Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 137-151

ArticleVertical Ground Deformation Associated with the Volcanic Activity of
Sakurajima Volcano, Japan during 1996-2010 as Revealed by
Repeated Precise Leveling Surveys
Keigo YAMAMOTO＊, Tadaomi SONODA＊, Tetsuro TAKAYAMA＊, Nobuo ICHIKAWA＊, Takahiro OHKURA＊＊,
Shin YOSHIKAWA＊＊, Hiroyuki INOUE＊＊, Takeshi MATSUSHIMA＊＊＊,
Kazunari UCHIDA＊＊＊ and Manami NAKAMOTO＊＊＊(Received October 5, 2011 ; Accepted March 2, 2012)
The recent vertical ground deformation in Sakurajima volcano and around Aira caldera associated with thevolcanic activity of this volcano is revealed by the repeated precise leveling surveys conducted in October-December
2007, November 2009 and April and November 2010. The ground uplifts are detected in Sakurajima volcano andaround Aira caldera to be centered in the caldera during the period from 1996 to 2010, as the previous results during theperiod from 1991 to 1996. From the analysis based on a spherical source (Mogiʼs) model, the inflation source is located
6 3
at 8.8 km - 10.8 km depth with the volume change rates of 6.5-8.2×10 m /year beneath the center of Aira caldera
generally through the period of 1996-2010. It is indicated that the magma storage at the inflation source inferred at 10km depth beneath Aira caldera is progressed during the period. In the period of 2007-2009, a shallow inflation source
6
3
is located at 4.3 km depth with a volume change rate of about 0.6×10 m /year beneath the northern part of Sakurajima.
It suggests the magma movement towards shallow part of Sakurajima volcano from 10 km depth beneath Aira caldera,
6 3although the estimated amount of magma input is as small as about 2.0×10 m .The magma storage at the magma
reservoir beneath Aira caldera has continued since around 1991 when the eruptive activity of Sakurajima volcano was
gradually decayed. Explosive eruptions are increasing at Showa crater in Sakurajima volcano especially since 2009.
However continuing ground uplifts are observed until November 2010. It is suggested that the amount of suppliedmagma overcomes that of ejected magma at the magma supply system beneath Aira caldera in spite of the increasing
volcanic activity. Considering the estimated volume increase at the inflation sources and the volume of the ejected
8 3
magma based on the observed amount of the ash-fall deposits, it is indicated that the total of about 1.2×10 m magma
is inferred to have additionally stored beneath Aira caldera during the period from 1991 to 2010. The ground upliftaround the northern part of Sakurajima caused by the progressing magma storage at the time of November 2010
recovers and further exceeds the height level in around 1973, when the intense summit eruptions during the 1970s andthe 1980s started. These results suggest the immanent potential of the next intensive eruptive activity of this volcano.
Key words : Sakurajima volcano, Aira caldera, precise leveling survey, vertical ground deformation

1．Introduction
Sakurajima volcano is an andesitic stratovolcano locatedin southern Kyushu, Japan (Fig. 1). It is situated on thesouthern rim of Aira caldera. At this volcano, the flank
eruptions with the lava flows and the summit eruptions
have been repeated during historic times, including thegreat eruption in 1914. The current eruptive activity at thesummit crater of Minamidake began in 1955, which ischaracterized by violent explosive eruptions of a Vulcanian＊

Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, SakurajimaYokoyama, Kagoshima 891-1419, Japan.
＊＊
Aso Volcanological Laboratory, Graduate School of
Science, Kyoto University, Kawayo, Minami-Aso, Aso,
Kumamoto 869-1404, Japan.

type, as well as the intermittent emissions of volcanic ash.
In addition, the eruptive activity at Showa crater on theeastern slope of the volcano started in June 2006 and theactivity has increased in recent years. The total number of
explosive eruptions since 1955 exceeds 9500 in 2010.
Repeated precise leveling surveys have been conductedin and around Sakurajima volcano since 1957 (Eto, 1967;
Eto et al., 1997; Yoshikawa, 1961). The results suggested
that the ground deformation associated with the eruptive＊＊＊

