Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 153-162

ArticleAbsolute Gravity Variation at Sakurajima Volcano from April 2009
through January 2011 and its Relevance tothe Eruptive Activity of Showa Crater
Shuhei OKUBO＊, Takahito KAZAMA＊,＊＊, Keigo YAMAMOTO＊＊＊, Masato IGUCHI＊＊＊,
Yoshiyuki TANAKA＊, Takayuki SUGANO＊, Yuichi IMANISHI＊, Wenke SUN＊,＊＊＊＊,
Mamoru SAKA＊, Atsushi WATANABE＊ and Shigeo MATSUMOTO＊＊＊＊＊(Received September 30, 2011 ; Accepted February 27, 2012)
We describe absolute gravity measurements performed from April 2009 through January 2011, and present
technical suggestions for carrying out continuous observations in a volcanic area. The results clearly show significant
gravity variations of as large as 30μgal during the observation period. Hydrological simulations reveal that about halfof the gravity change is attributable to groundwater disturbance. After correcting for this disturbance, the observed
variations in gravity can be divided into 5 separate phases. Phase I is a period with few eruptions, which extends from
April to late June 2009 when an abrupt 10μgal gravity decrease was observed. During the succeeding phase II, from
July 2009 to May 2010, gravity oscillated about a mean value with an amplitude of 5μgal, while the monthly number of
explosions at Showa crater dramatically increased from 50 to about 150. In phase III, which was a transient quiescentperiod, gravity increased by as much as 10μgal in a single month. This was followed by phase IV, during which there
was a steady gravity decrease until November 2010. During the final phase V, gravity remained almost constant until atleast January 2011. These five phases are closely linked to the eruptive activity at Showa crater. In fact, excellent
correlations are found among the records of absolute gravity, ejected weight of volcanic ash, ground tilt, and infrasoundair shock amplitude. The gravity data are transformed into changes in magma head height using a simplified line massmodel.
Key words : absolute gravity, magma head, open conduit, Sakurajima volcano, Showa crater

1．Introduction
Sakurajima volcano woke from a decade-long period ofdormancy in 2006. Many Vulcanian eruptions andpyroclastic flows from Showa crater have been recorded(Iguchi et al., 2010). The volcanic activity since 2006, andin particular since 2009, exhibited the following features:(1) Most of the eruptions occurred at Showa crater with
only a few exceptions at Minamidake crater, which had
been the major vent for a 50 year period since 1955.
(2) The annual number of explosive eruptions in 2010
marked the highest ever recorded in Sakurajima volcano
since 1955. Roughly 500 eruptions in 2009, 900 in 2010,
600 in 2011 as of September 1, 2011, are classified as
explosions with distinct infrasound pressure pulses.
Previously, only in 1960, 1974, 1983 and 1985 did thenumber exceed 400 although it never reached 500.
＊

Earthquake Research Institute, University of Tokyo, 1-1-1,
Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
＊＊
Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan.
＊＊＊
Sakurajima Volcano Research Center of Disaster Prevention Research Institute, Kyoto University, 1722-19,
Sakurajima-Yokoyama, Kagoshima 891-1419, Japan.

(3) Several parameters characterizing the intensity of
volcanic activity since 2008 were smaller by a factor of 3
to 10 than those during the last active period from 1974 to1985 (Iguchi et al., 2010). For example, the total weightof ejected volcanic ash in 2009 is estimated to be only 3.2
million tons while in the 1980ʼs it was 10 to 30 million tons
every year. In addition, the volumetric changes of theinflation/deflation sources for individual explosions since2008 were only 102 to 104 m3 while those in the 1980ʼswere 103 to 105 m3. Furthermore, the overall seismicity
since 2008 was lower than that before 2007; it was not
unusual to count more than 2,000 earthquakes per month
before 2007 whereas this decreased to less than 800 since2008.
To summarize, although there have been more frequent
explosions since 2008, both the degree of seismicity and＊＊＊＊

Key Laboratory of Computational Geodynamics, Graduate
University of Chinese Academy of Sciences, 19A Yuquanlu,
Beijing 100049, China.
＊＊＊＊＊
Sumiko Consultant Corp., 2-9-7, Ikenohata, Taito-ku, Tokyo
110-0008, Japan.
Corresponding author: Shuhei Okubo
e-mail: okubo@eri.u-tokyo.ac.jp

