  /*  (,**/)
 /  -21ῌ-3+  ,**. 
     ,**/  --*   ,**/  2+1  

Application of Single Force Model to the Volcanic Explosion
Earthquakes Observed at Asama Volcano in ,**.
Takeshi NISHIMURA and Higashi UCHIDAWe analyze ﬁve explosion earthquakes observed at Asama volcano in ,**.. The main phase consisting ofRayleigh wave is well explained by a vertical downward single force with peak amplitude of +*+*ῌ+*++ N and pulse
width of /ῌ0 s. These source parameters are in the range expected from a scaling relation presented by Nishimuraand Hamaguchi (+33-), which suggests that the ,**. explosions of Asama volcano are typical Vulcanianeruptions. The internal pressure built up beneath the crater is estimated to be *.,ῌ+./ MPa, which tends tobecome large after the formation of lava dome in the crater.
Key words : explosion earthquake, single force, AsamaE+   !"# $%&'()*+,-./0 1

#C!"530=>?"@ZVE ,**.  k# 3+  ,-  ,3

 +*+*  ++2 (Kanamori et al., +32.) 34"53# 67?

< !>?"@ 5!k#089:;#("53<= !>?" (Kanamorir?J0lRand Given, +32- ; Nishimura and Hamaguchi, +33-)@

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DY AZ[\C!"]

+  fE"##333,-   $<0 `!C m12$0CB4"33 EZVEcdef +32* g*Lhi*jE>)4"@E kl> mR < mR53<= !>?" J^_#R@ `> 5ab0 +322ῌ+323 +32- ZV+. qBCqf +32-  &'  +32. 3?( S( TUV , n4"53 A%&[,\C!"] + MPa o#p"53 q DW*¡J¢+, (MAJO) Xr:st4"530uR@

C Nishimura,**. k£¤#: +32- (+332)  vwx#y !Rz+{?"# %#5,b.¥04"@|}.~3 DWX0 58PF<:MAJO  ¦§ STS-+ ¨ /Li©ª«*¬­ ®

 ¯32*ῌ2/12 °±²³-´µ¶³- 0ῌ.S·S·¸·¹º/·»¼
Department of Geophysics, Graduate School of Science, Tohoku University, 0ῌ- Aramaki-aza Aoba,

y>

Aoba-ku, Sendai 32*ῌ2/12, Japan.
Corresponding author : Takeshi Nishimurae-mail : nishi@zisin.geophys.tohoku.ac.jp¶·¸¹º» ¼388

Fig. ,. Vertical component seismograms of volcanic
explosion earthquakes observed at MAJO in,**.. (a) Displacement seismograms. (b)Filtered seismograms (*.+ῌ*.,/ Hz). Brokenlines indicate the best ﬁt theoretical waveforms
calculated from a single force source.

Fig. +. Seismograms of volcanic explosion earthquakes
on September ,-, ,**.. (a) Displacement
seismograms (no instrumental correction). (b)Band-pass ﬁltered (*.+ῌ*.,/ Hz) seismograms.
(c) Particle orbits of the main phase that isindicated by a horizontal arrow on top of thewaveforms in (b). Note that the main phaseshows a retrograde motion as indicated byarrows with broken line on vertical crosssections (left and middle panels). Gray arrowon the NS-EW plane (right panel) indicates adirection to Mount Asama.

Fig. ,  3 Q@ ++ ;Nk/lmI-.῔ no/p 3  ,- [\q rῑ/sEtFu_`vIAab
Retrograde IJ - w,῔6OH῎IA!KLMLN:Ox< yΐAYZ[\z{| }GO
0+*~  /0 v pp ( 6 ,.3m Q@ ,.3+*/ m ;N, ῌ6 + Yq-ῌ ῏῍῎ῐῑUV/|6 ῔Fig. +  IRIS  3  ,- ῔! STS-+ "#$%&BCELtA / qO
 T ,**/ ;/ MAJO N