Institute of Seismology and Volcanology, Faculty of
Sciences, Kyushu University, 2-5643-29, Shinyama,
Shimabara, Nagasaki 855-0843, Japan.
Corresponding author: Keigo Yamamoto
e-mail: yamamoto@svo.dpri.kyoto-u.ac.jp

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO

Fig. 1. (a) Location map of Sakurajima volcano. Dashed box indicates the area shown in (b). (b) Leveling bench marks
measured in this study (solid circles). Solid and open triangles indicate the locations of the active summit and Showa
craters of Sakurajima volcano, respectively.

activity reflects the phenomena of magma storage and/or
ejection at the deep primary magma reservoir inferred atabout 10 km depth beneath Aira caldera and at thesecondary one inferred at about 3 km depth beneath Sakurajima volcano (Eto, 1989). The ground subsidence aroundthe northern and the central parts of Sakurajima had
continued due to the pressure decrease both at the primary
and the secondary magma reservoirs associated with theintense eruptive activity since 1973. The ground aroundthe northern part of Sakurajima was observed to be
uplifted when the eruptive activity of Sakurajima volcano
was gradually decayed since around 1991-1996 (Eto et al.,1997).
We conducted the repeated precise leveling surveys in

Sakurajima volcano and around Aira caldera in 2007, 2009
and 2010. In this paper, we discuss the recent verticalground deformation associated with the volcanic activityof this volcano.
2．Description of the leveling surveys
In Table 1, the descriptions of our leveling surveys,
including measured leveling routes, conducting organizations and used leveling equipments are summarized. The
leveling routes measured in the October-December 2007
survey are about 98 km long in total (Table 1 and Fig. 1b).
The leveling surveys were conducted by the Geographical
Survey Institute1 (GSI) and by the joint university team(Yamamoto et al., 2008).

Vertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan
during 1996-2010 as Revealed by Repeated Precise Leveling Surveys
Table 1.

List of leveling surveys.

In the November 2009, April 2010 and November 2010
surveys, Sakurajima coast route and Sakurajima western
flank route were repeatedly measured by the joint university team (Table 1 and Fig. 1b). In addition, we measured
Kagoshima Bay western coast route in the November 2009
survey (Yamamoto et al., 2010). In the November 2010
survey, a new leveling route of about 8 km long wasconstructed around the northern flank of this volcano (notshown in Table 1 and Fig. 1b) and was also measured.
The vertical displacements along this new route will beobtained at the next survey.
The leveling instruments used in these surveys weredigital levels (Wild NA3003 and NA3000) and Invar bar
coded leveling staffs (Wild GPCL3) for the measurements
by the university team and a digital level (Zeiss DiNi12)
and Invar bar coded staffs (Zeiss LD13) for the 2007
measurements by GSI. We conducted the precise levelingsurveys, so as to keep the difference error of double-run
below the allowable limit of 2.5×L1/2 mm and to keep
also the closed error of the level-circuit within 2.0×S1/2
mm if the leveling route is closed, where L is the distancein kilometers along the leveling route between themeasured two bench marks and S is that of the levelcircuit. The horizontality of the line of sight of levels andcircular level of the leveling staffs were checked and1

139

English appellation of the GSI is changed to Geospatial
Information Authority of Japan, from 1 April 2010.

adjusted before each survey, in order to decrease themeasurement errors. As a result, mean square errors of thesurveys were achieved with a good accuracy as the range
from±0.30 to±0.62 mm/km (Table s 2, 3, 4 and 5).
3．Results of the leveling surveys
From the data obtained in each leveling survey, we
calculated the relative height of each bench mark referredto the reference bench mark BM.S.17 which is located at
the western coast of Sakurajima (Tables 2, 3 and 4), except
for Kagoshima Bay western coast route where thereference bench mark is BM.2469 located in Kagoshimacity to the west of Sakurajima (Table 5). We measured
Sakurajima western flank route in the November 2009
survey, but only the relative heights of bench marks
between BM.S.101 and BM.OBS are shown in Table 3 andare discussed here due to the large measurement errors.
The calculated relative heights of the bench marks werethen compared with those of the previous surveys, resulting in the relative vertical displacements of the benchmarks between the corresponding surveys.
3-1 Vertical displacements during the period from1996 to 2007
Fig. 2 represents the relative vertical displacements ofthe bench marks during the period from October 1996 to
October-December 2007, where the 1996 data are takenfrom Eto et al. (1997) and partly (from BM.J.2797 to BM.
2785) from the 1997 survey results conducted by GSI. In

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO
Table 2. Relative heights of bench marks along Sakurajima coast route referred to BM.S.17 which
is located at the western coast of Sakurajima. Asterisk shows replaced bench marks.

Vertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan
during 1996-2010 as Revealed by Repeated Precise Leveling Surveys

141

Table 3. Relative heights of bench marks along Sakurajima western flank route referred to BM.S.
17 which is located at the western coast of Sakurajima. Asterisk shows replaced bench marks.

this figure, the vertical displacements during the period
from December 1991 to October 1996 (Eto et al., 1997)
are also plotted for comparison (Figs. 2a and 2b). The
resultant displacements inside Sakurajima (Fig. 2c) indicate the ground uplift at the northern part of Sakurajima,
similar to those observed during 1991-1996 (Fig. 2a). We
make clear that the amount of the ground uplift isincreasing towards the center of Aira caldera in thedisplacements outside Sakurajima (Fig. 2e), especiallybetween the bench marks of BM. J. 2797 and BM. 2785.
The total amount of the maximum uplift at the northernpart of Sakurajima during 1996-2007 is as much as about
14 cm referred to BM.2785, which is located the farther(about 30 km) apart from the center of Aira caldera. The
uplift is thought to reflect the inflation of the deep primarymagma reservoir beneath Aira caldera, suggesting that themagma storage at the deep primary magma reservoir isprogressed during the study period. Eto et al. (1997)
pointed out that the magma storage began since around1993 when the eruptive activity at the summit crater of
Sakurajima volcano was gradually decayed. On the other
hand, the localized subsidence is seen around the benchmarks of BM.S.36, BM.S.4 and BM.S.12 both in Figs. 2aand 2c (shaded zones in figures). The cause of thissubsidence is thought to be the ground compaction due tothe gravitational loading of the East Showa lava flow, theSouth Showa lava flow and the artificial mound from thelahar deposits, respectively (Eto et al., 1997). Thus theground subsidence is thought to be the phenomena which
does not relate to the volcanic activity. The resultant displacements of the Sakurajima western flank route (Fig. 2d)
are relatively small compared with those of the Sakurajima
coast route (Fig. 2c).
3-2 Vertical displacements during the period from2007 to 2010
Fig. 3 displays the relative vertical displacements of thebench marks during the periods from October-December
2007 through November 2009 and April 2010 to November
2010. The resultant displacements of the Sakurajima coastroute during the period from October-December 2007 to
November 2009 (Fig. 3a) indicate the ground uplift at thenorthern part of Sakurajima, similar to those observed
during 1991-1996 (Fig. 2a) and 1996-2007 (Fig. 2c). However, the amount of uplift at the northern part of Sakurajima during this period is about 7 mm referred to BM.S.17,
indicating that the uplift velocity was smaller than the halfof those in previous periods. The localized subsidence,
especially seen around BM.S.36 in Fig. 3a, seems relatively larger during this period compared with that measured in the previous periods. The resultant displacementsof the Sakurajima western flank route (Fig. 3b) show theground subsidence near the central part of this volcano.

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO

Table 4. Relative heights of bench marks along Kagoshima
Bay eastern coast route and Soo route referred to BM.
S.17 which is located at the western coast of Sakurajima. Asterisk shows replaced bench marks.

Table 5. Relative heights of bench marks along Kagoshima
Bay western coast route referred to BM.2469 locatedin Kagoshima city to the west of Sakurajima. Asteriskshows replaced bench marks.