154

Shuhei OKUBO, Takahito KAZAMA, Keigo YAMAMOTO, Masato IGUCHI, Yoshiyuki TANAKA, Takayuki SUGANO,
Yuichi IMANISHI, Wenke SUN, Mamoru SAKA, Atsushi WATANABE and Shigeo MATSUMOTO

Table 1. Geographic coordinates of absolute gravity station ARM, Arimura Crustal Deformation Observatory(see also Fig. 1).

crustal deformation have been much lower. This strongly
suggests that Sakurajima has switched from being aclosed-conduit system to an open one as its principal ventchanged from Minamidake to Showa crater in 2006.
When a volcanoʼs conduit is not tightly plugged for a long
period of time, it is expected that mechanical forces
associated with the supply of magma rarely build up deep
underground; instead, the internal overpressure can easily
be relieved by frequent but small-scale explosions. If thisis the case, ground deformations and seismicity may nolonger be good indicators for diagnosing the volcano. On
the other hand, continuous gravity monitoring provides us
with information on magma movement in the form of masstransport, particularly when it is not significantly affectedby ground uplift/subsidence as is the case for Sakurajima
since 2006.
In this paper, we present absolute gravity data from
April 2009 through January 2011. We also describe several
technical aspects related to carrying out continuous absolute gravity monitoring for more than a year, since this issomething that has never been attempted anywhere in theworld and so is worth communicating. The absolute gravity
records are interpreted in terms of changes in magma headheight using a simple line mass model.
2．Absolute Gravity Measurements
The gravity data were obtained using FG5 absolutegravimeters that are installed at the Arimura Crustal Movement Observation Vault, 2.1 km from Showa crater (ARM
in Fig. 1, Table 1); three gravimeters (Serial Number #109,#212, and #241) were used in turn to carry out continuous
observations without any significant interruptions due tomechanical problems. The acceleration rate of a freefalling target was measured every 10 to 15 seconds in a
vacuum chamber with a laser interferometer and a rubidium atomic clock (Niebauer et al., 1995; Okubo et al.,1997). The standard deviation for each measurement after
correcting for the Earthʼs tide and the barometric effect was
20 to 50μgal (1μgal=10−6 cm /sec2), depending on thelevel of ground noise. Since about 1,200 gravity measurements were performed every day, the error in the dailymean gravity is estimated to be less than 50/(1,200)1/2 ,which is 〜1μgal.
Since absolute gravimeters were originally designed tooperate in a clean and quiet laboratory with good temperature control, we faced considerable unexpected problems

Fig. 1. Location of the ARM absolute gravity station,“Showa Crater” and “Minamidake Crater”. A permanent GPS station of the Geospatial Information Authority of Japan, 960721, is denoted with a solid circle.
Contours are drawn every 100 meters above mean sea
level.

in operating the FG5 instruments for extended periods oftime (Table 2). For example, the laser tube lost power at
an unusually fast rate of 25 %/month; in ordinary laboratory conditions, the rate is 5 %/month or less (Fig. 2). In
addition, a personal computer used with the FG5 sufferedfrom frequent abnormal shutdowns a few months after
installation. These problems most likely arose due to finegrained volcanic ash creeping into parts of the gravimeterthat were exposed to the open air. After several periods oftrial and error, these problems were solved by constructing
a vinyl tent around the entire gravimeter; the environment
inside then became almost free from volcanic dust (Fig.
3a). The laser tube was treated separately and placed in acompact clean bench (Fig. 3b). These measures to avoid
the infiltration of fine dust have been working well since
March 2010, and there have consequently been no significant interruptions to the absolute gravity measurements(Table 2).
3．Gravity Changes
Fig. 4 shows the daily means of absolute gravity
measured at ARM from April 2009 through January 2011.
Unfortunately, the time series is interrupted twice (October
2009 and March 2010) because of unexpected instrumental
problems (Table 2). Nevertheless, the data provide us
with invaluable information on the mass transport within
Sakurajima volcano during the period of most frequent
explosions within the last 55 years. We must remember,
however, that gravity measurements are also sensitive tomass redistribution just around the observation site. In
particular, changes in groundwater level and soil moisture
content have a significant influence on local gravity.
Consequently, it is crucially important to eliminate theseAbsolute Gravity Variation at Sakurajima Volcano from April 2009 through January 2011
and its Relevance to the Eruptive Activity of Showa Crater
Table 2.

