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Three-dimensional P-Wave Velocity Structure and the Explosion Field
at the Shallow Part of Suwanosejima Volcano
Hiroshi YAKIWARAῌ, Masato IGUCHIῌῌ, Takeshi TAMEGURIῌῌ, Tomoki TSUTSUIῌῌῌ,ῌῌῌῌῌ
Jun OIKAWAῌῌῌῌ, Takahiro OHKURAῌῌῌῌῌ and Hiroki MIYAMACHIῌ
Three-dimensional P-wave velocity model beneath Suwanosejima Volcano, southwest Japan, was determinedby inversion of 10. rays from 3 artiﬁcial shots recorded on 31 seismic stations. We constructed -/,/*/ gridmodels in order to perform the inversion with the possible grid spaces. As a result of all inversions, thecalculations of the ,3,3+2 grid models converged. We also averaged the P-wave velocities calculated with theselected -** models with AIC (Akaike’s Information Criteria) smallness from all the resolved models to obtain theﬁnal P-wave velocity model does not depend on the grid conﬁgurations.
The ﬁnal P-wave velocity model is well resolved from the surface to about *.- km below sea level. A region
with high P-wave velocity up to ..- km/s exists below sea level at + km west of the collapse caldera, which may
represent silicic intrusive rocks. Besides, another high P-wave velocity, mountain-like, body that graduallyextends from sea level to the vicinity of the active crater of the volcano is revealed. The peak of the body,
however, shifts southeastward about *./ km from the active crater. We, therefore, suppose that the high velocity
body is a result of the accumulation of the dense volcanic blocks and/or agglutinates in the vicinity of past eruptive
vents through the growth process of the volcano ediﬁce. On the other hand, very-long period pulses lasting before
and after explosive eruptions and the explosion earthquakes occur at the inside of the high P-wave velocity, -.*ῌ-.2 km/s, area beneath the crater. It is, therefore, concluded that the explosion ﬁeld of Suwanosejima Volcano is
located in the high P-wave velocity area.
Key words : Suwanosejima volcano, three-dimensional P-wave velocity model, inversion, explosive eruption,
explosion earthquake ++-ῌ**-,  +ῌ+ῌ+23,ῌ*21+ +*20+
 !"#$ %&'(
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Earthquake Research Institute, University of Tokyo,+ῌ+ῌ+ Yayoi, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
Nansei-Toko Observatory for Earthquakes and Volcanoes, Graduate School of Science and Engineering,  230ῌ+.*. -./012#01345 /,2*
Kagoshima University, +*20+ Yoshino, Kagoshima (*6789
23,ῌ*21+, Japan.
Aso Volcanological Laboratory, Kyoto University, /,2* 23+ῌ+.+3
: ;* +1,,ῌ+3
Kawayo, Minamiaso, Kumamoto 203ῌ+.*., Japan.
 23*ῌ**0/
<=)!"(*>?6789
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Sakurajima Volcano Research Center, Disaster PreEarth and Environmental Science, Graduate School
vention Research Institute, Kyoto University, +1,,ῌ+3
Sakurajima-Yokoyama, Kagoshima 23+ῌ+.+3, Japan.
of Science and Engineering, Kagoshima University, *+*ῌ2/*, FGHIJ +ῌ++ῌ,+ῌ-/ Korimoto, Kagoshima 23*ῌ**0/, Japan.
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Faculty of Engineering and Resource science, Akita
Corresponding author : Hiroshi Yakiwara
e-mail : yakiwara@sci.kagoshima-u.ac.jp
University, +ῌ+ Tegata-Gakuen, Akita *+*ῌ2/*,, Japan.

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77

Fig. +. Right : Location map of Suwanosejima Volcano. Left : Map of seismic stationsof the active seismic survey. Solid stars and crosses represent artiﬁcial shots andseismic stations, respectively. Triangles are the summits of the peaks of the
volcanic ediﬁce (Tn : Tondachi-dake, Ot : O-take, Ng : Negami-dake, Su : Su-saki).
The center position of the active crater(Cc), O-take Crater, is also shown with anarrow. The black contours (contour interval +**m) represent surface topography.

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the grid model. (b), (c), (d), and (e) are the P-wave velocity distributions smoothedwith +**, -**, /** and +,*** models, respectively. Velocities are contoured every*.,/ km/s. White lines represent the shoreline. The regions with low resolution
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-  P  !"(Fig. , d) A +,*** :;<] (Fig. , e)-  P K:;<w{3 AIC  + IJ2=:;<9. u,v,:;<2J,
/ P K9]j/S m\0 R0 w{HJ,/K 9+/012 .῍, m

 / Resolution 9! H-.0 "z /** :;<  ] K

LM5N)

9j/

j/komPC K)g+C *.+*., km/s mn/ SS
IJ P Kw{2./:;<=9  2
-**



0K

H0 -** :;<9.-w{H-J,
9f P -  P K:;< H0

w{2./:;<=) +** ?@  #n/. 