We can find that the resultant displacements of the
Sakurajima coast route during the period from November
2009 to November 2010 (Fig. 3c) also show the ground
uplift at the northern part of Sakurajima. The amount of
uplift at the northern part of Sakurajima during this period
is about 11 mm referred to BM.S.17 and we can also findthat the most of the uplift occurs during the period from
November 2009 to April 2010. The uplift velocity during
the period of November 2009-April 2010 is estimated tobe larger than the twice of those in 1991-1996 (Fig. 2a)which is thought to be typical during the inflation stage ofAira caldera, while it decreased during the period ofApril-November 2010. It is suggested that the magma
storage at the deep primary magma reservoir is rapidly
progressed especially during the period of November
2009-April 2010. The resultant displacements of the partin the Sakurajima western flank route (Fig. 3d) are
relatively small compared with those of the Sakurajima
coast route (Fig. 3c) but show the slight ground uplift near
the central part of this volcano during the period of
November 2009-April 2010.

One explanation of the cause of this subsidence is that itreflects the deflation of the magma reservoir that is locatedbeneath the summit crater, caused by the recent increase of
the volume of the ejected magma associated with the eruptive activity at the summit and Showa craters. However
we also suspect the cause partly by other effects such as
artificial and/or natural mound from the lahar deposits,
especially around BM.S.108.

4．Discussion
4-1 Analysis based on Mogiʼs model and magma
supply rates
The relative vertical displacements during the period
from 1996 to 2007 are analyzed in the same method as Eto
et al. (1997) based on a spherical source model (Mogi,
1958), in order to calculate the locations and the volume
changes of one or two pressure sources. In the calculation,
the relative vertical displacement data of the bench marks
troubled during the study period and those observed the
localized subsidence (around BM.S.36, BM.S.4 and BM.S.

Vertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan
during 1996-2010 as Revealed by Repeated Precise Leveling Surveys

143

Fig. 2. Vertical displacements of the bench marks in Sakurajima volcano and around Aira caldera referred to BM.S.17
which is located at the western coast of Sakurajima. Shaded zones represent the regions where the localized subsidence
is seen (see text). (a) and (b): Sakurajima coast route and Sakurajima western flank route, respectively during the period
from December 1991 to October 1996 (Eto et al., 1997); (c) and (d): Sakurajima coast route and Sakurajima western
flank route, respectively during the period from October 1996 to October-December 2007; (e): Kagoshima Bay eastern
coast route and Soo route during the period from October 1996, July 1997 to October-December 2007.

12) are eliminated. The results of the pressure source
calculation are summarized in Table 6, in which thoseduring the period 1991-1996 (Eto et al., 1997) are also
shown for comparison.
The inflation source is located beneath the center of Airacaldera (source A in Table 6) while the pressure source
beneath the center of Sakurajima (source B in Table 6)cannot be found from the calculation, similar to the results
obtained in the previous period of 1991-1996. The location of the calculated pressure source and the comparison
between the measured and the theoretical vertical displacements are displayed in Fig. 4, in which the theoreticalcurve deduced from one inflation source accounts for theobserved data fairly well. The estimated volume increaseat the pressure source during the period 1996-2007 is

about 7.2×107 m3 (i.e., 6.5×106 m3/year) (Table 6).
At Sakurajima volcano, monthly amounts of volcanicash ejected by the summit eruptions have been estimatedsince 1978 (Eto, 1988). In the estimation, the empiricalequations as a distribution function of ash-fall deposits are
determined by using the observed monthly amount of ashfall deposits at monitoring stations around the volcano andthe integrations of the empirical equations result in themonthly mass of ash-fall deposits. The amounts of ejectedash-fall deposits have been compared with the deformation
volumes calculated from the leveling data in order toevaluate the amounts of supplied magma at the magma
reservoirs (Eto et al., 1997).
Hereinafter, the similar discussion is made with theresults during the period 1996-2007. The amount of the

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO

Fig. 3. Vertical displacements of the bench marks in Sakurajima volcano referred to BM.S.17 which is located at thewestern coast of Sakurajima. Shaded zones represent the regions where the localized subsidence is seen (see text). (a)and (b): Sakurajima coast route and Sakurajima western flank route, respectively during the period from OctoberDecember 2007 to November 2009; (c) and (d): Sakurajima coast route and Sakurajima western flank route,respectively during the period from November 2009 through April 2010 to November 2010.