Overview of absolute gravity measurements from April 2009 through January 2011.

effects from the raw data in order to establish a physicalmodel for magma transport in a volcano.
3-1 Modeling gravity disturbance due to groundwater
Kazama and Okubo (2009) proposed a physical method
to estimate the gravity disturbance due to variations in the
moisture content θ(t) of the top unsaturated layer and thewater table height h (t). The two variables are governed by∂θ∂θ x,y,z,t∂D θ;x,y,z+K θ;x,y,z ,
=
∂z
∂t
∂z



155



(1)

∂hx,y,t∂∂h=K hsx,yh−h bx,y
∂t
∂x
∂x
∂
∂h
+K hs
h−h b+Nx,y,t∂y∂ynx,y






(2)

where Dv and Kv respectively denote vertical diffusivity
and permeability, and n, Khs, hb, N in the second equation
stand for porosity, saturated permeability, height of theupper surface of the impermeable layer, and recharge fromthe unsaturated layer, respectively. In the following, we
assume156

Shuhei OKUBO, Takahito KAZAMA, Keigo YAMAMOTO, Masato IGUCHI, Yoshiyuki TANAKA, Takayuki SUGANO,
Yuichi IMANISHI, Wenke SUN, Mamoru SAKA, Atsushi WATANABE and Shigeo MATSUMOTO

Fig. 2. Laser power as a function of time. Power dropped by about 23 % each month until July 2009. The laser was covered
with a clean bench in a vinyl tent in November 2009. Since then, the rate of power loss was restored to the normal valueof 5 %/month.

Fig. 3. Protection for the gravimeter against fine-grained volcanic ash. (a) FG5 gravimeter installed in a vinyl tent. (b) The
laser was also protected from volcanic ash using a clean bench in a vinyl tent.

 D θ;x,y,z=D sexp −b 1−θ x,y,z−θ minθ max −θ min



(3)

 

K θ;x,y,z=K sexp −a 1−θ x,y,z−θ minθ max −θ min

,

(4)

Absolute Gravity Variation at Sakurajima Volcano from April 2009 through January 2011
and its Relevance to the Eruptive Activity of Showa Crater(Gardner and Mayhugh, 1958; Davidson et al, 1969).
We take Kvs=5.0×10−6 m/s, Dvs=1.0×10−5 m/s, a=7.
98, b=1.16, θmin =0.00 and θmax =0.50 as these arerepresentative values for the soil “Shirasu“ covering the
top surface around Sakurajima (Miwa et al., 1991; Sugioand Okabayashi, 1994; Kazama, 2010). Furthermore, we
assume that the porosity n(x,y) and horizontal permeability
Kh(x,y) are homogenous in space asn(x,y)=θmax=0.50,(5)Khs (x,y)=3.5×10−4 m/s(6)based on the averaged data of Oshima (2008).
Undulation of hb is ignored and hb (x, y) is taken to be−400 m because a low electrical conductivity layer at 400
m below the mean sea level (Kanda et al., 2008) is most
likely to be an impermeable layer. Recharge to thesaturated layer N (x, y, t) m/s is calculated based on thewater content profile at the bottom of the unsaturated layerusing the Buckingham-Darcy law (Jury and Horton, 2004),
as given byNx,y,t=D x,y,z+K x,y,zzht.