$782.- AIC  ++** :;<2 P 
KResolution 9Y@-¡10¢ Resolu-

tion ) *.0  P KY9¬H-e#]9l u78 P K H0 0kH e#]2
./ P K¯=

Resolution ) *.0  P 

~m3 £¤¥¦5§56¨©ªm«s)%.
  (Menke, +323 ; Wiggins,+31,) ­®

9H-  mB/ yzJ,/HB.9

K= ) w{2.0:;<= /*ῌ 9°m 0K

/?@  )D 2&/ YPH-²³H u)SHB.978 P K3¡1J,PI\0 H0 P| ].0

H0 -῍.

w{±23 
/ P KY9

9^_j/2n0l :;<2´µ0EY'o3.ῌ+ 

\-.P.

t±¶2·j/()* ¸U*Y@ K

w{2./:;<=)¹P.?@2|c}¨©ªmn/£¤¥¦5§56LM5NK

3 ¯ :;<45682c/pq)º/) »:;<9.-+,¼½9^_H K:;<¶

- P 

Fig. -. Results of the checkerboard test on six grid spaces. The velocity patterns are
depicted at the three depths that are arranged for the grid nodes on each grid
space. “AIC No.” is the number countered in the order of the AIC smallness for thegrid space. The parameters providing the grid space are also shown. Top : Depthof upper surface of the grid space. V. Int. : Vertical interval between the grid
nodes. H. Int. : Horizontal interval between the grid nodes. Disp. N : Northward
displacement of the whole grid space. Disp. E : Eastward displacement of the wholegrid space.

81 

82 

!"#"

$%&'()*+, -./0*1*23.e, 45 Fig. .g 678% Resolution  P +*9 -῍, % + :; P <)= >?<)=

@%AB CDEF$,GH@E +*ῌ %I,J*2AKE/ῌ+ 

7 L

L%MN7O BPQJ*/ῌ+ῌ+ *./ km

2%R7S7T U% V%I,OW3+*9

*./ km A4X@ 7&'()*+, -῍- @\ ]



P

 YZ[ (Cc) %!"#T$%A^  R%&#A'(_T`)  F$87 - :; P  <)=@-./0*(Fig. .a) ^%a*bYZ[ (Cc) @\*c

1*23+*9@,+dE -./0*1*2%3

N R%!"#Ae,` @-.7  YZ[ (Cc)+*9@f9(g'(%hi/,j

kAF$870 ,.2 kmῌsFig. -  - :; P <)=%Ql% 0 W%<)YZ[ (Cc) %1& !&AmXEn20=)go= AIC 3V + -* /* +** -** 45 /** %J*

A4,7.5k  ,.- ,./ kmῌs %TF$8

2,AWMEpq-./0*1*2&'(%rs,7 \ &#.5t!#.5k  YZ[ (Cc)u` &'(rs%R7S7%6 J*2T,7m877 AW7ET'k`  6%%#8

v  wx9m8 LA - J*2 A4X 

D YZ[ (Cc) m877

3+*9,u` MN7%<)=A4MEO yH

T'z $ $%A^{=>9m8'2

AwO|M  -./0*1*23+*9%hi/T:L}%8D,jEM 6@A%~`  6%6

AW7E2:;%<^?M AIC 3VA AIC Tz T AIC 3V /**@ %@AB; (Fig. .d) Ai7EM  %<)=%hi/T AIC 3V + %<)=%O%@+d YZ (Ot) %&#A ,.* kmῌs k%C'EODEAFqEM QAjKM \ wx(_TG$87  6%(_

9%J*2A I}%T$%AM

%,jE4BYZ (Ot) A+E$%A'$ -./0*1*23+*9%hiMN7%<)=
A4MEONOA?zM 

2:H| (Fig. +)@-J7 T .K@LMK%e,

 *./ km  % (Fig. +) m8.)*+A6%(_%:PH|t

J*2 6% xAQ` 5TM$

R9%ST @7EMM@-.7  6

hi/T'M@K87 %6@AKE ]T (*.1*.2 km) U%

.ῌ,  

'*=%,VE,F$EM ]WpX%%YZ[  %J*2A%EF$AC'kT:i7EM T/T

$87  P @A %YZ[, ¡/2 :i` 6@AB %YZ[%T
F$87@ \¢£

6%, Resolu-

tion @¤` Resolution  * ]WTNz_M m8 +]WT©N %`¦,@

 Fig. --./0*

1*2&'(%3+*9A Resolution T *., «%f

/ῌ+ῌ, *., km

*., km A4X

 YZ[ (Cc) H|m

8!"#T$%A^ 0 -./ kmῌs -.* kmῌs ]^%$%A'(_TR%¥¦,§B¦¨ ^%a*b%aªYZ[ (Cc) %!"#0 *./ km H|
Ae,`@K87  YZ[ (Cc) m877

9+*,¬­Pc b®,+d R%rs Fig.

