  //  (,*+*)
 .  +3-ῌ+33   µ¶·¸¹º»¼½®

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Extensional Strains around Mt. Fuji and Hakone Volcanoand Low-Frequency Earthquakes
Masatake HARADA, Kohji HOSONO, Akio KOBAYASHI,
Yohei YUKUTAKE and Akio YOSHIDA
Temporal changes in dilatational strain and the activity of low-frequency earthquakes around Mt. Fuji andHakone volcano are investigated. It is shown that both cumulative strain and cumulative number of lowfrequency earthquakes around Mt. Fuji have been increasing since the end of ,**0. The tendency is more notable
for relatively larger earthquakes rather than smaller earthquakes. The b value for earthquakes during the period
after November ,**0 is signiﬁcantly smaller than the b value during the period from January ,**. throughOctober ,**0. These facts suggest that the crustal stress surrounding the source region of low-frequancy
earthquakes has been increasing. We think this is the cause of the dilatational strain observed around Mt. Fuji.
On the other hand, a clear relationship is not seen between extensional strain events observed three times since,**+ and the activity in low-frequancy earthquakes around Hakone volcano.
Key words : Mt. Fuji, Hakone volcano, crustal strain, low-frequency earthquake, b value+    ;<=DEFGH67 ,**2 1 2 2IJK GPS L

67 ,**+ 189:;<=>?@ABC=>?:M :N1OPDQR ST@STUT:C=VW

XYZ[\]./^TZ_`@ab^TZ_` $%

6cdZ_`e>f-gh:N1OPDijdZ& ,**3 ; klm ,*+* ; *n o ,**3 p  AM @e>f-gh@:q rDstu`

BC=>?67 ,**+ 1 ,**0 1 ,**2ῌ,**3 1L vwxyzL{ |D}M ]./^T~ ( o,,**3 ;  o ,**3 ; *n o ,**3 ;<=klm (,*+*) LXT ,**2 1 2 2IJK;<\ABvwxy:*=:67e>f-]=>?: GPS 6./^TZ_`GH:[\7 ;<cdZ  +32+ ; Ukawa, ,**/ L;<===JK:^ +/.1 km L`{ |67 ,*** 1Le>f-gh]LgcPd

xw66`@]^TZ_` S6 ;<~ ST@L;<=>?DF ¡¢==JKD£¤@u`¥ ,* km 8z:¦6M ]./^T~@_§¨]` © o

GPS ./ª:O«JK¬­~ S:¦:®¯°±²,**0,/*ῌ**-+ /20 !"#Hot Springs Research Institute of Kanagawa Prefecture,/20 Iriuda, Odawara, Kanagawa ,/*ῌ**-+, Japan.
-2+ῌ+,-, '()*+ -/++$%&,-./0
Matsushiro Seismological Observatory, Japan Meteorological Agency, -/++ Nishijo, Matsushiro, Nagano-2+ῌ+,-,, Japan.

:N1OPD Fig. + Lu ³´L7klmLX-*/ῌ**/,  +ῌ+$%&$%!"#Meteorological Research Institute, Japan Meteorological Agency, +ῌ+ Nagamine, Tsukuba, Ibaraki -*/ῌ**/, Japan.
Corresponding author : Masatake Haradae-mail : harada@onken.odawara.kanagawa.jp

  194   

Fig. +. Temporal change in the dilatational strainaround Mt. Fuji. In calculating the strain change,
we used average coordinates in -* days and thosein the succeeding -* days at GPS stations in thecircle shown in the inlet map.

Fig. ,. Same as Fig. + except for the dilatational strainaround Hakone volcano.