/'() + BCELtz{|N= T*+, -.῔ / /0 1UVRSe  ON

23'456 7 8  P 9 S 6fghN6/z{

:;<=ῒ>?@8 A BCDE8F mJBCELt   ¡s¢£L

GH῎I6 Retrograde IJKLMLN:¤ ¥?/ ῏῕6 +32- ¦ §¨TOAPQ (Fig. + (b), (c)) ORS6 TUVN(+32.) A|/J[\©ª«¬­{® P #$WX YZ[\N: O / ]^0 kmῌs, S #$ -.. kmῌs, ¯$ ,,1** kgῌm-$_`Aab!KLML:699c_`Nishimura and Hamaguchi (+33-) [\e'ῐ°±²IAdeA fgh6OidG 6Rj῍ ³/ ;/ R´45/ ῕8  ¡µ6 *./ km ³/A |!Au

,**. 4+d3 ¦´J.ῌ ῍389

῎"opq3IJ
+, F " t &Frpr,P(+)

rv

(,)

trcsV<t(J (Nishimura and Hamaguchi, +33-)
VV3 r ,tu-v& P wx+./ =Ny& v ,t0]z1!& c ,t0] 2 C=>/J3g{ - 3IJ (+)& (,) |C=,tu-v$}~sN&ῌv῎log(p῍c῏(-)

logFr,logtlogP,log<3=NJ Fig. . & ,**. 4+d
"opq$& 5wde (CruzAtienza et al., ,**+ ; Nishimura and Hamaguchi, +33- ; 

65& ,**,- k & (-) | 8  (-) |C=7 ,NJ F  t  83
Fig. -. Comparison of the observed seismograms on
September ,-, ,**., with the best-ﬁt theoreticalwaves calculated from a single force source.
The seismograms are ﬁltered from *.+ to *.,/ Hz.

IJ VV

3& cr-& vr+** mῌs  & P h  *.+& +& +* MPa
 - 9$%  C=& ,**. 4+dq Pr+ MPa +8:C=@;& wdGwd& wdyg{<C,$J  V<WCJ !"#$%3 E + & b"g{<3I!"' ()*& +,- F "J<& +,5 F , & b&

 t $. /0& "$ + s 12345 
6$76 8 &69:9$3;

*

 3 E ,- = + k, VN& (-) |>? , ? - MNJ

<=" t $>   & ?@AB"opq+<IJV$ 8C=& +, F $D  Fig. -  3 E

MNJ c & @A 2,- 7*.- ijk=NJ /& ,t1! v & ,**. 4+

6

6$F G

H 3IJ?@AKA (dB,w

L- MNJ +ῌ.

C$iJ  /*ῐ,** mῌs0NO& P$J ? -(+.+ῐ+..) k3 -m aNOk& 5wd!

=NJ3&QRS/TU3+ JV<WCJ X& K(-) |? -  ,.*m*.- ¡D¢£/J ¤Y &AB< ,  !9Y  VN& GH

3 E + +,<,CY& ?

"#Z$<[%&\]^_`$Sa& b'(, MNJwxE@/ =NJy<,CY*+d$ef)cC= -* !gO0N

D3+J Vy+, ¥& Kanamori)c<7(Fig. + (c)) =NJ Table +  / het al. (+32.) ¦§¨a©ª!$J F  t $/ /& Fig. , (b) ?@AkJ F  5 (,**.) & 3 E +0 «¬­6$ F +, +*+* N C=SAR e& wxGH®<¯°±²<Y76+*++ N, " /ῐ0 s ijk=N l&6V$I³

J V´SH®¯°µ9:9C=& +, F  ,  !& " tJB,w¶·¸)KI³,N Vm+ sC=& 3 E + B<bN)¹!n <IJD,NJ F ,H®¯°´Jª!º©<,tu$»3CYD¼,NJ¸¹º»-¼ }390
Table +.

+Source parameters of explosion earthquakes.

Data of air wave and tilt motion are taken from Seismological and Volcanological Department ofJMA (,**/).

 ,**. 

+32-  .  2       (+32.)
Nishimura and Hamaguchi (+33-)  ! "#$%&'()*+,-./012 3 45
6  789:%;<=>?@ABCD
E (MAJO) F18GHIJ!

