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,**/  1  ,,  ,**0  ,  ,2 Precise Hypocenter Determination for Volcanic Earthquakes
Observed at Asama Volcano, Japan
Jun OIKAWA῍, Yoshiaki IDA῍῍ and Hiroshi TSUJI῍῍῍
Hypocenters of volcanic earthquakes in Asama volcano were reexamined and precisely determined using
additional four seismic stations that had been installed close to the summit crater from +330 to +333. The mostsigniﬁcant result obtained in the present study is that the groups of shallow earthquakes beneath the summit crater(B-type and N-type earthquakes), which show broad distribution within the depth of ,./ km according to the old
calculation without the four summit stations, are compactly concentrated into a shallower range within the depthof 0** m underneath the west side of the crater. In addition to the conventional method of the hypocenter
determination, the double di#erence method was applied but its application turned out not to have a signiﬁcant
inﬂuence on the result. The summit stations were also quite e#ective in constraining the seismic velocity structure
and the P-wave velocity of Asama volcano was determined to be -./ km/s so as to minimize the variance between
the observed and calculated travel times. Most volcano-tectonic earthquakes (A-type earthquakes) are distributed near the sea level under the west ﬂank of the volcano. The precise hypocenter determination has revealed
that there is a seismic gap between the shallow and ﬂank earthquake groups. It is inferred that this seismic gap
may represent a magma reservoir because the gap agrees with the location of the pressure source that wasestimated from a previous study of the ground deformation.
Key words : Asama volcano, volcanic earthquake, hypocenter distribution, double di#erence method, P-wave
velocity+ῌ ῐ ῎ ῑ ῏y2- %z{|?2nql +33*  1  ,* %}X !"#$%&'()*+,-.

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 S-  #V?2 WX#YZ2[\%]^_

0H ?21r#{|:#>()

2[` 97`>@aPA ,* bc%de_ +30* fg,**. #?2 +31- #?21r+ hi/?2>ῌj%klma_OPn oRDpz

qgl1rs#tk0uvwnxuPA +31-  +32,+#+iaP2#s+ % ++-ῌ**-, ¡¢£ +ῌ+ῌ+V¤¥¦§Earthquake Research Institute, University of Tokyo,
Yayoi +ῌ+ῌ+, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
 01+ῌ,,*+ ¨©ª«¬­® ,+01¨©¯V¤°¤¥¦±Graduate School of Life Science, University of Hyogo, ,+01 Shosha, Himeji, Hyogo 01+ῌ,,*+, Japan.

O__u) ,**/A Fig. +  +31*  ,**/ g -2.ῌ**0+ ²³¬V´]µ´¶·¸ 0-*V¤¥¦§²³2¹¤¥¦º»
Komoro Observatory of Volcano-Chemistry, Earthquake Research Institute, University of Tokyo, Kamasu-aza-Sakahomachi 0-*, Komoro, Nagano -2.ῌ**0+, Japan.
Corresponding author : Jun Oikawa
e-mail : oikawa@eri.u-tokyo.ac.jp

ab º]_`»]

118¼

Fig. +. Monthly frequency of the volcanic earthquakes observed at Asama volcano from +31* to,**/. Arrows represent the time of eruptive
activities.

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Fig. ,. Network of seismic stations in Asama volcano. (a) The whole network. (b) The network near the summit crater.

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>

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&3{$< +! -/ m 1=Q &¢Fig. -. Hypocenter distributions of volcanic earthquakes at Asama volcano using the network (a)without and (b) with four stations near thesummit crater.

rK% 7\]£#k#=3+ A 67+ B 67 |'¤ ¥(#¦1 Sv+ +33*  FG%Q>$@+ §)¤ wx#{ $=Q_®¯67+,3%Q+ + >- 

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·¸ ¹uº!»/uµ120¼

k02* l ,- M"XY%m
 !& 01nHao\L -& ./& 3?0@ apq c]$F rstuv!& +33+w pqxb -.
"E. 

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 ./& !V}~#

MNnE !#FG& %E)& !V}~#

$> m !#$%(Fig. .. Variance between observed and calculated
travel times as a function of shallow seismic
velocity for B and N type earthquakes () andfor A and F type earthquakes (').

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0 kmῌs

& N  !01^_2*QH& DD " Fig. .

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 ,- !"& ./45+ ,!

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3?0@ <AB P "CD / B @ ¡f Fig. 1 (a) QI\ (b) % DD "!& N 

!/ P  -./ km ῌ s

& ¢£¤¥Q-z#¦§¨©%# !

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MNO"EPQ

H& -./ kmῌs "- R( P& S
T

& 3?0@<AB P  -./ km ῌ s "]^_``aIH2* & ab

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I(EH& 01^_2*

U%
P  -./ km ῌ s "FG!V Fig. /"E P  ,./ km ῌ s "!V(Fig. - (b)) " 7%"& !W M"XY E

EP

Z& #­jE- !"EH #"E®¯°/ A  !,]& ­ 8Agh>& !V+Z[3>EP- B

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!QI\ N ``aIH!,]& !W01^_+ 0** m cH A  ! ,- ¬() - ¬& F  !

/& ./& ²ab /** m & b-** m cHd)& 01^_2*+ 1**/m  E .,]2*e]"EP/ῌ+ 

- A  !e]a & f gh

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Fig. /.

 

121

Hypocenters determined using the P-wave velocity of -./ km/s that is considered to be of the best estimate.

Fig. 0. Hypocenter distributions of volcanic earthquakes at Asama volcano determined by the double di#erence(DD) method.

