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,**/  +  +1  ,**/  +,  +3 A New Seismic Observation System at the Kusatsu-Shirane Volcano
Takehiko MORI῍, Jun’ichi HIRABAYASHI῍῍, Kenji NOGAMI῍῍ and Shin’ya ONIZAWA῍῍,῍῍῍Flow properties of volcanic ﬂuids beneath the Kusatsu-Shirane volcano have been studied at the Volcano
Fluid Research Center, Tokyo Institute of Technology, by using geochemical methods. A novel seismic
observation system, in which seismic signals from two borehole stations newly constructed at the summit arecontinuously acquired with the data from our previous observation system, started to monitor the activity of theKusatsu-Shirane volcano in November ,**+. The seismic signals are digitized with a ,* or ,, bit A/D resolution
and a sampling time of *.*+ s, and are integrated with the seismic data from the Earthquake Research Institute,
University of Tokyo. As a result, the detection capability of earthquakes and the accuracy of hypocenter
determination are signiﬁcantly improved especially in the Yugama region. Volcanic earthquakes in excess of +**occurrences per month / to +* times the number of earthquakes observed by the Japan Meteorological Agency
 were observed by the new system. The observed volcanic earthquakes are tectonic type, with distinct P- andS-waves. A total of 0*, earthquake hypocenters were determined between November ,**+ and March ,**-.
The regions of Yugama and north Motoshirane are seismogenic regions in the vicinity of the Kusatsu-Shirane
volcano. Seismic activity in north Motoshirane, which was unknown in the past, is equivalent to the activity in
Yugama.
Key words : Kusatsu-Shirane volcano, seismic observation system, volcanic earthquakes, seismic activity+ῌ ῐ ῎ ῑ ῏;VMN-t$eK iBu%IvVl/ !" # $% &'()*

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;%|}eK ~0M{Z)->/%D/E0%BFGHI)JKLMN-2 O' -* )zVl (Ossaka et al., +32-)2 +32/ )

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 )-Z Minakami/E^_`a@A/E0 ^bcdeK/fghiB(+3-3) )- +3-1Q+3-3 %Uk01-2 U;jLMN-2 kl mhD/E%n)

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 ¦-**ῌ*2-1 §¨a(©ª +ῌ+ῌ+ «¬ 1­®¯°±d²³"
Geological Survey of Japan, AIST, Tsukuba Central
1, +ῌ+ῌ+, Higashi, Tsukuba, Ibaraki -*/ῌ2/01, Japan.
 ¦-11ῌ+1++  ´µ¶·q¸ 0.+ῌ-0
q/ ¹º»¼Volcanic Fluid Research Center, Tokyo Institute ofTechnology, 0.+ῌ-0 Kusatsu Agatsuma, Gunma -11ῌ
+1++, Japan.

 WX : ¦-**ῌ*2-1 §¨a(©ª +ῌ+ῌ+ «¬ 1­®¯°±d²³"
Present Address : Geological Survey of Japan, AIST,
Tsukuba Central 1, +ῌ+ῌ+, Higashi, Tsukuba, Ibaraki-*/ῌ2/01, Japan.
Corresponding author : Takehiko Morie-mail : mori.may*,+,@aist.go.jp . ¡¢.£¤¥.¦§¨©

42
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 +321 CB7^_`:a

( B- 5:;0

Fig. +. Location of the stations around Kusatsu-Shirane Volcano. Solid circles () are new stations installed byVolcanic Fluid Research Center, Tokyo Institute of Technology (VFRC). Borehole-type seismometers andtiltmeters are installed in KSW, KSE and KSS, and seismometers buried near the ground are featured at KSJ
and KSVO. Solid squares () are data relay stations of VFRC. Open circles () are seismic stations ofEarthquake Research Institute, University of Tokyo (ERI). Data from ERI stations are transmitted throughsatellite communications. A solid triangle () is seismic station of Japan Meteorological Agency (JMA).
Solid lines in the upper and lower ﬁgures are contour lines of topography and prefectural boundaries, respectively.

)*+,-01<45 ¡¢£¤ 43

VFRC 'rs  + my 1 m\zZ@ ?8? #n

  !"#$%&'o ERI {|? 0 m45m]4}{|'( )*+,--./0123456 

~FD  JMNOFr \J@ 

784945:;<=>?

