Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 115-135

ArticleCharacteristics of Volcanic Activity at Sakurajima Volcanoʼs
Showa Crater During the Period 2006 to 2011
Masato IGUCHI, Takeshi TAMEGURI＊, Yusaku OHTA＊＊, Sadato UEKI＊＊ and Shigeru NAKAO＊＊＊(Received September 27, 2011 ; Accepted January 8, 2013)
Eruptive activity at the Showa crater of the Sakurajima volcano has steadily increased since it resumed in June
2006, and 2718 vulcanian eruptions occurred during the period from 2008 to 2011. In this paper, we clarify thecharacteristics of vulcanian eruptions at the Showa crater based mainly on ground deformation. A long-term extensionof the ground of the Aira caldera and Sakurajima, repeating quasi-annual cycles of minor inflation-deflation, wereobtained by GPS, tilt and strain observations. The inflation event that started in October 2009 was the largest. Majorpressure source was estimated to be located at a depth of 12 km beneath the Aira caldera for the inflation and a minor
source was obtained a depth of 5 km at the northern flank of Kitadake in addition to a source beneath Minamidake
which has been know by previous studies. The magma plumbing system is composed of a major magma reservoir at a
depth of ≈10 km beneath the Aira caldera and additional magma reservoirs at depths of around 5 km beneath thesummit area from the north flank of Kitadake to Minamidake. Strain changes which indicate inflation were detectedprior to explosions and the inflation strain lasted mostly 1 h. The strain changes were caused by a shallow pressuresource less than 1.5 km. The inflation occasionally continued for more than 7 h with an addition of inflation of a deep
source (4 km), which corresponds to the magma reservoir beneath Minamidake. The conduit to the Showa crater may
be branched from the magma reservoir beneath Minamidake or from the major conduit connected to it. When
inflationary ground deformation progressed at a high rate, the eruptive activity reached a peak from December 2009 to
March 2010. This suggests that the accumulation of magma beneath the central cones of the Sakurajima volcano
progressed simultaneously to a discharge of magma. The simultaneous progress of the accumulation and discharge ofmagma and the frequent occurrence of small vulcanian eruptions may be related to the small open conduit.
Key words : Sakurajima volcano, Showa crater, Minamidake crater, ground deformation, magma supply, vulcanianeruption

1．Introduction
Sakurajima is a post-caldera volcano formed at thesouthern rim of the Aira caldera. The central cones are
Kitadake, Nakadake and Minamidake. Kitadake is theoldest cone. All the eruptions since the 8th century haveoccurred at the summit area of Minamidake and its flanks,
including the sea area.
Eruptive activity has taken place continuously at the
Minamidake crater since 1955. The eruptive activity is
characterized by frequent activities of a vulcanian type,
and the crater was quite active during the period from 1972
to 1992, with 4919 vulcanian eruptions and 2.4×108 tonsof volcanic ash being emitted. Of particular note, 498
vulcanian eruptions occurred in 1974, 415 in 1983 and 452
in 1985. The mechanisms of the vulcanian eruptions havebeen studied mainly by seismological and geodetic obser＊

Sakurajima Volcano Research Center, DPRI, Kyoto Univ.,
1722-19, Sakurajima-Yokoyama, Kagoshima 891-1419,
Japan.
＊＊
GraduateSchool of Science, TohokuUniv., 6-3 Aramakiaza Aoba, Aoba-ku, Sendai 980-8578, Japan.

vations. Tameguri et al. (2002) analyzed the explosion
earthquakes associated with the vulcanian eruptions andproposed a four-source model with moment tensors to
explain the waveforms of the explosion earthquakes.
However, this seismological method cannot cover theentire process of vulcanian eruptions due to the insufficientfrequency range of even the most broadband seismometer.
Ishihara (1990) detected inflation of the ground prior to thevulcanian eruptions and the deflation associated with themby using water-tube tiltmeters and extensometers sited in
an underground tunnel, and interpreted the inflation anddeflation as being caused by magma intrusion at a depth of
2-6 km and by ejection of the magma product, respectively. Iguchi et al. (2008c) proposed a model for theprocess of vulcanian eruptions in which an explosive
eruption is triggered by minor deflation, which is induced＊＊＊