Table 6. Results of the pressure source calculation. A and B: pressure sources beneath the center of Aira caldera andbeneath the center of Sakurajima, respectively. X and Y: horizontal position of the pressure source, where the origin of
the X,Y coordinates is at the center of the summit crater of Sakurajima volcano. Z: depth of the pressure source. Uzo isthe vertical displacement above the pressure source. ΔVp is the deformation volume at the pressure source.

ejected magma (where the density of magma is assumed tobe 2.5 g/cm3) based on the observed amount of the ash-falldeposits during the period 1996-2007 is estimated as about
2.8×106 m3. Combined with the calculated volume increase at the pressure source, the total of about 7.5×107
m3 magma is inferred to be supplied into the deep primarymagma reservoir beneath Aira caldera. This amount corresponds to about 6.8×106 m3 /year of the average magma
supply rate. The magma supply rate during the period of
1991-1996 was estimated as about 1.0×107 m3/year (Etoet al., 1997) and was equivalent to the typical average
supply rate that had been investigated from the grounddeformation in the non-eruptive period of 1919-1932(Ishihara, 1981). In the period of 1996-2007, the magma
supply rate is inferred to become slightly smaller than thatin the previous period, or the typical average rate,suggesting that the almost ordinary but slightly smaller
rate magma supplying process has been continued at themagma reservoir beneath Aira caldera.
Although the relative vertical displacements are avail-Vertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan
during 1996-2010 as Revealed by Repeated Precise Leveling Surveys

145

Fig. 4. (a) Horizontal location of the calculated pressure source (solid circle) by using the relative vertical displacement data
during the period from 1996 to 2007. Z and ΔVp represent the depth and the deformation volume at the pressure source,
respectively. Solid rectangles denote the bench marks used for the calculation. Solid and open triangles indicate thelocations of the summit and Showa craters, respectively. (b) Comparison between measured vertical displacements(solid circles) and theoretical vertical displacements (solid line) versus radial distance from the pressure source.

able only in Sakurajima during 2007-2010, and theobtained displacements are as small as of about millimeterorder, we also tried the pressure source calculations basedon a spherical source model. The data of the bench marks

troubled during the study periods and those observed the
localized subsidence are also eliminated from the calculations. The results of the pressure source calculations are
summarized in Table 6 and are displayed in Fig. 5.

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO

Fig. 5. (a), (c) and (e): same as Fig. 4a but by using the data during the period from October-December 2007 to November
2009 ((a): A-source and (c): B-source in Table 6) and those during the period from November 2009 to November 2010((e): A-source). (b), (d) and (f): same as Fig. 4b but by using the data during the period from October-December 2007 to
November 2009 ((b): A-source and (d): B-source in Table 6) and those during the period from November 2009 toNovember 2010 ((f): A-source).

Vertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan
during 1996-2010 as Revealed by Repeated Precise Leveling Surveys

147

Fig. 6. Schematic illustration showing the locations of the calculated pressure sources projected onto North-South crosssection crossing the summit crater of Sakurajima volcano (open circles) and inferred magma movement (see text for
discussion). The results during the period of December 1991 - October 1996 (Eto et al., 1997) are also plotted forcomparison. The area of the each circle is drawn in proportion to the absolute deformation volume at the pressure source.

From the data during the period from OctoberDecember 2007 to November 2009, the inflation source islocated at 4.3 km depth beneath the northern part of
Sakurajima (Figs. 5a and 5b) and the deflation source isfound at 1.5 km depth beneath the western flank of this
volcano (Figs. 5c and 5d). The North-South cross sectional view of the locations of these pressure sources canbe seen in Fig. 6 with that calculated during 1996-2007.
We suppose that the inflation source at 4.3 km depth
reflects the magma accumulation around the location(Figs. 5a and 6), whereas the estimated location and theshallow depth of the deflation source (Figs. 5c and 6) lead
us to the speculation that it does not relate to the magma
reservoir. The volume increase at the inflation source isestimated as about 1.1×106 m3 (i.e., 0.6×106 m3 /year)(Table 6). While the amount of the ejected magma basedon the observed amount of the ash-fall deposits during thisperiod is estimated as about 0.9×106 m3, the total of about
2. 0 × 106 m3 magma is inferred to be supplied into the

