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,**2 @ , A / BC- ,**2 @ 3 A +1 BC$Relationship Between the Explosive Activities and the Associated Volcanic Tremors
observed at Nakadake Summit of Aso Volcano³³ The Temporal Variation in Amplitude of Tremors Recorded by Wiechert Seismograph ³³
῍῍
Hironori SAKAGUCHI῍, Yasuaki SUDO῍῍, Yoshihiro SAWADA῍ and Shin YOSHIKAWA῍
Aso Volcano is one of the active volcanoes in Japan. Seismic wave associated with volcanic activity had beenrecorded by Wiechert seismograph at Aso Volcanological Laboratory, Kyoto University from +3,2 to ,***.
Some records of volcanic tremors related to large-scale explosive events had been already analyzed, however, many
events were not yet examined. In this study, the previous volcanic activities with explosion are re-examined andclassiﬁed into the following four types based on the seismographic record : (A) The amplitude of tremor was small
prior to an explosion. After the explosion, the amplitude of tremor increased ; (B) A phreatic explosion suddenly
took place without any precursory signal. The tremor amplitude was less than - mm before the phreatic explosion,and then decreased to be less than *./ mm after the explosion ; (C) An explosion occurred after decreasing in
amplitude of volcanic tremor. After explosion, volcanic activity had been increasing ; (D) The volcanic tremor
was increasing and changed into continuous tremors. An explosion occurs among continuous tremor. In
Nakadake crater, types C and D are major types of volcanic activity after +30-. The above classiﬁcation could
be an important criteria for the prediction of eruption at Aso Volcano.
Key words : volcanic tremor, volcanic eruption, Wiechert seismograph, Aso Volcano, phreatic explosion+ῌ ῐ ῎ ῑ ῏

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03*ῌ2/*.  +*0*
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Interdisciplinary Faculty of Science and Engineering,
Shimane University, +*0* Nishikawatsu, Matue, 03*ῌ
2/*., Japan.
203ῌ,,-, :; +3-*ῌ+<45=>?
Aso Volcano Museum, +3-*ῌ+ Akamizu, Aso, Kumamoto, 203ῌ,,-,, Japan.

203ῌ+.*. 
'( ! !)$!*+,-./01!*+234
5*+6789
Aso Volcanological Laboratory, Kyoto University,
Kawayo, Minami-Aso-son, Aso-gun, 203ῌ+.*., Japan.
Corresponding author : Hironori Sakaguchi
e-mail : s*13,*0@matsu.shimane-u.ac.jp

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144

Fig. +. Locations of AVL and Nakadake Crater. AVL is Aso Volcanological Laboratory of Kyoto University.
Solid lines are topography contour intervals of ,** m (Geological Survey Institute in Japan, ,**+).

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, -./0123456!#O$ +3-* $«¬\c@ ABd ,,! ¸s t¦¹  AB

Type A <ῌZº» Type B <῍Zº» TypeC  =¼Zº» Type D  ½Zº»  . y

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145

Table +. Classiﬁcation of Nakadake explosive events +3,2ῌ,***, based on the amplitude variationof volcanic tremors by Wiechert seismograph(EW component)at AVL. * represents the casethat heavy rains were observed before explosions.  and in the column of crater lakeindicates the presence and absence of lake water, respectively, at the explosive activity.

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,ῌ+Type A ῌῑῒῌ῏῎ῐ῍Type A I,-7JKLMNO*PQ

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V_`aI +3-- ] + b ,1 cdVW(Table +) +3-- ] +. beV,-./> Fig. , H5 6fgI,-./ hijk`a>l Periods ῌ ῍ ῎ ῏Lm . ;<=Afg367V_
Period ῌ+3-- ] + bnF^Ihiodep7qr

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+3.* 6sfI~&&'7

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L   +3-3 6fg +3-- ] + bf,-IJK , mm FGJ,-V: 6mM + b ,1 c +. s +hL \PQR7S

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Period ῍+ b ,1 cPQRXO*,-

Fig. ,. Amplitude variation of volcanic tremors whichwere recorded by Wiechert seismograph (EWcomponent) at AVL from January to April, +3--.
The vertical axis is the daily average amplitude. The
period I shows the phreatic explosions from thesmall crater lake remaining at +st Crater. The
period II shows the Strombolian eruptions at +st
Crater. The period III shows the Strombolian
eruptions at ,nd Crater. The period IV shows theactivity of explosion activity at +st and ,nd Craters.
The arrow indicates the phreatic explosion at +st
Crater which is classiﬁed into Type A.