GraduateSchool of Science and Engineering, Kagoshima
Univ., 1-21-24, Korimoto, Kagoshima 890-8580, Japan.
Corresponding author: Masato Iguchi
e-mail: iguchi@svo.dpri.kyoto-u.ac.jp

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by a decrease in pressure in a conduit as the result of asudden outgassing of water-saturated magma deep in the
conduit and the ensuing cap failure at the top of the
conduit.
After 1993, explosive activity at the Minamidake crater
gradually decreased and the annual number of vulcanianeruptions fell below 10 from 2003 onward. In contrast to
the declining eruptive activity at the Minamidake crater,
eruptive activity resumed at the Showa crater on June 4,
2006. The Showa crater is located at an elevation ofaround ≈ 800 m on the eastern flank of Minamidake.
Previous eruptive activity at the crater was initiated by
explosive eruptions with pyroclastic flows in October andNovember 1939 (Tsuya and Minakami, 1940). After anincrease in eruptive activity in January and February 1946,
lava effused from the crater in March and the lava flow
reached the east and south coasts in one month (Hagiwara
et al., 1946). The total amount of effused lava was estimated to be 0.18 km3 (Ishihara et al., 1981). The Showa
crater was dormant during the violent eruptive period of
the Minamidake crater that started in 1955, and theeruptive activity in 2006 at the Showa crater marked aresumption after 58 years of dormancy following minor
eruptions in 1948.
The eruptive activity at the Showa crater was of aphreatic type in 2006 and 2007 (Miyagi et al., 2011), thenvulcanian eruptions began to occur on February 3, 2008.
The number of vulcanian eruptions increased rapidly in
October 2009, and the annual numbers in 2010 and 2011were almost twice those at the peaks of vulcanian eruptions at the Minamidake crater. However, the scale of thevulcanian eruptions at the Showa crater was one to twoorders of magnitude smaller than the eruptions at the
Minamidake crater - from the viewpoints of explosion
earthquakes, infrasonic waves, ground deformation andamounts of volcanic ash ejected (Iguchi et al., 2010). The
eruptive activity at the Showa crater up to 2011 wascharacterized by more frequent occurrences of smaller
vulcanian eruptions than the eruptivity at the Minamidake
crater.
Following a decline in eruptivity at the Minamidake
crater that started in 1993, the magma reservoir 10 km
beneath the Aira caldera has become inflated, as revealed
by precise leveling (Eto et al., 1997; 1998a; Yamamoto etal., 2008), EDM (Eto et al., 1998b) and GPS observations(Kriswati and Iguchi, 2003; Iguchi et al., 2008b) conducted since 1995, and the amount of magma accumulated
in the reservoir was estimated to be 8×107 m3 during theperiod from 1996 to 2007 (Iguchi et al., 2008b). During
the process of inflation, eruptive activity at the Showa
crater resumed in 2006 and has continued to increase sincethen (Iguchi et al., 2010). Considering inflationary processes at the Aira caldera prior to the 1946 eruption at theShowa crater (Sassa, 1956), and prior to the frequent
occurrence of vulcanian eruptions at the Minamidake