inflation source. The averaged magma supply rate ofabout 1.0×106 m3 /year corresponds to one tenth of thetypical magma supply rate mentioned above. The locationof the inflation source seems to move shallower and closer
to Sakurajima volcano compared with that in the previous
period, which may suggest the magma movement towards
shallow part of Sakurajima volcano from 10 km depth
beneath Aira caldera, although the amount of magma input
is small (Fig. 6).
During the period from November 2009 to November
2010, the inflation source is located beneath the center ofAira caldera (Fig. 5e), similar to the results calculated in
the period of 1996-2007. The theoretical vertical displacements deduced from one inflation source account for themeasured data with a good agreement (Fig. 5f). The estimated volume increase at the inflation source is about 8.2
×106 m3 (Table 6) and the amount of the ejected magma
based on the observed amount of the ash-fall deposits
during the period is estimated as about 3. 0 × 106 m3,

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO

Fig. 7. (a) Relation among secular change of relative heights of BM.S.26 (located in northern Sakurajima) referred to BM.S.
17 (located in western Sakurajima), (b) yearly weights of estimated volcanic ash-fall deposits and (c) yearly number of
explosive eruptions at the active summit and Showa craters of Sakurajima volcano.

resulting in the total of about 1. 1 × 107 m3 magma isinferred to be supplied into the deep primary magma
reservoir beneath Aira caldera. The magma supply rate (ofabout 1.1×107 m3/year) corresponds to nearly the same tothe typical average supply rate suggested by Ishihara(1981). The magma supply rate may be doubled during
the period between November 2009 and April 2010,
considering that the most of the vertical displacements is
produced during this period. These shorter period
fluctuations of the magma supply rates may cause thefluctuations of eruptive activity and thus need the further
examination of these relations.
Combined the results of this study with that of Eto et al.
(1997), it is indicated that the total of about 1.4×108 m3
magma is inferred to be supplied into the magma reservoir
at about 10 km depth beneath Aira caldera (Fig. 6) during
the magma accumulation period from 1991 to 2010. On
the other hand, only about 0.2×108 m3 of this suppliedmagma has been ejected by eruptive activity, resulting in
the possibility that about 1.2×108 m3 magma has additionally stored beneath Aira caldera during the period from1991 to 2010.
4-2 Secular change of ground deformation andrelation to the volcanic activity
The ground deformation around the northern part of
Sakurajima located close to the center of Aira caldera wellreflects the inflation or deflation process of the deep
primary magma reservoir associated with the eruptive
activity of this volcano. In Fig. 7, the secular verticaldeformation of BM. S. 26 (located in northern part of
Sakurajima) referred to BM.S.17 (located in western partof Sakurajima) resulted from the repeated leveling surveys
is plotted since 1957 with yearly weights of estimatedvolcanic ash-fall deposits and yearly number of explosive
eruptions at the summit and Showa craters of Sakurajima
volcano. It is clearly seen that the ground uplift wasVertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan
during 1996-2010 as Revealed by Repeated Precise Leveling Surveys

149

Fig. 8. Secular change of relative heights of BM.2474 (located at western coast of Kagoshima Bay) referred to BM.2469(located in Kagoshima city to the west of Sakurajima) and volcanic activity of Sakurajima volcano (modified from Fig. 1
in Eto et al. (1997)).

