gI J$

/ 5 X JK6  34IR1&Y,

2Gq*²Y&³´Hµ  ,N+P ,* s @ZVZ[\]^U_`+PaVb7cI def d!x:Pzp&9$ 2G#$TmnfT ,**-

 3  / g ῌ) 6Vhij5|# ++ ¥

k8

2





X

MAXSEP  +/* m gl°

Jk8

l



 /+ ¥l d7!"mno XN   [U_& T}g l  dJT9#)I$* [_[Dhj^¶l|"+>/ῌ ῎ῒῑῐ῍ῐ῏῔ΐJ+/ UcUg

M  NI9 " -** m 3B

Ch&

|! 2  -+  +/ J[_[Dhj^   cq ** 9r&gB 3  + ;&ST X)

U& [U_gl°)IJ&·¸@ ¹iI ds tu&<$T ,* q *, 9X!:p!&( g v J$"+#$=I>R1#$ S 2XgB?@

B w!"&9A  "+!xBC , " -** m 3BT"

9J º&>@MAXSEP»+** m q3BN)IJTXB³´M)
.&±;



[

 m¨o¼

 MAXSEP  +** m & [,**. :4X 6D&I²0lm

ab

J

cd

399

e-1.f2+,L "ghi j`ka8/+/0lm
6! D

e-2+,L62U+

1

0 "`ab cd e-3n1o45+,p6 +1** m 3nq9:;
!" ' K 

r"

+,V

6! sbsRt2U67 l8 9uv Pw

=?N +

 ,**, : 3 xy ,**. : 2 x ,. ;z<&q X{|=+,E(" '(}F8=+,E/8>~?8E $@" =?N ," -(/ r ,**. :/ x8 0 x" =Fm(,E q/" '
1 xA8 6BCD+," 6B

J(J+/o Fm+,E ' '" ,**. :/ x8 0 x8 ,67E50lm

11 _'&"F!U" Gw"Fig. 2. Distribution of earthquake swarm occurredjust before the eruption on September +, ,**..
Blue circles represent the hypocenters determined with DD algorithm. Green arrows represent the vertical single-force location of the
explosion earthquakes determined by Ohminato
et al. (,**/). The red and blue ovals represent
the source region of type + and those of types ,and - estimated with the particle motions, respectively.

EH+ sbs E]/" }8 IJK
LM(N+,E/OB12&=?N +ghi j`ka, 

 MAE VE," 678P+)&@ 1
E,E ' &g  #1Q?&o`aN Pw@+," 6745 
C(L q/" , R

S(L(" " 6745 Tv!" U +V@E1 C01¡&Q?OL q/" =?N + 





+*     6

¢, :R!8£W=+,E/'"  =0( 6!sbsRtv!9u !" #$  %/67 X¤v!4E'TL]//'

&'() MAXSEP*+** m +,-.+/1Pw[¥¦" 

1 Fig. 2 2& -+ 30(-."045 6+/67  -** m 8 2**

m 9:; & ' <=>?@-./ B 0** 3C+/ 

/0ῌ" D/ῌ +8E +8+" AFG1HID

JK B// 3CA+/

L!" '01C1

ª(oPw«&E¬¥ '&Pw" 1 xA

Fm1¯

5­

°±I²0lm

sbs®ZB  =?N +

LC (!³M/'" sbs®Zv!673n©(Rt+," =?N +=0L [(´µ+/'v],Q?&'( .M' 6! NO1PQ&R

S"LTU V" P #$ ,.0W-.,2 kmῌs, Vpῌ
Vs  +.1-W+.31

9:&X" %18+,YM/Z["0 ('/1ῌ ῎ ῍ ῏,**-W,**. :4X67§

/¶]¤

ῑ

'/D

nY\-·(^w/_|I²0lm

1^*+/ ' I²0lm
0ῌ ῐ_'&"+," 67§ nY ¨©X¤HZ+/ [ ¸`a  ,**,

: 3 x / ;8=+,E/'(¹b / q/"0(,**/) \],^*/+(Fig. 2) " (),_0L`J/ ,**. :1 xºqc|

6B»¼$d+,E(" 1 xºy" c| 6B4LTUL-QLNLIJL¼@L$pLZLL @L[400   

 2  ,.     

!- " ## $%&' + , -(῏

῎4vwx5

67 +0 #8yz{yzvw59|} $:vw;~5( ,**. #e@+)  * %&' +  +,-@)<./01 2

+02****,( /E D >F)C! -3.   +4

= $>?

: $@AB CD567,849:;<=>? @A

G4%/EH )

B C D 2E/?D

KAC, KAH, MAE

F ><GHI* C1 %&' + 
@

JK

 L M)NOPQ< R" @STU BVJ)NMPQ< %&' ,  WX) ! Y YNZ) +*3,* [\!!!"]^O  ,**. # / < 0 )_`$ )
 > %&' + N1OaabY c%0N
2 ,-**3,.** m )& %&' -deX) !'f (./) 
> %&' , [g*?) & 2  -+ < 3  + )_ 0** hij+@D ,3 dee) <jZ
k ,l

-*

 -+ h./>?/ Double

Difference 0)Nmno p+@@AB

c%0N2 +1**3,,** m )[gq)& <  +@1rsrit))Z@2 23u)NOPQ<ῌ ῑ ῐ ῍
IJK L L28 L M L N  L @O(,**/) ,**. #e+@)O ZP' @/* $(-QRSLTU LVWXLYLN L
MLZ L[ \ (,**/) ,**. e+@))Z]F ,**/ # ^_yz`z¡*-¡¢£a¤`G (,**.) e67 +0 # $,**. #( @¥x¦§b @+@ac`d¡¡e 23 ++ῌ,-.
S¨f© (+331) 4ghiª«h¬@)0_­jrsr®¯9-5°±k @ ., S+1/ῌS+21.
²³ l (,**-) e@¥mm= n´-zvwµoe 12 ,2-ῌ.0-.
pi q (,**/) GPS `r ¶·¸¹e@rsr®¯º @ /* -.1ῌ-0+
Waldhauser, F. and Ellsworth, W. L. (,***) A Double
Di#erence earthquake location algorithm : method andapplication to the Northern Hayward Fault, California,
Bull. Seis. Soc. Am., 3*, 0, +-/-ῌ+-02.
$s`tu vpw»(