crater starting in 1972, it can be forecast that the current
eruptivity at the Showa crater will continue to increase.
Typically, abrupt starts of eruptions of the magmatic
type are immediately preceded by an increase in theseismicity of volcano-tectonic earthquakes, see: 1977-1982
Usu (Suzuki et al., 1980), 1983 Miyakejima (Uhira et al.,1984), 1986 flank eruption at Izu-Oshima (Yamaoka et al.,1988), 1991-1995 Fugendake at Unzen (Umakoshi et al.,2001), and 2000 Miyakejima eruptions (Sakai et al., 2001).
During the eruptive period at Minamidake, an increase in
A-type earthquakes (volcano-tectonic) and a migration of
their hypocenters prior to the occurrence of frequentvulcanian eruptions was reported (Kamo, 1978). In addition, swarms of B-type earthquakes occurred a few days
prior to an increase in vulcanian eruptions (Nishi, 1984).
In contrast to these eruptive activities, a gradual increase ineruptive activity at the Showa crater had not hitherto beenaccompanied by remarkable increases in seismicity, but by
ground deformation.
By analyzing ground deformation data, the long-term
processes giving rise to the gradual increase in eruptive
activity are clarified, and the characteristics of individual
vulcanian eruptions at the Showa crater are examined.
Finally, the mechanisms of such a slow increase in eruptive activity are discussed from the points of view of theshort-term and long-term processes of ground deformation.
In this study, we refer to a “vulcanian eruption” at the
Sakurajima volcano simply as an “explosion.”
2．Observations
The Sakurajima Volcanological Observatory (SVO,
Volcano Research Center; now the SVRC) operated by theDisaster Prevention Research Institute (DPRI) of Kyoto
University has observed Sakurajima since 1960. The locations of measuring instruments are shown in Fig. 1. Shortperiod (1 Hz) 3-component seismometers have been installed at 10 seismic stations (8 in boreholes, 2 on the groundsurface) around the summit cones, 5 of which are equippedwith broadband seismometers (STS-2: Streckeisen, JCP-1:
Akashi, Trillium 120 PA: Nanometrics; frequency range 0.
0083-50 Hz). Of the 8 borehole stations, 6 are equippedwith borehole tiltmeters (Akashi, JTS-23). The seismometers for the horizontal components and the tiltmeters inthe boreholes are oriented to the direction of the center ofthe Minamidake crater (radial component) and to thedirection rotated clockwise by 90° from it (tangential component). A set of 2-component (radial and tangential)
water-tube tiltmeters and 3-component (radial, tangential
and oblique) extensometers is installed over a length of 28
m in two underground tunnels respectively. One is locatedat Harutayama (HVOT) 3.4 km from the Showa crater, andthe other at Arimura (AVOT) 2. 1 km from the crater,
operated by the Osumi Office of River and National
Highway, Ministry of Land, Infrastructure, Transport andTourism (MLIT). Infrasonic microphones (type 7144:

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011

117

Fig. 1. Location of observation sites at Sakurajima. Dots indicate seismic stations. Stations HAR, KOM, ARI, KUR, SHN
and OKO are equipped with borehole tiltmeters. Open circles are GPS observation sites. The campaign sites are shownby smaller circles. Gray triangles are underground tunnels, HVOT and AVOT, where water-tube tiltmeters andextensometers are installed. Infrasonic microphones are installed at sites shown by diamonds. Inverted triangles are tidegauges. At Station KUR, the concentrations of CO2 and H2 at a hot spring are measured. The letters “M,” “K” and “S”
indicate craters Minamidake, Kitadake and Showa, respectively.