¶· ¸ ¹º»122¼

2 %(J/KL3 (-./ km/s) M2DE
KL N P GHKL3O %(%(P CQRSTU P GHGEKL  &'
JV W P GHXYM2CGE Z[ %(KL3GE\ ]^_L`a 9:<GHKLF
 ,X<bGHKLF-  cHE_/ /^_L,C
cHde ]N %("#$KL+3M2fgb$] QRS` Fig. 2 h A @ F @9:<A+ -%(J/=>TU

iFig. . hj klmn$ -.2 kmῌsI. kmῌs oHK

L$p- A @ F @q&r

SspEt`2 u v$] -rS,^_Lw+ %(spEx)F -/$]yz/ῌ, 

{| (+33*)  }~+
+3-3I+3/*ῌ/-  +3/.I+3/2  +310ῌ11I+32+ 

~d tC%&

 v , km `a^ (Fig. 3) M$p -A+ -^ 
A^$0  B @ W 
A @ WW]- 

+ !u T$]
2 b+E W Q
RS

{| (+33*)  +3-.I+3-3 ~

n}~ %&, km, v 1 km`a ^ (Fig. 3) $$p + A @^M2 ¡¢$]\ C£¤  iD

(+33*) +32*  ,**/j ,**. ¥ GPS Mi¦!

(+33*)  ,./ kmῌs 

WrSv-^w+ +33* I,**. FFig. 1. Hypocenter distributions near the summit
crater determined (a) by an ordinary methodand (b) by the double di#erence (DD) method.

-¡¢ ,**/j (Fig. 3) KL`2 }~` rS§¨ A©

A @ ,W-§¨"u! "#$ %&',Xªb#«F WQRS()*+ ,-$ ./ "# ¬­®MF Q

$ %(01234

RS56782 9: , ;<=>0

W&¯°$X$+Fig. 2

? B @ N @9:<A+ 
9:BCDEF

P GH ,./ kmῌsI-.- kmῌs u B @ N @ & 
Mz0R}~$ %&'(f± TU$C
 ²³´bµ`2 &r

:^ R

cd>eD123;ῐ<=>B?-GHIABJK*- A  B LMNOPQ
RSG/ῌ-,**. 

T # $ (,**/) UVWXYZ[ (,**/) (,**. \ 3 ]:^ _`

2 ] -+ ab 3 ] + a

RLcd

>eD*A? > 234 -** m b 1** m fgABh A ( TYZRh B N ῏@B**ijkl
3 ] + a _`mn*

T#$ (,**/) U

VWXYZ[ (,**/) (2o/p KAH,)

KAC qrstABuv 
Fig. 2. Variance between observed and calculated
travel times as a function of shallow seismic
velocity for B and N type earthquakes (») andfor A and F type earthquakes (¼) using the
network without four stations near the summit
crater.

3_

2E

Fwxyz{|o/qrst*- TYZRtqrsB}~c81>eD6 Bv 

T#$ (,**/) U

VWXYZ[ (,**/) ( ,**. \ + ]b ,**/ \ +
]

Ro/6



*A?

) DD 0>eD234b4 + km 78

04 + km R*>h A ( TYZRb@+
>B?1 ,**. \ 3 ]9 =(b ' # $ ,**/ (Fig.
3)

῍34b ) 234;ῐ<=>
1` 

Bv &' # $,**/ (Fig. 3) A01 A  1 B  N ῏
@RLA1BCD6 01@R 

0G

?1 Bv   9`( )o/p
?o/-
Fig. 3. A vertical view illustrating the relation between pressure sources revealed by analysis of
crustal deformation data and earthquake locations. Solid circles, a solid triangle and solidstars are isotropic pressure sources determinedby Miyazaki (+33*), Oikawa et al. (,**.) andTakagi et al. (,**/), respectively. A solid rectangle is an intruded dike determined by Aoki etal. (,**/).

¡¢R(£¤¥¦ῑAB(§
0

TYZ( :^ * ) 2EFo/p¨©
 1Bc80ª>eD«¬Bv



+330 \ ++ ] ­+332 \ +, ]qrst :^R

>eD*A? 94®

¯b
+ :^ 4:° P ±²8( -./ kmῌs Rῌ ῌ  B  N
"#$ (,**.) %&'#$ (,**/) (,-./῎01

)



 !

)*+ GPS23456 + km 78

9:;ῐ<=>?῏@ABCD(Fig. 3) A ( B  N ῏@EF,B  N (2%%0-34R

U³ .** m 78 ´µ³ ,** m 78¶·¸ 56() 234b 0** m 78 Rfg
-

B  N ῏@34( 

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  
῏



῎
!"#$%&'$

()*+, -./ !."01
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Hirata, N. and Matsu’ura, M. (+321) Maximum-likelihood
estimation of hypocenter with origin time eliminatedusing non-linear inversion technique. Phys. Earth Planet.
Inter., .1 /*ῌ0+.
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Minakami, T., Utibori, S., Hiraga, S., Miyazaki, T., Gyoda,
N. and Utsunomiya, T. (+31*) Seismometrical studies of
volcano Asama part +. Seismic and volcanic activities ofAsama during +3-.ῌ+303. Bull. Earthq. Res. Inst., .2,,-/ῌ-*+.
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Waldhauser, F. and Ellsworth, W.L. (,***) A doubledi#erence earthquake location algorithm : method andapplication to the Northern Hayward Fault. California,
Bull. Seis. Soc. Amer., 3*, 0, +-/-ῌ+-02.
Waldhauser, F. (,**+) hypoDD
A program to computedouble-di#erence hypocenter locations. U.S.G.S. openﬁle report *+ῌ++-.
Wessel, P. and Smith, W.H.F. (+332) New, improved version of the Generic Mapping Tools released. EOS, Trans.
Amer. Geophys. Union, 13, /13.
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