(7)Equations (1)-(2) can be numerically solved for the soil
moisture content θ(t) and the water table height h(t) underappropriate initial and boundary conditions using the
Finite Difference Method (FDM). In our FDM computations, vertical and horizontal grid sizes are set at Δz=1.0m, Δx = 59. 3 m (east-west) and Δy = 46. 3 m (northsouth), which match those of the digital elevation model ofthe Geographical Survey Institute (1997).
The boundary conditions are the same as thosedescribed in Kazama (2010). We take∂θD ∂z +K 




∂θ

D ∂z +K =Rt−Et,zh gxy

(8)

(9)

θ x,y,ht,t=θ max =n,(10)hx coast,y coast,t=0,

(11)

K hs

∂h



zh bt∂h ∂x e + ∂y e xcomponents of the vector normal to the water divide.
In brief, Eqs. (8) -(9) are derived by considering thevertical soil-water flux on the ground and at the top of theshallowest impermeable layer (i. e., bottom of saturated
layer). Equation (10) is obtained based on the fact that thesoil at the interface of the saturated/unsaturated layers
must be saturated with water. Equation (11) specifies thatthe sea level should match the water table along the coast,
while Eq. (12) implies that groundwater does not flow
across the water divide.
The initial conditions for the soil moisture content θ(t)and the water table height h(t) are obtained in thefollowing manner. We first derived steady state solutions
to Eqs. (1)-(2) by carrying out an FDM simulation with
arbitrary initial conditions and with boundary conditions(8)-(12), with annual mean values substituted for R(t) andE(t). Whatever the initial conditions, the FDM solution
converges to a steady state solution within 500 years. We
then carried out an FDM simulation again using real data
for R (t) and E (t) obtained since January 2006 as theboundary conditions and using the steady state as theinitial conditions. The FDM results for January 1, 2009
can be used as the initial conditions; a two-year run-up
computation is usually sufficient to suppress transient fakesolutions.
Once the soil moisture content θ(t) and the water tableheight h (t) are obtained by the FDM simulation, it isstraightforward to estimate the gravity disturbance gw(t) at
a given station (x0,y0,z0) as the sum of g1 and g2, which
represent hydrological gravity effects arising from theunsaturated and saturated layers, respectively.
g wx 0,y 0,z 0,t=g 1x 0,y 0,z 0,t+g 2x 0,y 0,z 0,t,
g 1x 0,y 0,z 0,t



=−Gρw
=0,



y

=0

x dy d(12)where hg (x,y) represents undulations of the ground as
obtained from digital elevation models (Geographical
Survey Institute, 1997). R(t) is the precipitation measured
using a rain gauge while E (t) denotes the evapotranspiration estimated from daily means of surface temperature, relative humidity, wind speed and hours of sunshine(Penman, 1948). (xcoast, ycoast) and (xd, yd) represent thehorizontal coordinates of seacoast and water divides
around the target area while ex and ey are the horizontal157

h sxyhxytθ x,y,z,t



z−z 0
dz dxdy,r3

g 2x 0,y 0,z 0,tnx,y=−Gρwhxyth bxyz−z 0dz dxdy,r3(13)r= x−x 0 +y−y 0 +z−z 0
where G, ρw, and hs are the gravitational constant, water
density, and ground surface elevation, respectively.
Fig. 4 presents the absolute gravity change before andafter correcting for the hydrological disturbance. It can beclearly observed that the peak-to-peak fluctuation is reduced from 30μgal to 15μgal once the groundwater correction is applied to the raw data. In general, however, a small
fluctuation does not necessarily guarantee that the correction is adequately performed. We may, however, certainly
say that a significant part of the ground water disturbanceis successfully eliminated because the same formulation
gave excellent agreement between the observed and
2

2

2

158

Shuhei OKUBO, Takahito KAZAMA, Keigo YAMAMOTO, Masato IGUCHI, Yoshiyuki TANAKA, Takayuki SUGANO,
Yuichi IMANISHI, Wenke SUN, Mamoru SAKA, Atsushi WATANABE and Shigeo MATSUMOTO

Fig. 4. Gravity and precipitation.
(a) Gravity changes at ARM (Fig. 1) from March 2009 through January 2011. Red crosses and blue circles denote raw
gravity data after subtracting a nominal value of 979, 437, 796μgal and the estimated groundwater disturbance,
respectively. Gravity after eliminating the groundwater disturbance is indicated by black circles.
(b) Accumulated precipitation at ARM since January 1, 2009. During the period shaded in yellow, a large error might bepresent in the estimate of the groundwater disturbance owing to unusually heavy rainfall (more than 1,000 mm in amonth).