ACO) are installed at four sites. The microphone at
AVOT is used to detect explosions at the Showa crater.
The seismic and infrasonic signals are sampled at a rate of
200 Hz at a 24-bit resolution using an analog-digital converter (LF-2000 or LS7000-XT: Hakusan) at all relevantsites. The signals from the borehole and water-tube
tiltmeters and extensometers are sampled at 1 Hz using ananalog-digital converter of the same type. The digitizeddata are transferred to the SVO data center via an ISDN orADSL network and by radio-frequency wireless connections. Continuous and online GPS instruments are installed at 8 stations at and around Sakurajima. GPS data aresampled at 1 Hz by a dual-frequency receiver (GRX1200
or GMX902: Leica Geosystems). The GPS data are transferred by TCP/IP connection to SVO and analyzed automatically using Spider software (Leica Geosystems).
Campaign observations by GPS have been repeated at
Sakurajima and around the Aira caldera at least once ayear.
3．Precursors to the resumption of eruptive activity in
2006
Although no drastic change, such as a sudden preliminary increase in seismicity, was detected prior to theresumption of eruptive activity of the Showa crater on June4, 2006, three precursory phenomena were noted: a slightincrease in A-type earthquakes, an inflation of the Sakura-jima and Aira caldera areas, and an increase in geothermal
activity around the crater.
The monthly numbers of A-type, B-type and explosion
earthquakes during the period from 1991 to 2011 areshown in Fig. 2. Explosion earthquakes after 2008 wereassociated mainly with explosions at the Showa crater except for 6 Minamidake events. The seismicity of the Atype earthquakes was low during the active period of
eruptivity at the Minamidake crater, i.e., before 2000. The
seismicity increased during the period from November
2003 to January 2004 and during December 2004. Fig. 3
shows the hypocenter distribution of A-type earthquakes,
assuming a half space with 5 layers for the velocity of Pwaves based on Hidayati et al. (2007). The hypocentersof the A-type earthquakes were concentrated in the summit
area. In addition, A-type earthquakes occurred at depthsof 6-9 km in the southwestern part of Sakurajima and at
depths of 4-10 km below the northeastern rim of the Airacaldera. The seismicity in November 2003 was locatedsouthwest of Sakurajima. It was reported that A-type
earthquakes sometimes occurred in the same area in
1976-78, accompanying the migration of the hypocentersfrom a deep depth to a shallow one beneath the southwestof Minamidake prior to an increase in explosive activity,
typically in May 1976 (Kamo, 1978). As lower seismicity
was recorded in the 1980s and 1990s, the seismicity in thesouthwest of Minamidake became active in November

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

Fig. 2. Monthly number of A-type, B-type and explosion earthquakes during the period from 1991 to 2011. All the
explosion earthquakes prior to January 2008 were accompanied by explosions at the Minamidake crater. Most of the
explosion earthquakes after February 2008 were related to events at the Showa crater and there were only 6 events at the
Minamidake crater.

2003 for the first time in more than 20 years. From
January 2004, the seismicity at the summit area increased
after the occurrence of earthquakes southwest of the
volcano and northeast of the caldera. The seismicity of Btype earthquakes increased from February 2006 relative tothe seismicity in 2001 to 2005. However, the increase in
the seismicity of B-type earthquakes was much lower thanthat in the eruptive period of Minamidake in the 1990s.
A second precursor is inflation ground deformation.
Fig. 4 shows the temporal change in the horizontal distanceof baselines in the north-south (FUT-ARI) and east-west(SVO-KUR) directions. The number of explosion earthquakes reached a peak in December 1999, with 104
explosions at the Minamidake crater (Fig. 2). Prior to theincrease in explosive activity, extensions of the horizontal
baseline lengths of 6 cm and 4 cm were detected in thenorth-south and east-west directions, respectively, during
the period from November 1997 to October 1999.
Although a slight extension in the east-west direction and acontraction in the north-south direction were noted after
2000, the deformation rates were low. The baselines extended again, starting in October 2004, as shown by thearrows in Fig. 4. The extensional deformation corresponded to an increase in seismicity of A-type earthquakes
at the summit area in December 2004 (Figs. 2 and 3). The
extensions of the baselines had reached 8 cm and 9 cm inthe north-south and east-west directions, respectively,from 1995 to 2006. Eto et al. (1998a; 1998b) showed thatground of Sakurajima and the Aira caldera turned inflationin 1993. Extension of baselines detected by GPS (Fig. 4)
is a part of the inflation and the inflation had continued
with variety of deformation rate and the eruptive activity
commenced at the Showa crater in 2006.
An increase in geothermal activity is a third precursor.
An increase in fumarolic activity around the Showa crater
was observed visually in January 2006, and the increase
was confirmed by geothermal measurement using an
infrared scanner in March 2006 (Yokoo et al., 2007). The
geothermal temperatures around the Showa crater measured by infrared scanners decreased during the eruptive
period at Minamidake (Kamo et al., 1995); however, thetemperature in March 2006 was higher by 15-20℃ thanthe last observation, made in 1992.
4．Eruptive activity at the Showa crater from 2006 to
2011
Eruptive activity at the Showa crater commenced with
an emission of volcanic ash around 10 : 00 to 11 : 00 on
June 4, 2006. The first eruptive activity in June 2006
continued only for two weeks, accompanying a weakemission of volcanic ash; while a second bout of activity in
May 2007 continued for three weeks (Iguchi et al., 2008a).
The eruptive activities were of the phreatic type (Miyagi etal., 2010). The eruptions did not accompany explosion

