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,**/  /  +-  ,**/  ++  . !"Ground Deformation Associated with the ,**.ῌ,**/ Unrest of Asama Volcano, Japan
Yosuke AOKI῍, Hidefumi WATANABE῍, Etsuro KOYAMA῍῍, Jun OIKAWA῍ and Yuichi MORITA῍
Ground deformation associated with the most recent eruptions in Asama volcano started on September +,,**., is reported. The ground deformation observed by continuous Global Positioning System measurements ismodeled by dike intrusion for two di#erent periods ; one is between July, ,**., to March, ,**/, which represents
overall deformation during the unrest, and the other is between November, ,**., to March, ,**/, which represents
the deformation during the latter half of the unrest. We assumed a rectangular dike opening uniformly in elastic,
homogeneous, and isotropic medium to model the deformation ﬁeld. To solve a nonlinear optimization problem
in which model parameters are nonlinear to observed deformation ﬁeld, the Simulated Annealing inversion was
employed. The uncertainties of and trade-o#s between the model parameters are estimated by the Bootstrap
method.
The results show that the deformation ﬁeld is well modeled by a dike striking roughly east-west, strike ofwhich is consistent with the regional stress ﬁeld. Shape of the dike, that is, length, width, and thickness, is notwell constrained due to the small amount of deformation, up to +* mm, and the scarcity of GPS sites, but volumeof the dike is well constrained to be 0.2, and ..0- million cubic meters for the whole period and the latter half,
respectively. The estimated depth of the dike tip is roughly + km below the sea level ; the depth of hypocenters is
consistent with a theory of dike-induced earthquakes that they occur near the dike tip due to the stressconcentration. However, the comparison of the location of the modeled dike and the distribution of earthquakes
clearly shows that the hypocenter distribution is inconsistent with the theory described above, that is, thehypocenters are distributed only in the eastern half of the modeled dike tip. The possible reasons for thisinconsistency are either +) earthquakes exists in the west half of the dike tip as well, but they are not detected dueto the sparse distribution of seismometers at the west of the ﬂank, ,) the western half of the modeled dike is notcapable of generating earthquakes because the temperature is too high for brittle failure of rocks, or -) thedi#erential stress in the western half is so low that the area cannot reach the critical stress ﬁeld even by the
introduction of dike-tip stress concentration. Current geophysical observations cannot identify the reason but
future development of geophysical observations is expected to solve the puzzle.
Key words : ground deformation, Asama volcano, dike intrusion, Global Positioning System, geodetic inversion+ῌ ῐ ῎ ῑ ῏

8fOPQ%ghRia`gh&jk0bI lQ%g#$%&'( )*+,-./0 1234567

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Aoyama and Takeo, ,**+"I]Q%R^V_0`abc d8e +31- +32, +32- #$%-ra`&}~gh FA sts8u

++-ῌ**-, 9 +ῌ+ῌ+
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Earthquake Research Institute, University of Tokyo,+ῌ+ Yayoi +, Bunkyo-ku, Tokyo ++-ῌ**-,, Japan.
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9N}~#$%

Asama Volcano Observatory of Earthquake ResearchInstitute, University of Tokyo, ,+,/ Nagakura, Karuizawa, Nagano -23ῌ**++, Japan.
Corresponding author : Yosuke Aoki
e-mail : yaoki@eri.u-tokyo.ac.jp

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6 (,**.) 7 GPS 8  (+33*)   !)* +330

 , km  / km !"#$ %&'( !)@A1 3B* +, -./)'0 1( 2,**.

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!( 711} ) Aoki et al.,+333 ; Dzurisin, ,**- *+,1abC¡C1} ,**.

C

E¢`! GPS 8!)[!KB-£ *¤¥% ,**.ῌ,**/CH!.B

(!'B ;<;7/  !( 1}Fig. +.

(a) Tectonic setting around Asama volcano.
Triangles indicate the spatial distribution ofactive volcanoes. EUR, NAM, PAC and PHS
stand for the Eurasian, North American, Paciﬁcand Philippine Sea plates, respectively. A rectangle shows the area of Asama volcano shownin Fig. +b. (b) Spatial distribution of continuous
GPS sites analyzed in this study. The area isshown by the rectangle in Fig. +a. Circles andrectangles are the sites operated by Earthquake
Research Institute, University of Tokyo, andGeographical Survey Institute of Japan, respectively. A dashed line denotes the prefectural
boundary.

,ῌ GPS ῒ῔῎ῐ῍῏ῑΐ*¤¥!&B GPS 801 Fig. +b !2
 5

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\¤¥1 +33*

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6 ,**.  EG® *¤¥1 ,**.

