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Ground Deformation Detection at Kuchinoerabujima Volcano by
Continuous GPS with Simple Atmospheric Correction
Eiji SAITO and Masato IGUCHI
Kuchinoerabujima, located in southwestern Japan, is an andesitic volcano where explosive eruptions haverepeatedly occurred. Seismicity increased in +330 and +333, and inﬂation of the volcanic body was detected by
GPS surveys during the period from +33//30 to ,*** (Iguchi et al., ,**,). We established a continuous GPS
observation network in April, ,**. to study the relation between seismicity and ground deformation. Verticalcomponent of a baseline with a particularly large elevation di#erence is inﬂuenced by water vapor in theatmosphere. In this study, a simple atmospheric correction method is proposed to detect an upheaval componentof + cm order. After the correction, ground deformation starting at the beginning of January, ,**/ is clearlyrecognized at an observation site near the summit crater. The ground deformation has progressed at a rate ofabout + cm per +** days, and it may be caused by a presumed pressure source at a depth of -** m beneath thesummit. The deformation corresponded to increase in seismicity of high-frequency events at depths shallower
than /** m. It is inferred that these phenomena were caused by hydrothermal activity.
Key words : Kucinoerabujima volcano, GPS, ground deformation, seismicity, atmospheric correction(2 X$ o$pq!&62r[$st+  

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Geological Survey of Japan, AIST, 1-Central, +ῌ+ῌ+Higashi, Tsukuba -*/ῌ2/01, Japan.
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Sakurajima Volcano Research Center, DPRI, Kyoto
University, Sakurajima-Yokoyama, Kagoshima 23+ῌ+.+3, Japan.
Corresponding author : Eiji Saito
e-mail : e.saito@aist.go.jp

¹BºCy-»¼

22

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Fig. +. Location of continuous GPS network. Single-frequency type receivers are installed at stations KUC1,
KUC3, KUC++ and SDW. GSI is station #30*1,/ of the GEONET by the Geographical Survey Institute.
The data at SDW are retrieved by cellular modem communication, and data at GSI are downloaded via theInternet. The other stations’ data are recorded in loggers. Atmospheric temperature and humidity wereobserved at KUC++. Seismicity is monitored by a station indicated by a square.

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

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Fig. ,. Relative changes of positions in three components for all baselines. The horizontal components are shown by
a couple of normal plots and inverted ones (R) to enhance small changes. The changes for the northwestward
appeared in the horizontal components of SDW from January, ,**/. No remarkable changes in the baselines at
the foot of the mountain were recognized. It is impossible to detect the changes of the U component because alarger seasonal variation was included in the baseline with a larger vertical distance.

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25

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Speciﬁcations of meteorological data loggers.

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26

^ KUC++-SDW  Z_` *abcde,A=fghY +, RHijPZ_`,kB,
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/Fig. -. Temporal changes of water vapor pressure andrelative vertical component of the position. e : water
vapor pressure, UD_o : observed vertical component
and UD_c : vertical component corrected by Uῌe(See Fig. .).

/ῌ+

Jan*/





GSI-SDW  ,**/

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SDW  + !

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Fig. .. Correlation of vertical component with water
vapor pressure.

 KUC++-SDW ¢B"Z~D
 , RGi ]d= !H8= opL E>£B , R 0 P / R?`eq¢ E +.*0 cmῌ+** 

cm  +.. cm  0 

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%@ABC%& U ῌ e D EFGHI ", Fig. / 8=ª -** m, ,**/

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B SDW/0" . %&AXABY Jan*/  =Z[\V %&]*+ !

 A © ®¯°45±%¹9 SDW'.

 //* m ," $%ª'  ,/* m

GPS  - 

27

Fig. /. Relative changes of positions in NS, EW and corrected vertical components for all the baselines during theperiod from April ,**. to June ,**/. Perturbation due to atmospheric vapor on the vertical component ismostly removed by the atmospheric correction method, and a slight upheaval of SDW is identiﬁed in the .
baselines related to SDW.



28

Fig. 0. Displacement vector of SDW in horizontal plane (top) and vertical crosssection A-Aῌ (bottom). The
average displacement vector is calculated from the data of GSI-SDW from January to March, ,**/.
Horizontal displacement vector is almost oriented outward from the center of the summit crater. The dip
angle (.-) of the displacement vector is obtained from the average upheaval rate from February to May,,**/. Cross is estimated position of the pressure source, assuming isotropic inﬂation beneath the summit
crater.

0P6QF# GPS #\\~,-I  - »¼   ,UV  /E{INhij &|J &/ῌ, 

#$GPS  SDW 

29   !"#$%&'(&)# GSI-SDW *+
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