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146

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Period ῍, ' +*

WX  , 3(Y2+3GG=$>Z[< 345 0\].#$ ,-^(> /* mm ($/ A 0 ._`# >  , 3HaKbc?E36$/de<9$+2 , ' ,.
0)- - ' .

()*+>345

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0>Fig. -. Amplitude variation of volcanic tremors from
April to July, +3/2. The vertical axis value is thedaily average amplitude. From April to June, theactivity of boiling and phreatic explosions had
been occurring in the small crater lake remainingat +st Crater. The arrow indicates the time of thephreatic explosion which is classiﬁed into Type B.

ijklm no?p!W" q8 `rV/s +3-3 , ' ,0,1(>  , 3(Y*`)V/ / 'WX>t#:2/A<$uvh $3Gw%,x/u* 1/* y?@/ A&R(TU$ 0 ''z(>$uvh>3 '{G'( |}( <2~x `34), 3 ;<*c,x/ NOPQ +3.*

mm W<=>`)V/ %$+ 0 ' ,.

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08R(TU$+2V/

0><-+3-- 0WX A e DE` 1`34G>2

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0WX

86$+2`2 +3--G1rV/A<>  4 (,**0)8,x+2,ῌ,Type B ῌ

fL$/ (Fig. -) ijk;m no(DO3>
$uvh$/de 345 0>q 8- mm ./rV/ 5)->

Type B > 0 (*  - mm)*  , mm ./vh +3/2 0 >  0
6`7`2f 1`6$/<2345O,' 88  rh 39v

x+2de(:;<$+6C34G

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7`2f6CA ¡ ¢ >¥=$ HIJK73G(>>$`2 +3-* 0 3¦(?§C¨@5q8©ª 34A« ¬5(Y2' ./ +3/* 0 . ' +/

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4 +3/3 345 08 *+ mm <,2

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?$+2/ / ''zDh$uvh IJ$

03'0

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8 - mm ./<0,2f(>~h +3/1 0 +, ' .
++ ' ,

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CD EF?'G

147

HIJD'D&G K EF"#D'GL +#M !

NO$P&'QD'D&GK1RSTD'/ UV ,**1 KWXYZ[+\NO$P&' #$]^_ `abG E 3 @AcNd
 3  0  +-  *0

$]^_eNfghi jk1D'l
DG Fig. . ; m
Period ῏
3  0 Fig. .. Amplitude variation of volcanic tremors from
July to October, +313. The vertical axis is thedaily average amplitude. The period I shows theactivity of volcanic ash emissions and fragmentejections. The period II shows the activity ofstrong ash emissions and strong fragment ejections
with continuous tremors. The period III shows
the attacking of Typhoons 13++ and 13+, in the
Aso region. The period IV shows small explosions, ash emissions and fragment ejections. The
arrow indicates the phreatic explosion which isclassiﬁed into Type C.

Rop\$ q/

!"#

$%&'

(n

p'G b$ +* rsFt u_ DlD'G K$%&'3 @A ,/ mm vFcwRu_ DG
+313 xFy qz{ YZ|O$P&'#}$ ~gcD 3

IJ$&G1

F/ Type C $ b/$ $e$P&'3 IJ$/1 /  $3 IJ$1H #}
BeF +313 x YZ$P/[+\NO$P&']^_ `ab E|$3 @Aw
Cd

IJ lq/1RG&G

K Type C 1&'/ +313
 (x1$NO$P&'3

Rr$z/+313 x 1  +* F3@ABR Fig. . $Sq Kx+, #$-./ ( / e]^_] wC _ tblq/P$&G 0 +s$ !  #$`ab
G

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,ῌ.

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IJ

BD'3 @A N{&'/1{$lD'/ +3// x 1  ,/  +3/2 x +*  +-  +30x ++  +1  +30/ x +*  -+  +31. x 2  . 

Period ῌ
1 $3 @A cND E$%&' 1$§¨DG  d ©N  (Type D ῌΐῒ῔ῑ῏῎ῐ῍Type D  

$y q/1 Period ῌ ῍ ῎ ῏ . X\¥v$./K1 F{/

IJD lDG +311 x 1  1 +313 x ++  F  / (Table + ; “ῌ”(n 

$&G ® +32*

Type D $ª«q/ (Table +) ¬­F¬HfN{

e&G +323 x (Rr$z/+323 x 2  +, F3

Period ῍
1 ©$/1 b$ (23 @AcNDEl¯H°¢ep¢91±+311 x ++  2  +323 x 1  +0 ++  ,0  2 ¦$BD'&G 2 $~/1 +* mm R³´/P@AHµb/P$&G 2 ©P_ (n1 
$wCDG   ¶D_ (FHK´/P$&G ® +32*
Period ῎
2  ,0,2 $YZ ++ ·  3  -. $YZ +,qx +* e+, 

@ABR Fig. / $S#}F§¨?