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V¯° Nwx!KB£3FB GPS 8X ,**.

!56!±²3 2¦§, W! ASM .

\¤¥ ,**.

(Fig. +b) ³´3 ,**.

3 W + Lm

CGV ASM/MASM0 (Fig. +b) 7!&B ,**.
3 Wµ)*³´3 5

6 ,**.

,**.ῌ,**/ de"#$

577

Fig. ,. An example of GPS time series for ASM. and ASM0 with respect to 3/*,1,, locations of which are
shown in Fig. +b. Vertical bars mark the dates of moderate-sized eruptions. (a) Time series for ASM.. The
middle panel, the north-south component, shows that the deformation started in late July, ,**., and has moved
southward by approximately +* mm until March, ,**/. (b) Time series for ASM0 built in response to theﬁrst eruption on September +, ,**.. An apparent jump seen in all three components around February, ,**/,is due to snow removal on the GPS antenna. Time intervals a#ected by snow accumulation on the antenna
are shown by a solid line in the middle panel.

 *,*3/1*,*32/*-S*.0  (Fig. +b)   ,**.

 ,**/ 

!"#$%&'()*+,-. %/012.
,3 -* 45 = + > 5)67?@(Zumberge et al., +331)

A

89:;9<
GIPSY-OASIS ῌ,**/  + q , qz{n.||wxX +/ mm $\]%= 23 GPS }9~x.!,Y= ].!,3 

l , qz{3 ,**.  +, q |xX/ mm $n.%= 23].!

jk2.

BC ?@D3 ITRF,*** (Altamimi et al., ,**,)= GPS EFGHIJ3&-  'K A  ?@LMNOPQ3RSTU, --ῌ+ 

 / mm, VWTU,X +* mm ,Y. Fig. ,  ASM

,3/e.ASM0 .?@$+ZD[

].()*\] ^$n BC= 2

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3defUg2

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 !"#$3

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q + > +* >v,?@Stu

+* qr-s$n=,**/  - qv,wxX / mm yxX +*mm $

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578 

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 

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34

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+32/)

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 : MNG/  ;&$) OP@K  "#./&AQRDS TU @;&V )WXY @ ;&Z. Q0KL [ 1 \KL ;&$) ^* _`a]. !  ! [e. ! f` ! gbc>d

h!iAj kabc>d , lm!no 2 lKL /Gabc>d$NG/&
Okada (+32/)"#>d/Gabc>d pVDqr L

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*

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)NG/ EDabc>d^pVwxyFD<DGDz{ 15>d|}>~| kN Cervelli et al., ,**+m Simulated Annealing$LE 

(Basu and Frazer, +33*)

pVwxy& Simulated Annealing//abc>d l*G/& !Kst
EDabc>d  OPK&uv  abc>d

abc>d  Table +  / )u"y$;DE

E

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U ^*G/&.!!Z.FG^*G/:K |}>~|abc>dKD
pVxpV Simulated AnnealingK

D<Dabc>db|> )FG pV;pVxy<^*$_y$vst abc>d ; < & b|>$El/st ¡abc>d¢<;<G £¤¥>
^*G/Fu"y :¦§ y$

Annealing©ªKD OPK& (Efron and Tibshirani, +33-) ^*&&

Cervelli et al. (,**+)|}>~|)15>d$L //abc>d¨t«4K D<abc>d'¬?­>¥®
¯^*:K`  Bootstrap &

°E

;

±./&abc>d¨t^* + "#>d u"Dabc>d  ²³

E%$L pV|}>~| ^*G/&u"y¨t^*Fig. -. Comparison of the observed deformationﬁeld with the modeled deformation ﬁeld with a
intruded dike for (a) period + (between July,,**. and March, ,**/) and (b) period ,(between November, ,**. and March, ,**/).
Thick lines in each panel indicate the horizontal
projection of the modeled dikes. +-s error
ellipses of the observed deformation are also
shown. Dashed line marks the prefectural
boundary.

Simulated)^*G/&u"y¨t Bootstrap

st r´dobs-dcal ^*K d obsd cal//"# u"Dabc>dFG,-./ ²³KL 

,**.῍,**/ p¶{jijkl ·,Rl
Table +.

Possible bounds for dike parameters and estimated model parameters.