¯qET'k`- %MN7% ¡/2AWMEO Resolution T *.0x%(_  -./0*1*2&'(%3+*9%

BO±tm

°L

A

 *./ km (Fig. . a) @   *./ km 
 !#.5k  B'

hi/T?M6@Tmq 6%(_>  °L,u` &#.5k¥cA4MEO ¥¦Btt

I% %3+*9tId%MN7OT?zhi7'EM @K87  ¯qE \¢£  *.0 , Resolu-

°L,u`B R7T&ALmqE²TB,OWQ/ῌ+ῌ-  *.+ kmtion %M[@E³e*., km (Fig. .b) %@´B $%A/  !"!"#$% P

\¢£

&'

f87 - :; P , Fig. . Au

^(_ -.1 kmῌs x%(_ TYZ[ (Cc) %& !&#@"#A²T(Fig. .c) µAYZ[` Fig. . A *./ *., 45 *.+ km A4X

 (Cc) ,¶¨&!&῍"1"L@ YZ[ (Cc) ,B; (Fig. .ac) @YZ[%aª (Cc) ,g

g8M1"῍!&L% , W%·,OWQA¸@AB; Fig. .d, 45 Fig. .f ,u @A

K  ¹®%·xA a?YZ (Ot) hºi@2»

B;A Resolution %,¬­PcEu Fig.

¼A?Ahº@K87

j  ½\ -  P  !

83Fig. .. Final three-dimensional P-wave velocity distributions. (a), (b) and (c) are theplan view of the obtained P-wave velocity distributions contoured every *.,/ km/s
at depths of *./ km, *., km, and *.+ km, respectively. P-wave velocity distributions on west-east (A-Aῌ), and southwest-northeast (B-Bῌ) cross-section are also
shown in (d)-(g). These sections pass through the center of O-take Crater (Cc). (d)and (e) are the west-east (A-Aῌ) cross-sections contoured every *.,/ km/s, and every*.+ of diagonal elements of the resolution matrix, respectively. (f) and (g) are thesouthwest-northeast (B-Bῌ) cross-sections contoured every *.,/ km/s, and every *.+of resolution. The region with low resolution is masked by gray.
 +32- "#(Ng) $%&' ()*+,-./

01 23456+78 -./ kmῌs 9:  ;<..+ kmῌs = ;<> (Cc) ?@ *./ km AB?C

> (Cc) DDE ?D?FGH?C?IJK5?@ + km 5ALM$%&' ;<> (Cc) NK

$%O ;<> (Cc) ABPQ'5ORSM/ -.2 -.3 kmῌs =TU 678/$%O+V

GU6K&'WXYZ&..*  

84

P 

/ῌ,

Fig. .e   (Cc)  (A-A῍)

Fig. .d'()* P  (Cc)
(B-B῍) '.d!"#$%&%

+  Fig. . f Fig. . g,-. (Ng)    ῌ ()* P+ /0 12 Fig.

+ ( *.,/ kmῌs 34 56

Fig. .f  P+.* kmῌs 34 76 89:;<=>?@2A8
Fig. . e

Fig. . g  P+

 ( Resolution 8 *.+89:;<BCDE2A8FG HIJ8'
KJ LMJI2 P+ NF> O@

JIPQR8STUVWS *.- km

O@JIUVWXY-F  (A-A῍) ῌ (B-B῍)
8Z[I()Z\A]^(_+`a(Cc) 8bFcdK`.e(fY-gh0 2ij H8'()*fY-Y8k<l (Cc) XYAm *.-*.. km (Fig. . d) (noj  (Cc) S(pj\Z`Z 
 (Cc)  S8+  ,.2 -.* kmῌsqQI2 r ,.* -.* kmῌs 89:;<gs0atu0ano0 Fig. .d, )Xv Fig. .f  0`Dw  (A-A῍)()Z\ ,.* -.* kmῌs 89:;< xyz{|! gmFg