detected around the northern part of Sakurajima during theperiod around 1967-1973, which is caused by the inflationof the primary magma reservoir associated with themagma storage during the period of decreasing eruptive
activity. The ground subsidence was observed during theperiod around 1973-1991 due to the pressure decrease at
the magma reservoir associated with the magma ejection
during the period of intense eruptive activity. The ground
uplift was identified again since around 1991 when theeruptive activity of this volcano was gradually decayed(Eto et al., 1997). The current relative height of BM.S.26
in November 2010 recovers and further exceeds the heightlevel in around 1973, suggesting that the volcano has thepotential of the similar intense eruptive activity as that
occurred during the 1970s and the 1980s. The increase ofyearly number of explosive eruptions in 2009 and 2010
(Fig. 7c) is attributed to the recent increase of the eruptive
activity at Showa crater, and the amount of magma
ejection is correspondingly enlarged (Fig. 7b), while theuplift velocity momentary decreased during the period ofApril - November 2010 (Fig. 7a). However the continuing
ground uplifts suggest that the amount of supplied magma
overcomes that of ejected magma at the magma supply
system beneath Aira caldera until November 2010. It issuggested to be necessary of ground deformation monitoring associated with the volcanic activity in order toevaluate the eruption potential of this volcano in the future.
Fig. 8 represents the secular change of the relative heights
of BM.2474 (located at western coast of Kagoshima Bay)referred to BM. 2469 (located in Kagoshima city to thewest of Sakurajima) resulted from the repeated levelingsurveys since 1891, where BM.2474 is located relatively
close to the center of Aira caldera and displays the uplift or
subsidence associated with the inflation or deflation processof the deep primary magma reservoir. The leveling data
are acquired during more than one hundred years and thelava flows of the 1914 great eruption and the 1946 eruption
occurred on the way in this acquisition period (Eto, 1967;
Eto et al., 1997; Kamo, 1978; Omori, 1916; Sassa, 1956).
The ground uplift of BM.2474 associated with the magma
storage at the primary magma reservoir has progressed to
be about 75 cm after the 1914 eruption to November 2009,
and about 85 cm subsidence is inferred at the eruption(Sassa, 1956), where uplifts during 1891-1914 are assumed
mainly from the averaged uplift rate measured during otherperiods of dormant eruptive activity. The ground subsidence
at the eruption is suggested to be already recovered about
90 % for about 100 years after the eruption.
Lu et al. (2010) investigated the ground deformation at
Okmok volcano, which is a basaltic central volcanic complex in Alaska, measured with InSAR observations fromthe end of the 1997 eruption to shortly before the onset ofthe 2008 eruption. From the source volume changes, they
estimated that magma storage at about 3.5 km depth
beneath the caldera floor had increased by 3.7-5.2×107
m3 during 1997-2008, which corresponds to 85-100 % of
the magma volume erupted in 1997 (i.e., the volume ofmagma withdrawn during the 1997 eruption was mostly

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Keigo YAMAMOTO, Tadaomi SONODA, Tetsuro TAKAYAMA, Nobuo ICHIKAWA, Takahiro OHKURA, Shin YOSHIKAWA,
Hiroyuki INOUE, Takeshi MATSUSHIMA, Kazunari UCHIDA and Manami NAKAMOTO

replenished by 2008). They also suggested that the effectsof continuing magma supply and vesiculation of stored
magma caused a critical pressure threshold to be exceeded,
triggering the 2008 eruption. The case of Okmok volcano
is suggestive to other volcanoes including Sakurajima. If
the magma storage is progressing and the resultant ground
uplift around BM. 2474 continues at the recent rate, therelative height of BM. 2474 will reach the hypothesizedheight level at just before the 1914 eruption after about
10-20 years, suggesting that the volcano will gain theimmanent potential of the similar eruptive activity as that
occurred in 1914.
5．Conclusions
The vertical ground deformation in Sakurajima volcano
and around Aira caldera associated with the volcanic
activity of this volcano is revealed by the recent repeatedprecise leveling surveys conducted in October-December
2007, November 2009 and April and November 2010. We
discuss the ground deformation in the period of 1991-2010
including previous leveling data. The results are summarized as follows:(1) The ground uplifts are detected in Sakurajima
volcano and around Aira caldera to be centered in thecaldera during the period of 1996-2010. The uplifts arethought to reflect the magma storage at the magma
reservoir beneath Aira caldera, and have continued sincearound 1991 when the eruptive activity of Sakurajima
volcano was gradually decayed. Explosive eruptions are
increasing in Sakurajima volcano since 2009. However
continuing ground uplifts suggest that the amount of
supplied magma overcomes that of ejected magma at themagma supply system beneath Aira caldera until November 2010.
(2) From the analysis based on Mogiʼ s model, theinflation source is generally located at the 8. 8-10. 8 km
depth with the volume change rates of 6.5-8.2×106 m3/year beneath the center of Aira caldera through the periodof 1996-2010. It is indicated that the magma storage
beneath Aira caldera is progressed during the period. In
the period of 2007-2009, a shallow inflation source iscalculated at the 4.3 km depth with a volume change rate ofabout 0.6×106 m3/year beneath the northern part of Sakurajima. It may suggest the magma migration towards shallow part of Sakurajima volcano from 10 km depth beneath
Aira caldera.
(3) It is indicated that the total of about 1.2×108 m3
magma is inferred to have additionally stored beneath Airacaldera during the period from 1991 to 2010. The progressing ground uplift of BM.S.26 around the northern partof Sakurajima due to the magma storage at the time ofNovember 2010 recovers and further exceeds the heightlevel in around 1973, when the intense summit eruptions
during the 1970s and the 1980s started. The ground upliftof BM.2474 around the western coast of Kagoshima Bay is