 ,**, ;  ,**1 ;  ,**3  (,**3) 
 GPS!"# GEONET, F- $ %&'( )8 /'9

:/;<"=>?>1

*+,-.(/01234 56 - 7
@% A!B 4CD/%E'(F!G">/23H+IJK09=<LM%NO P+%QR'( #3S T$UV  - 

K09 +331ῌ+333 ,**,ῌ,**- ,**1ῌ,**2 % &W

Fig. - (a)  ,**+ %XYZ['\]9^_+K09 +331ῌ+333 ,**1ῌ,**2 % `aV(((bcde&Wcf)*g+ hh,-4ibj +K09 +332ῌ+333 ,**,ῌ,**- %%= Fig. - (b)  P+k+*g+ hl"K09

<LMmnIJ&' =<LMIJ._opqrst"M (M) u%390](/C.9v>%Kw9 x'9NO( y( /z{,-%E| M *./ X5 h%09}01\(+333) 2'(;~q%09  w

NO( b 3(bc*g+) -* 4J

&W!*g+x53H PJ/z%mn'( +/ 4KH'G,HP1'( + J6hhK09 +.1,hK09 ,.+3 4 0+

 4789/P42-'( h:hl"H

/z%m'9 P/zzm%;0(,NO( b 3 x3 *.2.   ,**0 \G<Fig. + 4i (bcg=4 ,**0 %>H0H

C0

@,-.9 P+ ,**3 % +*  '?|,390Fig. , &Wc%/'9PDH@Ci GPS§HNO(Fig. - (c), (d) 2-{zm/ M-T ,- Fig. - (e),(f) _84 D+HH *g+ hl",* km X

x!G/z

J!G¡¢/2-£_¤A¥'9 Bc¦4
DG¨0,iD/,9/+ |G©ª (bc¡%@4i 0( GPS !"#\e/z*g+

m¬1)­(bcg=/z%NO(/EiF'9 &W!*g+ h ,**0h ,**0 %>X 2-{,«.4i /z¡%@H HG\1 &W%>XY '2-{,®]90 y( (bc!C/Pf=4 ,**+ % 0 H +* *g+w9 ,**0 % 1 H ++ w9 P'9 ,**2 %&W!*g+0 ¯HG ,**3 % - w9/  - 7,\°°0-.90
D@%²C³D'(J´¥'9 µ¶·h_ +*ῌ,* km 4iF'9h_ ,*ῌ,/ km /P+Fig. - (b) H M , * Hf h|±9,^I4

C90G0/H+4 Fig. - (c) zm¸4]J´¹!°*+¹!º»0($¼,PK9(bc*g+hl" ,**0 %Xl2@

LG_+90, #3S M ,**3 ; i/½N|OP,i PD4 Fig. . 

195

Fig. -. (a) Epicentral distribution of low-frequency earthquakes beneath Mt. Fuji and Hakone volcano.
(b) Number of low-frequency earthquakes above respective threshold magnitude. Black and whitecircles represent number of low-frequency earthquakes beneath Mt. Fuji and Hakone volcano,respectively. (c) Cumulative number and magnitude - time diagram for low-frequency earthquakes
beneath Mt. Fuji, and (d) those beneath Hakone volcano. (e) Depth - time diagram for low-frequency
earthquakes beneath Mt. Fuji, and (f) that beneath Hakone volcano. Data are taken from the JMA
catalogue.

M !"#$%&'()23 45671 ,**0 89:;<=6>?@A*+ ,-)"./0 1 ,**08B=6CD) EF!G HI5E+ M J#K)"#%&71GLMNOP

*Q ,**0 8BRSI!

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 AIC+ q^_OX

 AIC,  (AIC+,196



Fig. .. Cumulative number and magnitude - time diagrams of low-frequency earthquakes around Mt. Fuji larger
than or equal to each of the threshold magnitude Mc (*.+, *.,, *.-, *.., *./, *.0, *.1, *.2, *.3, and +.*).

 !"#$%&'()*+

197

Fig. /. (a) Epicenters of low-frequency earthquakes near Mt. Fuji in the period from January ,**. toOctober ,**0 (white circles) and those in the period from November ,**0 to December ,**3 (blackcircles). Gray triangle indicates the summit of Mt. Fuji. (b) Number-magnitude relationship for
low-frequency earthquakes in the two periods. Note that the b value for the latter period is signiﬁcantly
smaller than that for the former period.