computed soil moisture content θ(t) (Kazama 2010).
One point is worth noting concerning the correction
during the heavy rainfall period, i.e., from the middle ofJune 2010 to the end of July 2010 (Fig. 4). In Fig. 4, arather unnatural bump is visible during that period. We
believe this is due to the shortcomings of our hydrologicalmodel for the period in question. Equations (1) and (2)may not fully represent actual hydrological processes in
unusual circumstances. In fact, precipitation during thisperiod totaled 1, 500 mm, three times larger than therainfall during the same season in 2009. It is therefore
likely that gravity remained almost unchanged if thegroundwater effect was eliminated.
3-2 Gravity Change as an Indicator of Volcanic
Activity
It is worthwhile reviewing the volcanic activity that

occurred during the period of our gravity measurements.
For this purpose, we show the ejected weight of volcanicash, ground tilt, and infrasound impulse amplitude asfunctions of time in Fig. 5. It is clearly seen in Fig. 5(a),(b) and (d) that the entire period (from January 2009 to theend of January 2011) is divided into five phases, namely
phase I to V. This division agrees in principle with Iguchi
et al. (2011a), who divided the same period into 4 phases;
they actually divided the period after September 30, 2009,
into 3 phases.
Phase I lasted until the end of June 2009 without any
significant ejection of volcanic ash. It was followed byphase II (inflation period of Iguchi et al. (2011)) when
explosive eruptions began to occur frequently, accompanied by a remarkable increase in the volume of ejectedvolcanic ash. Phase III (from early May 2010 to the middle

Absolute Gravity Variation at Sakurajima Volcano from April 2009 through January 2011
and its Relevance to the Eruptive Activity of Showa Crater

159

Fig. 5. Gravity change (after eliminating the groundwater disturbance) compared with indices of volcanic activity. The
entire period is divided into 5 phases (I to V) based on the volcanic activity. (a) Weight of volcanic ash ejected eachmonth (Iguchi et al., 2013). (b) Ground tilt toward the center of Showa crater (Iguchi et al., 2011). (c) Absolute gravity
variation at ARM with correction for groundwater disturbance. Black circles are daily means while the red curve and theblue dotted line represent the smoothed gravity variation allowing for a probable overcorrection in June and July, 2010.
(d) Infrasound impulse amplitude at a microphone site around Arimura (Iguchi et al., 2013). (e) Accumulated
precipitation since January 1, 2009.

160

Shuhei OKUBO, Takahito KAZAMA, Keigo YAMAMOTO, Masato IGUCHI, Yoshiyuki TANAKA, Takayuki SUGANO,
Yuichi IMANISHI, Wenke SUN, Mamoru SAKA, Atsushi WATANABE and Shigeo MATSUMOTO

of June 2010) can be identified as a short but distinctive
quiescent period with few explosions. It is likely to be atransient period because it was followed by phase IV(deflation period of Iguchi et al., (2011)), characterized bysignificant deflation of the volcano, which strongly suggests a decrease in the supply of magma deep underground. Phase IV lasted until the middle of November
2010. Since then, volcanic activity has been in phase V(re-inflation period of Iguchi et al., (2011)) and all of theindices have been gradually restored to their phase II
levels.
The gravity changes at ARM, following correction forgroundwater effects (Fig. 5 (c)), are compared with thethree volcanic indices in Figs. 5 (a), (b), and (d). We
would like to draw attention to the following points:(1) Gravity decreased very rapidly (ca. 20μgal/month)during a short transition interval from phase I to the more
active phase II.
(2) Gravity oscillated about a mean value with anamplitude of ca. 5μgal during phase II.
(3) Gravity increased rapidly (ca. 5μgal/month) during
the short transient phase III.
(4) Gravity decreased steadily during phase IV.
(5) Gravity ceased to decrease during phase V.
The above observations strongly suggest that movementof magma in the conduit of Showa crater is reflected in
specific gravity changes.
4．Estimating Magma Head Height from Gravity
Change
The elevation around ARM during our observational
period remained almost unchanged, with a random observational noise of 1 cm or so according to the F3 solution1of the nearest permanent GPS station 960721 of the Geospatial Information Authority of Japan (Fig. 1). The rather
insignificant amount of crustal deformation despite thefrequent Vulcanian eruptions in 2009 and 2010 (Fig. 5)
implies that overpressure in the magma chamber was noteffectively transmitted to the surrounding medium. In
other words, it is very likely that Sakurajima volcano was
erupting as an open conduit system during our observational period. Even if we were to take 2 cm as theelevation change at ARM during our observation period,
its effect on the gravity is still only about 4 to 6μgal,
depending on the choice of the Free Air or Bouguer gravity
gradient. Since this is smaller than the observed 20μgal
gravity change by a factor of 3 to 5 (Fig. 5), we are left
with two possibilities to explain the observed gravity
change: (1) temporal variation of mass in a magma
reservoir, or (2) movement of magma head in the conduit.
Ishihara (1990) and Iguchi et al. (2013) argued that there
are two principal magma reservoirs: a deeper one locatedat 10 km depth beneath the center of Aira caldera (〜10 km
1