\ VFRC m] N\z ~

@ A7 BCD$45EFGH?IJ)*+]MNO] JF MNO'P,-KL/0145MNO'PQ(&Q]A^IJJa'87 5RST 6

 5RSTUVWXYZ@[\] <=>A^4523_`'( abZI<c7^deA^a'F8Z@ ab87 VFRC ])*+,-01 J45 UV
J5RSTa'

?I , E' %&87 ,**- f - gh\)*+,-4

NOm<{ ERI MNO'PQ

56 i@<?J ¡¢£¤y¥?@{|¦E] ῌ )*+,-§¨©v6 Fª«\, 

¬­a' ῍ 455R/]®ª«KLza,**+ fjk)*+,-0145Tlm' p¯°± +323 ; ²± +330s ῎ ³´ ] #no + m BCD$45E pjq ERI 'r

NmFª«µ (KSS) za' - m¶s  0 m BCtuD$v- wxON pjq

? KSS '<{ , m'·¸Fª«¹º»¼|

Table +. Speciﬁcation of observation stations of Volcanic Fluid Research Center, Tokyo Institute of Technology (VFRC).

Fig. ,. Schematic illustration of the new observation system. A/D conversion of the data is carried out with a ,*or ,, bit resolution and a sampling time of *.*+ s. The digitized data are transmitted from the data relay
stations to Kusatsu-Shirane Volcanological Observatory (KSVO) by optical communication. ERI’s observation data are acquired by satellite communication. All the data are integrated at KSVO.

· ¸> ?@¹ A=¸º »B¼C

44 (KSW)  2** m    (KSE)

  2** m0  KSE  ,**+/* m  KSW  % VFRC,** m  !"#$ & '()*+, -. /kl(2 . /0 )C


 ERI

N+OVN+O;EFGH# + VFRC

![*[EFGH . /0 +*!" & +0

  N+O,%U[ 012 3452 678

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9:;< (KSJ) =>?# L-.C, Mark Product

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

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7@ $  # .

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Z[ID   £¤6v S$, km 34%U 32¥7[hF)ijF<T (KSR) klmK (KSM)¦n e-mail &:;[  %  &noVWo:%U pqrXY

IJ)_§+}¨ + abCD£}¨

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s.

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>V=>?# L-. C,

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,. ab

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N+O;EFGH#!2qD VFRC]`F,){0M   ab±²

N+O $P5 % . /[   0 9b F,){³012 t0%&'12±´<= CRT =0M¬°[ F,){#µFig. -. Top : The daily earthquake frequency observed by VFRC. Bottom : The monthly earthquake frequency observed by VFRC and JMA.

N+O¨ ab¶ 

Fig. .. Seismic velocity structure used for the hypocenter determination.

@n¶kfv./01RBSE"»¼

45

Fig. /. The hypocenter distribution around Kusatsu-Shirane volcano from November, ,**+ to March, ,**-.
Solid and open circles are stations of VFRC and ERI, respectively, which are used in the hypocenter determination. The hypocenters of 0*, earthquakes are determined during this period. Circles numbered + and ,represent Yugama and north Motoshirane regions, respectively.

CRT 

ῌῌ   !"#$ %KSS  + Hz, KSW & KSE  , Hz '(F"./Y@F f./&] .//]F3v2f Fig. - u῏"./G(\)*(+, -./01#$23!4 RMS 56

,**+ N ++ T23 ,**- N - TF VFRC 

7 "0182393 :;<=>?@AB#$1fU./ Fig. - T"ῌ C *$DEF"GHIFJ

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01RBSE ,**+ N +* T +0 U23VWXῐYZ./C]¢01 nK + `TF +** Y[ \*A]-^_"B`a"bcd$£¤TC¥&¦§FJf ¨" ©ª«fg hN ++ T + U23῍iXῐCjZ

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g"Yµ¶· Tpq ,* 

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qrC 0* o"5ppqr" , stu&vK w"x

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46

Table ,. Magnitude-frequency relations of theearthquake that occurred in Yugama andnorth Motoshirane.

  KSW  S-P 10 *.- N

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ῌ 12 tAuB (+323) #00 HI?.FA0 0

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st (,**/) ou v - w 0-/ p.
Minakami, T. (+3-3) Explosive activities of volcano
Kusatsu-Shirane during +3-1 and +3-2 (Part +). Bull.
Earthq. Res. Inst., +1, /3*ῌ0,-.
Minakami, T., Hiraga, S., Miyazaki, T. and Utibori, S.
(+303) Fundamental Research for predicting Volcanic
Eruptions (Part ,). Bull. Earthq. Res. Inst., .1, 23-ῌ3.3.
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Ossaka, J., Ozawa, T., Nomura, T., Ossaka, T., Hirabayashi, J., Takaesu, A. and Hayashi, T. (+32*) Variation of chemical compositions in volcanic gases andwaters at Kusatsu-Shirane Volcano and its activity in+310. Bull. Volcanol., .-, ,*1ῌ,+0.
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