BoundsLength (km)Width (km)Depth (km)aM
Dip (degree)bM
Strike (degree)cM
East (km)dM
North (km)dM
Open (m)Volume (m-)eM

579

Optimum parameters : period +Optimum parameters : period ,

min.

max.

min.

opt.

max

min.

opt.

max

+4*
+4*
*4+
./4*
!+/*4*
!+*4*
!/4*
*4*
NῌA

+*4*
+*4*
/4*
+-/4*
!-*4*
/4*
14*
+4*
NῌA

+4./2,+4,*1,*4-+/134-,.3!2*4--*/!24-//3,4*131*4+*/.40-µ+*0,4+*.2.4*3*,,43//,+*+4./..
!0.4,+/,!/41+.,-4**31*413,1042,µ+*0

341123241232.423+++.4/*,/!./4.22+!,4///1/4/+1,*43.1/+*4.µ+*0+4+02,+4++/.
*4,1/.
.34+100!++3412,1!+*4****+4-1*0*4*-.1*430µ+*0.4+33++4+1*/-4*0,*
0*40-/2!3*4.31.
!141+**
04+-,,*43.+,.40-µ+*0

34.22+
34,,+0.422*.
+,24*/.0!-341.*,,4*0*304/,/3*421,*
14.*µ+*0

Column + and , : Minimum and maximum possible values for each model parameter, respectively.
Column - to / : Optimal model parameters for the period + that represent the whole period of the activity (July, ,**.,to March, ,**/).
Column . represents the optimum values estimated by the Simulated Annealing, and columns - and / represent theminimum and maximum possible values within 3/ῌ conﬁdence intervals estimated by the Bootstrap method.
Column 0 to 2 : Same as column - to / except for that these columns are for the period , that represent the latter halfof the activity (November, ,**. to March, ,**/).
a) Depth to the top of the dike from the surface. Because the altitude of the surface in the area is about ,.* kmabove the sea level, the estimated depth must be subtracted by about ,.* km when it is translated to the depth below
the sea level. For example, the top of the dike for the period +, ,.30 km below the surface, should read about +.* km
below the sea level.
b) 3* degrees for a vertical dike. A value less than and larger than 3* degrees indicate a north-dipping andsouth-dipping dike, respectively, for an east-west striking dike.
c) Measured clockwise from north, that is, a value of !./.* indicates a northwest-southeast striking dike.
d) The location of the center of the dike measured from the summit of Asama volcano.
e) Calculated by multiplying length, width, and thickness of the dike obtained by the inversion.

r 

, !+   p oM st /linear trend ud 8vC.r  "wNbxyhefgh" 8zO-)s#$% p* & "'( p [p(+) p(,) p(-) p(.) p{R|*/ud""3 _PQ` }~RS

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580

Fig. .. An example of covariance scatter plots for the period +. We show some selected parameter pairs only.
Each panel indicates covariance between (a) area and thickness, (b) depth and volume, (c) east component of
the location and length, (d) east component of the location and depth, (e) width and depth, and (f) width andvolume, of the modeled dike, respectively.


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 (Fig. .a) Period , J123|jf%(x)* +,-DUD V]^_}D~%&' !( )* +,-.
Table +    !

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Fig. /.

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581

Same as Fig. . except that these plots are for the period ,.

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Fig. .e m .f /0;Bf9,-./0ijkl "9)¯°DD=I8"9
Fig. 0{[

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/ 

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Altamimi, Z., Sillard, P. and Boucher, C. (,**,) ITRF,*** :A new release of the International Terrestrial Reference
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Aoyama, H. and Takeo, M. (,**+) Wave properties andfocal mechanisms of N-type earthquakes at Asama volcano. J. Volcanol. Geotherm. Res., +*/, +0-ῌ+2,.
Basu, A. and Frazer, L. N. (+33*) Rapid-determination of
the critical-temperature in Simulated Annealing inversion. Nature, ,.3, +.*3ῌ+.+,.
Cervelli, P., Murray, M. H., Segall, P., Aoki, Y. and Kato,
T. (,**+) Estimating source parameters from deformation data, with an application to the March +331 earthquake swarm o# the Izu Peninsula, Japan. J. Geophys.
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RG***+*1.
Efron, B. and Tibshirani, R. J. (+33-) An Introduction to
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Geophys. Res. Lett., ,0, -,,+ῌ-,,..
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501¸¸21 211 2ῌ+0.
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concentrated seismicity caused by deformation of Kilauea’s
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Mogi, K. (+3/2) Relations between the eruptions of various volcanoes and the deformations of the ground surfaces around them. Bull. Earthq. Res. Inst., -0, 33ῌ+-..
     (,**/)  ,**. 
!"#$%  & /*& -*-ῌ-+-.
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wx yz{|
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'()*+,- (+33+) ./012345
678239:;<% 0=>?& 31& .0+ῌ.1..
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ST>UV>WXYEW& V*//ῌ**,.
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