http : //terras.gsi.go.jp/ja/index2.php

north of Sakurajima) and a shallower one at 2-6 km depth
below Minamidake crater (Fig. 1). Gravity changes due tomass increases at these reservoirs, without crustal
deformation occurring, are estimated to be 3 to 10μ
gal/year if we take 107 m3 /year as a reasonable magma
supply rate (Ishihara 1990, Iguchi et al. 2013). Since these
rates are an order of magnitude smaller than the observed
rate of ca. 10μgal/month (Fig. 4), we conclude that
temporal variation of mass in a magma reservoir is not amajor source of the observed gravity variation. In thefollowing, we investigate whether the observed gravity
change can be well explained in terms of variation of themagma head height, similar to the approach taken by
Okubo (2005) for Asama volcano.
Let us first assume the conduit to be a vertical cylinder
in which rock fragments and/or solidified magma form a
porous matrix with a porosity ϕ, as shown in Fig. 6(a). A
melt with a density ρmelt is assumed to rise and fall within
the conduit through the matrix. Since the diameter a of the
conduit (〜100 m) is much smaller than the horizontal
distance L (=2,100 m) between the conduit and the ARM
observation point, the attraction Δg due to the magmacolumn should be well represented by that of a line mass as
1
1− +Δg 0ΔgHt=πGa 2ρmeltϕ2
2L L +Ht−H 0

Δg 0≡ΔgHt=H 0

(14)where G is the gravitational constant, H (t) is the heightof the magma head above mean sea level (MSL) at a time t,
and H0 is the height of the gravity station (Fig. 6 (a)). We
may solve Eq. (14) for H (t) in terms of Δg (t) as  Gσ+Δgt−Δg L  −1

Ht=H 0±L
σ≡πa 2ρmeltϕ

Gσ

2

0

(15)

We should be aware here that Eq. (15), which links Δg
to variations in H, is not single valued. We may, however,resolve this ambiguity by remembering that H (t) is acontinuous function of time and by choosing an appropriate initial condition. By substituting L=2,100 m, H0 =80 m (Table 1) into Eq. (15), and assuming the physicallyplausible values ρmelt=2.67×103 kg/m3, a=100 m, and ϕ=0.75, we can determine the height of the magma head asa function of time (Fig. 6(b)). As an initial condition, we
assumed H0=80 m (i.e., the height of the gravity station)
in June 2009 when the gravity was a maximum.
Fig. 6(b) shows the following interesting features. The
magma head was 400-800 m above the MSL during themost active phase II, which is quite high in the conduit
because the elevation of Showa crater is ca. 800 m (Fig.
1). Phase III is characterized by a sudden fall of the headfrom 800 m to 0 m, implying temporary weakening of thedriving pressure in the conduit. This is consistent with the
1 month quiescent period with few explosions. Phase IV
is characterized by a steady fall in magma level, which isAbsolute Gravity Variation at Sakurajima Volcano from April 2009 through January 2011
and its Relevance to the Eruptive Activity of Showa Crater