8k<l XY(}~ s08(^j\ mF

tu0 (Fig. .d) 12 ῌ (B-B῍)g-.* km ῌ s 89:;<

()Z\ ,.*(Cc) KJmFg

mFg(}~s08(^j\(tu0 (Fig. .f) UVWXY

Fig. /. Comparison of P-wave velocity distribution and the locations of geophysical phenomena location on VLP, CEX and the EEX are
the source areas of very long period phases(Iguchi et al., ,**2), moderate contraction andsharp expansion of the explosion earthquake(Tameguri et al., ,**.), respectively.
The
P-wave velocity is contoured every *.,/ km/s.
The P-wave velocities of the gray area are
equal to or higher than -.* km/s. A highvelocity block (Vp ..* km/s) is hatched. The
demagnetized block (Ueda et al., ,**1) nearlycorrespond to the hatched block. The hypocenters of A-type earthquakes (Yakiwara etal., ,**/) are shown by circles.

F + LMJI2a8"} F
Resolution  *.1-FG Fig. .e, )Xv Fig. .g ῌ (B-B ῍)()Z\  (Ot) 8m8

0 



#$%.()*&FLMJI2 -\ ,.* kmῌs '(8+ qQI )` P-+ aFG (Fig. .f) HI /ῌ+ῌ+ F2XYm(no0)`_+ a  UV(Ot) m8.,b8)`-+WXY-8b(cdK(fY-_+a8'a8 , JI(DI22M O@JI
r UVWXYS()Z\ - `+
/MJI  (A-A῍)()Z\ 

 (Cc) XYAm0(_+aF -.1..- kmῌs 8_+ "M8(^j\ m0( ..* kmῌs '(8-+ῌ (B-B ῍)+ 8*+ j\ UVW,bS()Z\0ῌ+  P 

#$%8KJm8UVWS .KJ
 *./ *.0 km S

( ..- kmῌs (0 P_+ ano0 Fig. .d, )Xv Fig. /  H8_+ aFG (Fig. . d) 

1 O¡F>¢2 j\ £¤JI()Z\  (Cc) XY#$% ¥8"8£¤¦3§¨j2©4ªm0(_+..+ kmῌs 5

8_+(,**1)   £¤«{<;( . ¬8 6 ­r£®

/MJI8(^j\ mF -., kmῌs ..*¯°±²l ³{z78j\´°±²l8£®µkmῌs 5F+

aF -./

¶·-S0 (Fig. .f) HIJ8

a8 Resolution  *.0 *.1 FG Fig. .e, )Xv .g 

9¸LM KJ,-.,bS8.¬ .
PQ *.- km KJ¹ - ¬ PQ *.* km *.1 km (º + Aῌm 58]^`£®µ-Sa8no/j2 .¬*» H8£®µ -Sa :¼½8 P

. - /0 P -234) 5678

<=>

9

%& ' (,**1) .()*?

A; D E-F;./61E-F;G
, >L34EM

N52;%& ' (,**1) OPQ2

7& ' (+333) T89(U12

85:!.."#$; Volcano-Tectonic 2@+P - A;,BC

H0IJ;52EK1 2
6IJ A *2 P -

2(RS

5:!.V:2W,X

6Y;Z[ \!G<#$.=0ῌ,  P@+2]IJ522 ./6  >^?. # +331 ; Tomatsu et al., ,**+


_@.

 +2+- A(B`)52 , >EaM;(Nishi, ,**,) CD. (Yamawaki et al., ,**.) bE

!C"7&' (+333)  T89. Fc ' ,**-

(HdI E 2;JeKdI2f:!. (Tanaka et al., ,**,) G

L. (Onizawa et al., ,**1)  >^9.

gh  E 2;H9(dI2MN;eKkCH9(dI<#$PB`)2

1lmnopIJ;

QqC A; %& ' (,**1) 6r1- E 2)WC678

;

91Su"i9jA P -O

Fc (,**/) 5 P 

O sR , >to2+ >vN;wTREx)yG + >UJC I" ./6 VWz2C52

,CXREx)y .