+.+.+.) ,-./012 34  5678.9:'()*+;<= >3?@A b BC11DEFGH042

IJK LM N1OC42  *+<Pb QR0*+<PSTUVWXYZ[S\.]123;_4 ` (Warren and Latham, +31* ; Wyss etal., +331) b ;/013WX;QR1 g21TU;'13Ylmnc4

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 ,***12}~042

Fig. 0 !"hijk:'()*+.?1  M 2 Fig. - (b) ,**- b  +.1.  M> 34  ,**0
 ,**-

 ,**+

I1b /01Lc2

xy.z_04

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!"#( ,**+,**3

 ,**0 ,**.

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 4  ,**2ῌ.$%&;04>; 4.M !"hijk:'()*+;<= >de;fc1  34.?1

3 Fig. / (b).

 !"hijk:'()*+.?1

g2nc42 34   .

 b  +.,3 ,**+

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1D ,**0+<Pa;VVbc 12de;fc2 (Fig. / (a))

 KcYpCv 11; .

12 ,**0++ 78 M *.. 7:*+z

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 ,**0

 !"#(04 12$¸³¹4}~0423g2 º»  ¼

198

Fig. 0.



Same as Fig. . except for low-frequency earthquakes around Hakone volcano.

 
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!

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46.7891  /81=96>7?@ !A:;7B<9!
1G ; ,**+ &HA1,'

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- m:5. V MNOPBY0 km IY 4/6789!

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UVG £k_T ¤I_T _T ¥_T¦§_T ¥_T f_T ¨_T _T ©ª¢2UVG yo p| R ¥
«R CDERSF TUVG ¬0­ £TUV'®¯xbcOah xbc¡T7°,p.±². . GPS Oah PAT-ME x# +333 . µ¡;)!¶E·¢¸GX Zd*£¤µ ¡ ¥a¡;)!¨¹¢¸K9©ª1ºr\XYPw_«c¬. »¼­½l199ῐ ῕ ῔ ῑ "# $ (-./%&*+ (,**3) ,**+ &,'*+ 3,-4+5   /. ,,-ῌ,-..
:56;7<.
012 (+32+) 34
    ,0 +.-.
$4<=>C?DEF@> (,**3) ,**+
 IJK9
KL
&,'
Q+  CDERSF TUVGHW .+ 1ῌ+..
C ? DE (,**0) \]^_`abc J  ( d K ePgahLMNij k -H 03 +1+ῌ+10.
# OPQ R$lSTU
W$nXF@> (,**3) CDERSF TUVG
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yo p| (,**,)  V  Q+  e`f.gssH 2* -.ῌ.*.
`yo p| (,**1) 6~1 s Q+ f.gssH 11 +-,ῌ+.+.
`yo p| (,*+*) 6~1 s Q+ f.gssH 2- +,-ῌ+-..
x#y (+333) GPS Oahlz]P (SEISGPS) ' |H } +* ,/1ῌ,00.
"#Ef 
 (,**3) GEONET X-:5.  VGL! Q+  [P5 Tsd++, muvsa 0+ῌ0,.
 # X> (,**,) ,**+
Uj +- & ,'   CD
ERSF TUVG:5XY /, +ῌ..
Ukawa, M. (,**/) Deep low-frequency earthquake swarm
in the mid crust beneath Mount Fuji (Japan) in ,*** and,**+. Bull. Volcanol., 02, .1ῌ/0.
Utsu, T. (+333) Presentation and analysis of earthquake
size distribution : a historical revie and some new approaches. Pure appl. geophys., +//, /*3ῌ/-/.
Warren, N.W. and Latham, G.V. (+31*) An experimental
study of thermally induced microfracturing and its relation to volcanic seismicity. J. Geophys. Res., 1/, ..//ῌ
..0-.
Wyss, M., Shimazaki K. and Wiemer S. (+331) Mapping
active magma chambers by b values beneath the o#-Itovolcano, Japan. J. Geophys. Res., +*,, ,*.+-ῌ,*.,,.
F@>4<=>³ (,**0)   ,*** &^1´'5.K , /2 .*+ῌ.*0.
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