161

Fig. 6. Method of estimating height of the magma head in the conduit. (a) Line mass model used to calculate the elevationof the magma head. A gravimeter is installed at an elevation H0 above mean sea level. The distance from the conduit to
the gravimeter is L. (b) Magma head height H (t) derived from gravity change Δ (t) using Eq. (15). Phases I to IV (?)
correspond to those in Fig. 5.

consistent with the deflation of Sakurajima volcano (Iguchi2011). It should be noted, however, that systematic bias
might have been introduced into the data since July 2010
by a significant ground water disturbance during JuneJuly 2010 (Fig. 4). If the gravity in phase IV is biased by−5 to −10μgal, the estimated magma head height includes an error of a few hundred meters.
5．Conclusions and Discussion
We carried out absolute gravity measurements for 21
months from April 2009 through January 2011 at Sakurajima volcano, paying special attention to avoid interruptions to the record. The gravity exhibited characteristic
variations associated with 5 distinct phases linked to thevolcanic activity. The absence of any significant elevation
change at the gravity station enabled us to infer the magma
head height by using a simple line mass model. The
estimated height until July 2010 is consistent with theeruptive activity of Showa crater while that after August2010 might be biased by our possibly inappropriatehydrological model. We should be also aware that several
assumptions used in the modeling must be justified infuture studies, including the conduit radius and theporosity. However, our estimates of the fluctuations in themagma head height can serve as a basis or a working
hypothesis to better understand the eruptive activity ofShowa crater of Sakurajima volcano.
Acknowledgments
We express our sincere thanks to the Osumi Office ofRiver and National Highway, Kyushu Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism for allowing us to use the facilities of the
Arimura Crustal Deformation Observation Vault and providing us with hourly precipitation data. Special thanks
are given to members of the Sakurajima Volcano Observatory, Kyoto University for providing us with technicalsupport to continue our research. We appreciate discussion with the members of the Institute of PhysicalGeodesy, Technical University of Darmstadt during thefirst authorʼs stay in Germany, supported by the Alexandervon Humboldt Foundation. This work was supported inpart by a Grant-in-Aid for Scientific Research (20244071)
and in part by the Collaborative Research Program of theDisaster Prevention Research Institute, Kyoto University(21G-04, 22G-07).
References
Davidson, J. M., Stone, L. R. , Nielsen, D. R. and Larue, M. E.
(1969) Field measurement and use of soil-water properties.
Water Resour. Res., 5 (6), 1312-1321.
Gardner, W. R. and Mayhugh, M. S. (1958) Solutions andtests of the diffusion equation for the movement of water insoil. Soil Sci. Soc. Am. Proc., 22, 197-201.
Geographical Survey Institute (1997) Suuchi chizu 50 m mesh(Hyoukou) Nippon III [CD-ROM]. Japan Map Center, Tokyo,
Japan.
Iguchi, M. (2011) Volcanic activity and prediction of volcanic
eruption by using ground deformation data in undergroundtunnels at Sakurajima volcano, Japan. Abstract of the Sym-

162

Shuhei OKUBO, Takahito KAZAMA, Keigo YAMAMOTO, Masato IGUCHI, Yoshiyuki TANAKA, Takayuki SUGANO,
Yuichi IMANISHI, Wenke SUN, Mamoru SAKA, Atsushi WATANABE and Shigeo MATSUMOTO

posium on Prediction of Eruption at Volcanoes under Openconduit, July 14, 2011, Kagoshima, Japan (in Japanese).
Iguchi, M., Yokoo, A. and Tameguri, T. (2010) Intensity of
volcanic explosions at Showa Crater of Sakurajima Volcano. Annuals Disas. Prev. Res. Inst., Kyoto Univ., 53B,
233-240 (in Japanese with English abstract).
Iguchi, M., Ohta, Y., Ueki, S., Tameguri, T., Sonoda, T.,
Takayama, T. and Ichikawa, N. (2011) Evaluation of volcanic activity of Sakurajima volcano in 2010. Annuals Disas.
Prev. Res. Inst., Kyoto Univ., 54B, 171-183 (in Japanesewith English abstract).
Iguchi, M., Tameguri, T., Ohta, Y., Ueki, S. and Nakao, S.
(2013) Characteristics of volcanic activity at Sakurajima
volcanoʼ s Showa crater during the period 2006 to 2011.
Bull. Volcanol. Soc. Japan, 58, 115-135.
Ishihara, K. (1990) Pressure sources and induced grounddeformation associated with explosive Eruptions at anandesitic volcano: Sakurajima volcano, Japan. In Magma
Transport and Storage (M.P.Ryan, ed.), John Wiley andSons, 335-356.
Jury, W. A. and Horton, R. (2004) Soil Physics. 6th ed., John
Wiley, Hoboken, N. J. 370 p.
Kanda, W. et al. (2008) AMT electrical resistivity survey at
Sakurajima volcano. In Report on the 10-th integrated
observation campaign of Sakurajima volcano, issued by
Sakurajima Volcano Res. Center, Disas. Prev. Res. Inst.,
Kyoto Univ., 89-104 (in Japanese).
Kazama, T. (2010) Hydrological modeling of groundwater
disturbances to observed gravity data toward high-accuracy
monitoring of magma transfer in volcanoes. Ph.D thesis,