KLMN% OPQ Nishimura and Hamaguchi(+33-) % WWSSN R G  S

% F T +./ U +*+* N, t T . s  & ' ( ) *  + , - .
/012 VFig. . WXY3*    (+32.) % ASRO FZ[8.F189,\1[],HIG^MN _` 2.*U+*+* Ns a<+,-./012=b* " +1 cde+,-./012=fgh!b Gi Qe=jkl*4mn.2%

a% ,**. opqrdmms4o! +32- ,**. tuv

l*<w=<d Gi
xc CDyzF18  {|de} ,./ km 5>?@ F E~7% f%f

de{G "5e% *.*/ῌ*.+, micro radian 
 (Table +) +,-./012l*4

<%<  % ~G*  <'(!_!%Fig. .. Relation between peak amplitude and pulse
width of vertical single force. Solid circles
represent the data of Asama obtained in thisstudy. Open circles, triangles, and square symbols
are the data of the explosion earthquakes ofAsama (April 2, +32-), Tokachi and Mount St.
Helens (Nishimura and Hamaguchi, +33-).
Diamonds and plus symbols denote Tokachi-dakedata by Kato et al. (,**,) and Popocatepetl
data by Cruz-Atienza et al. (,**+), respectively.
Three ﬁne lines represent the relation predictedfrom single force models with a pressure of *.+,+, and +* MPa.

;< Gi _ " 2 km D CD45>l*4 (Table +)%#e5< 5% '( e5©! ª}«'¬ 1* m, ­4 .* m ®

¯°=

G±# V>?@{*Gl>WH$ * %1&'m-,**/Y ®¯°= 3  ,3 + (w ! -) m¡(¢$¡£¤b¥¦§=;< Gi¨

%³g,HI MN (0 s) ®eps4>?@% 3  ,3 c +*  + ^C²kn.2% + s < o´µ  ¡¶r= +** mῌs!·

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" 3 # +

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!

"7$J- K.LMDNOP
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qr'stGJluKG@d>v w;/ῌ ῎ ῍ ῏,**. CLxF

M / yJ- z{/ +,-, +*+*ῌ+*++ N K.
LMDNOPQR, /   0 x @NO D O

| }~\],^ Nishimura and Hamaguchi (+33-) 32P6/ ,-OP3QR AB$SlC% *.,ῌ+./ MPa

T> o AB$U, 563;
|/ C%)D|MV~ῐ ῕ ῔ ῑCruz-Atienza, V. M., Pacheco, J. F., Singh, S. K., Shapiro,N. M., Valdes, C. and Iglesias, A. (,**+) Size of Popocatepetl volcano explosions (+331ῌ,**+) from waveform
inversion. Geophys. Res. Lett., ,2, .*,1ῌ.*-*.
Kanamori, H. and Given, J. W. (+32-) Lamb pulse observed in nature. Geophys. Res. Lett., +*, -1-ῌ-10.
Kanamori, H., Given, J. W. and Lay, T. (+32.) Analysis ofseismic body waves excited by the Mount St. Helens
eruption of May +2, +32*. J. Geophys. Res., 23, +2/0ῌ
+200.
&')g'h i (,**,) +322ῌ+323 C
=Aj; ]  kl)q mnqUVo 0/ +13ῌ+32.
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WX AFr (,**/) LxF %8 +0 C %,**. C(Co( AF¨/©s %8 +0 C¨/(Nishimura, T. (+332) Source mechanisms of volcanic explosion earthquakes : single force and implosive sources.
J. Volcanol. Geotherm. Res., 20, 31ῌ+*0.
Nishimura, T. and Hamaguchi, H. (+33-) Scaling law ofvolcanic explosion earthquakes. Geophys. Res. Lett., ,*,,.13ῌ,.2,.
)tuY'vwx'y z'ª{|} (,**.) «¬­ SAR JlLxFAB~®P® 6AFq¢£¤q¥ ,**. C(¦§)¢ S*-.
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