YIJ # (+331) ?.>

^Zh{-VWz[~)52EK1 G678
 9./6WG2)C" \u2

VWz P -

[

{

!>6

78 f G2dC* : B`

O   WG<]$^ *./ km #$%&'~)52C_`) a 5 P ./61C,  P -|}!C.*O 

N5[~)2fg"J ; h."b92;. cIi P -p d2YeIJ :g

Chiarabba et al., ,*** ; Di Stefano and Chiarabba,)L34g- 678 h[iI*

,**,G @C p 4(2 jk:}.:lfg"J %mIJh  ^Z P -&

>G p dRS52x

' (,**1) 678

9&

jk:n1op)qCsrEs)
P -

52Eyft)2

5

IJ !C"  Z;d:B`)L34

h

- Okuma et al. (,**0)  lt



p

B`

uvh{-O
O

!"#$%&' (Fig. .d)uY3 *.1  *.2 km
srEwGe0.!".<!{ -uY3 6C2fg"J .

rxEy 678&opE; OZ.">^!" z{|4 C:[}"J;678  ¡.:.8~))E¢k (Fig. 0) O:£b[ .<::5

Nh ¤¥¦${|E .§C¨

2xIJ; (Okuma et al., ,**0) 552 ¡.:()*GW-678

9 %& ' ,**1

 h[678 iId:W

W©g P

uvb|ªJ; P -|}!C.*O  .

sIJ2fgE«¬)()*­®¯^Zh{C.:°jk}

±|9 *.- km W9 P ¤*Z p²³' mIJsIJ

 5 P -

1p

9g2fg

;2fg"JC!

T8 (Fig. /) < ' (,**/)  4g .(E´(@+^ZhWµ. ¶U6·tE;m¸52!"  

¹·2.4¯E¶ b9.-D.4B`)5

Y; ºu"J; A *¯ Fig. .d  P 

2ufg"J  Ec]#| (Fig. .

 -.2 km ῌ s -km () (Fig. /)9  I +.-  -./A *+",P - Tanaka et al. (,**,) d) h{ P 

1 (»ª(EK¼½y Fig. / x) (VLP) (Iguchi et al., ,**2)



86

!"

 #"$ %&   '   P !"  ( #(-.*-.2 km/s) )$*+

ΐῒ

%&, 

-.  ./

01'(*+23456 7 )*8+,-./ 901-.:2; <=301>7?56 7 9045 90*@6 A
7B C89 D:;=&<*+E7F ;
GHIJK8 ;L=8=>M01>7?56
7 67 N?@ A$ BCDO4 A7B 9045 
Fig. 0. Photograph of the inside surface of Otake
Crater. The broken ellipsoid marks the sti#
blocks.



C89E

1>7?567 %FPG HIQ 6 1
H.R

H 

HSTU - J F6 7

)*8+,  +,VO/KLM DNWO CQXYQ %&,E7F<=301>

 (CEX)

7?56 7 ^PT_U`V;W aX   (EEX)  , ,**. ;DO=>M01>7?56 7 bcAYO_U>de.R'

f4ZFN3+ P `V'g[=>M01>7?56 7PQRS

Z[\

]5

!" (-.* -.2 km/s) (#)$*+ h6F '

M+' P !"(#)$*

+pqMre+ PQ\rijse+Ck
.Quvwxyz  Pn .Ck'o
r uv=w*+2\r 6607rx=h
rTT3uv*+o3 >

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#$ (,**/) %& ' -  0-0pp.
()*+ (+32-) ,-./01
2  ,2 +*+ῌ++/.
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C7D4 (,***) %#G?+323 
+333  ' - QR$&STU \ +*  +* L +ῌ+*.
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XYZ[ .2 B, ,31ῌ-,,.
C7D4] +1 & (,**0) HI_`4a(! W% XYZ[ .3 B, --3ῌ-/-.
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y/ z{5| }~ (+333) 
HI_`6 78 W% XYZ[ .,
B-+ ++ῌ+2.
+ B] 2, & (+331) HI_`4a(!TUIO#2tu YZ[41, +01ῌ+2-.
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:;< (+3/.) =>X,YZ '* ?X=YZ" + +-ῌ,3.
 (+331) (!qh.JN@
 -D    ., +0/ῌ+1*.
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« ¬] +, & (,**.) 4ah.JNJ­HI_`(! L· ª¸ No. .2 ,-ῌ,0.
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¹KºL] +. & (,**,) ,**+ M»4a(!-  P   L· ª¸ No. -3

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+.ῌ,+.
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 (+3+2)    
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345678 (,**+)  +2+-  9 :;<;=>?  .0 /-ῌ1*.
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*C+f (,**/) X,g-h.JN%(!
ijkl  /* S+*+ῌS++..
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K YZ[4 1* 2+ῌ3*.
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/95B*5  (,**1) # - #  /,,++ῌ,,*.
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!"E F*C7D4@A B A
B
BC ¡¢£  ¤ (,**/) %#7D¥HeJK¦EMN -  §p I¨  F  X©XYZGHED¥YZ D¥Iª .,, ®¯°±²³´ \ +0  YZ\µ"¶ 00ῌ
1*.
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