about 75 cm for about 100 years after the 1914 great eruption to November 2009, and already recovers about 90 %of the inferred ground subsidence at the eruption. These
results suggest the immanent potential of the next intensive
eruptive activity of this volcano.
Acknowledgments
We thank the staffs of Sakurajima Volcano ResearchCenter, Disaster Prevention Research Institute, Kyoto
University, for help and assistance for the observations.
The leveling surveys are conducted with many other colleagues: T. Yamazaki, M. Tada, Y. Hiraoka, T. Enya, M.
Nemoto, A. Yokoo, T. Ohminato and A. Watanabe. We
are also grateful to Dr. F. Kimata and an anonymous reviewer for constructive and valuable comments and suggestions which improve the manuscript. We also acknowledge Dr. T. Hashimoto for his helpful editorial handling.
Some figures are drawn by using the GMT software(Wessel and Smith, 1995). This study was supported by
the National Project for the Prediction of Volcanic Eruption and by the National Project for the Prediction of
Earhtquake and Volcanic Eruption by Ministry of Education, Science, Sports and Culture, Japan.
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(Editorial handling Takeshi Hashimoto)

水準測量の繰返し観測による桜島火山の火山活動に伴う地盤上下変動（1996 年〜2010 年）
山本圭吾・園田忠臣・高山鐵朗・市川信夫・大倉敬宏・吉川
井上寛之・松島 健・内田和也・中元真美

慎・

桜島火山の活動に伴う最近の桜島および姶良カルデラ周辺域における地盤上下変動が，2007 年 10 月-12
月，2009 年 11 月，2010 年 4 月および 11 月と行われた精密水準測量の繰返し観測によって明らかとなった．
姶良カルデラ周辺の地盤は，1996 年から 2010 年までの期間において，それ以前の 1991 年から 1996 年までの期間に得られていた結果と同様に，カルデラ内部を中心として隆起したことが確認された．球状圧力源（茂木）モデルに基づく解析を行った結果，1996 年-2010 年の期間において，姶良カルデラ中央部地下の深さ
8.8 km-10.8 km に増圧源の存在が推定された．この期間，姶良カルデラ地下に推定されるマグマ溜りにおいてマグマの貯留が進行したものと考えられる．2007 年-2009 年の期間においては，桜島北部地下の深さ 4.3
km に増圧源の存在が推定された．このことは，姶良カルデラの深さ 10 km から桜島の浅部方向へのマグマの移動が生じた可能性を示唆するが，そのマグマの移動量は小さい．姶良カルデラ地下におけるマグマの貯留は，桜島火山の山頂噴火活動が静穏化した 1991 年頃から継続している．2009 年以降，昭和火口における噴火活動が活発化する傾向にあるが，観測された地盤隆起の継続は，噴火活動が活発化しつつある 2010 年
11 月の時点においても姶良カルデラ地下においてマグマの供給量が放出量を上まっていることを示唆している．計算された増圧源において見積もられた容積増加量および観測降下火山灰量に基づき見積もられた
8 3マグマの放出量を考慮すると，1991 年から 2010 年までの期間において姶良カルデラの地下に約 1.2×10 mのマグマが新たに蓄積されたことが推定される．また，マグマの蓄積に伴う桜島北部付近の 2010 年 11 月の時点における地盤隆起量は，1970 年代および 1980 年代の活発な山頂噴火活動が開始した 1973 年頃の状態を回復し更に隆起が継続した状態となっている．これらの結果は，桜島火山の次の大規模噴火活動についての潜在的なポテンシャルを示唆するものと考えられる．