University of Tokyo, pp. 196 (in Japanese with English
abstract).
Kazama, T. and Okubo, S. (2009) Hydrological modeling ofgroundwater disturbances to observed gravity: theory andapplication to Asama volcano, central Japan. J. Geophys.
Res., 114, B08402, doi: 10.1029/2009JB006391.
Miwa, K., Nanba, N., and Wakumatsu, C. (1991) Measurementof unsaturated hydraulic conductivity of mixed soil: wetting
process. Bull. Fac. Agric. Kagoshima Univ., 41, 81-87 (inJapanese with English abstract).
Niebauer, T. M., Sasagawa, G. S., Faller, J. E., Hilt, R. andKlopping, F. (1995) A new generation of absolute gravimeters. Metrologia, 32, 159-180.
Okubo, S. (2005) Gravity changes associated with volcanismobservation, theory, and analysis. Bull. Volcanol. Soc. Japan,
50, S49-S58 (in Japanese with English Abstract).
Okubo, S., Yoshida, S., Sato, T., Tamura, Y. and Imanishi, Y.
(1997) Verifying the precision of a new generation absolutegravimeter FG5 -comparison with superconducting gravimeters and detection of oceanic loading tide. Geophys. Res.
Lett., 24, 489-492 doi: 10.1029/97GL00217.
Oshima, H. (2008) Groundwater in the shallow part of
Sakurajima volcano-archived data. In Report on the 10-th
integrated observation campaign of Sakurajima volcano,issued by Sakurajima Volcano Res. Center, Disas. Prev.
Res. Inst., Kyoto Univ., 191-193 (in Japanese).
Penman, H. L. (1948) Natural evaporation from open water,
bare soil and grass. Proc. Roy. Soc. Lond. Ser. A, 193,
120-145.

(Editorial handling Takeshi Nishimura)2009 年 4 月〜2011 年 1 月の桜島火山における絶対重力観測─昭和火口からの噴火活動に関する考察
大久保修平・風間卓仁・山本圭吾・井口正人・田中愛幸・菅野貴之・
今西祐一・孫 文科・坂 守・渡邉篤志・松本滋夫

2009 年 4 月から 2011 年 1 月まで，桜島火山の有村観測坑で行った絶対重力の連続観測により，測定誤差を有意に超える，30 マイクロガルの重力値の減少が検出された．陸水流動シミュレーションを用いて，降雨等により発生する地下水起源の重力擾乱を取り除くと，重力は両振幅で 15〜20 マイクロガルの変動を示した．変動の様相から観測期間が 5 つの特徴的なフェーズに分けられることが明らかとなった．これらのフェーズは，火山灰放出量・有村における傾斜変動・爆発回数などの火山活動を示す指標の変動と，きわめて良い相関をもっていた．重力の火道内のマグマ頭位の変動によって重力変動が引き起こされると考え，線質量モデルに基づいて，頭位標高の推移を推定した．その結果から，マグマヘッドは 2009 年 7 月〜2010 年
5 月半ばには標高 400〜800 m と高い位置にあったが，その後の 1 か月間で標高 0 m 程度まで急降下していると推定された．これらの推定は，昭和火口からの爆発回数の消長ときわめて良い対応を見せている．