²lD'G E]^_R`aq/K1 §¨1 (n Ru_ D'G 1 $/1 0$ 1 (nRN $q/lD£G +323 x 2 | (

@AB1 #}¤e Period ῌ ῍ ῎1¥v$./K1 F{/
Period ῌ·  )* (¸¹ºF> ,20 mm $H»q/[+\+323 x 2 NORHGDG 2  ,0 | 3 @AH½¶$w

$3 @A(23 @A¼1$cD 3+* mm R³´/P$&G(2 



148-ῌ ῎ ῍ ῏

  !

 "#

 $ %&'()!*+")!,-./ 1, 01 ,, 21*+3 Type A ῌ45
6 Type B ῍456 Type C  745
6 Type D  456 1 . 456(8
9:;<=>! ? @ 14563AB1C>;D
Fig. /. Amplitude variation of volcanic tremors from
August to December, +323. The vertical axis is thedaily average amplitude. The period I shows theactivity of ash emissions and fragment ejections.
The period II shows the activity of Strombolian
eruptions. The period III shows phreatic explosions and Strombolian spatterings. The arrow
indicates the phreatic explosion with pyroclastic
surge which is classiﬁed into Type D.

Type A 3 %&J;)!K

1E;FGH=<IE;!L456(D Type B 3 %& 1E;FGH=<I
J;L; ?1KM E;FG456
(D Type C 3!1%& ;N<=1"#(OP) ?1KIJ;L456(D Type D 3 1$K"%)!8> E;&! 456(DAQ1 . R14561(S'(GM1; Type C (3 T U Type D 1 2 T Type B 3 . T Type A 3 +1)VM 2 WXY3Z*[()G!

; 3 W+:[_`)! 91ab3 !,1%&e.f;/0)G!  ,**1^ Type C: Type D 1c 1d(D! Type A "-C%&c3./ +** 0(gM!h1>c (Di 1h1;Fj>! +3-- A\3 Type A 1*+3

Period ῍+* W32%c;klmnoV2%p:_`)! %& 1E3 +/ mm AQ:>i )VM
q[

T(D! +30- A\1*+]:)r4)G>G
Type C 1 7A$12%c :E33 Type D

C>! ++ W:5)16_")G! 9 3 st<14561

,+  ,/ 73IJ:klmnoV2%"uv

;LawGM 89=<1xyxz%&{

|i:Lc ;)  ,**1 ++ W ,0 73k1}~; c>G9:")w%"-CIJ;)! Fig. / ; ; i Type C 1c 13 % <9()!N<

Period ῎=1="21>?$ ;7L

+, W>:%& 1E3 2 mm @A(9:;<  Table + ( ῌ "B)!

P)!; % wc 3IJ:)G)3 91="21>;0>\%CD2E"uv|i:)G!  ,**1 [

Fp1G+,1 (Di .>G+ 3 +, W$H(r4 !; ?1K I

!% 1J; 7"89)!:<)G! +, WKH3%& 3 / mm @A1E



p:P)! (Fig. /)

%& :E3"#)!3 KL

Type D 3K1456:3>i 1*+1

L1D2%c 164"`CQ(M>#NO$K(%& 3!8>E"&t  E1>

1SR:>: qa%&r4"`C 1M";¡Pd< >Gc (D 2%c 30>)G) g03klmnoV2%c _`L
'12%c 3 SQ0R¢i1P£i
S¤2f CD2Ec  klmnoV2% R¢i1©¥ W :GCc

. @ ;D X ^ Ono et al.,+33/ ;   K ,**0 91C>cType D 3 '12%cῑῐp

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Mori, T., Sudo, Y., Tsutsui, T. and Shin, Y. (,**2) Characteristic of isolated hybrid tremor (HBT) during a calm
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(+33/) Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan. J. Volcanol. Geotherm. Res., 00,+-1ῌ+.2.

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Sassa, K. (+3-/) Volcanic micro-tremors and eruptionearthquakes (Part + of the geophysical studies on the
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Takagi, N., Kaneshima, S., Kawakatsu, H., Yamamoto,
M., Sudo, Y., Ohkura, T., Yoshikawa, S. and Mori, T.
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