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011

119

Fig. 3. Hypocenter distributions of A-type earthquakes during the period 1997 to 2011. The epicenter distribution is shownat the upper left graph. Hypocenters on vertical cross-sections in the directions north-south and east-west are shown in
the upper middle and lower left graphs, respectively. The upper right graph shows temporal changes in north-south
locations during the period 2001 to 2011. The Minamidake crater and the Wakamiko submarine volcano are indicated by“M” and “W,” respectively.

Fig. 4. Temporal change in horizontal distance of baselines in the north-south and east-west directions measured by GPS during the period from 1995 to 2011.
Arrows indicate the beginning of extensions. See Fig. 1
for location of stations. Note that fluctuation of dataplot became small after replacement of GPS receivers
and antennas in June 2005.

earthquakes, and the strength of the infrasonic waves
accompanying the eruptions was <3 Pa at Station AVOT.
The style of eruption changed to a vulcanian type on
February 3, 2008, when eruptive activity restarted after
eight months of dormancy. An eruption at 10 : 17 on
February 3 was accompanied by an explosion earthquake
and a clear infrasonic wave (16 Pa at Station AVOT). We

counted the number of explosions based on the intensity of
infrasonic waves of ≥10 Pa at Station AVOT. An explosion occurred at 15 : 54 on the same day, accompanied by apyroclastic flow having a flow distance of 1. 1 km. A
pyroclastic flow was similarly produced by an explosion at
11 : 25 on February 6.
The temporal change in eruptive activity from 2008 to
2011 is shown in Fig. 5 by the amplitude of infrasonicwaves, the monthly number of explosions, and the monthly
weight of volcanic ash ejected from the Showa crater. The
monthly weight of volcanic ash is estimated from the sumof the patterns of decrease in volcanic ash weight according to distance from the crater, approximated by an
exponentially decaying curve, along 8 directions (Ishikawa
et al., 1981). The decrease patterns are determined bymeasurements at 58 points, which have been operated by
the Kagoshima Prefectural government since 1978.
In 2008, 18 explosions occurred from February 3 to July
28. No volcanic eruptions occurred after August, however,gas emissions from the surrounding areas of the crater
were quite intensive, sometimes reaching a height of 1 km
above the crater.
On February 1, 2009, explosive activity resumed. During
the period from February to April 2009, eruptive anddormant periods were alternatively repeated at a timeinterval of around two weeks. Activity, consisting of explosions, gradually increased from July 2009 onward. On
October 3, an explosion occurred at the Minamidake crater.
The volcanic ash cloud reached 3000 m above the summit

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

Fig. 6. Temporal change in horizontal distance of baselines in the north-south and east-west directions measured by GPS during the period 2006 to 2011. Monthly
numbers of explosions and monthly weights of volcanic ash ejected from the craters of Sakurajima are
included in the upper and lower graphs, respectively.
Fig. 5. Temporal change in eruptive activity during theperiod from 2008 to 2011. Top: Amplitudes of infrasonic waves generated by explosions. The infrasonic waves are observed at the station AVOT. Middle:
Monthly number of explosions. Bottom: Monthly weightof volcanic ash ejected from the Showa crater.

and the amplitude of the infrasonic waves was >200 Pa at
Station AVOT and 837 Pa at Station HAR. The scale ofthe explosion was classified as “large” relative to most
explosions at the Minamidake crater. After the explosion,
eruptive activity at the Showa crater increased, reaching apeak during the period from December 2009 to March
2010, when more than 130 explosions occurred per month.
The monthly weight of volcanic ash exceeded 1×106 tons
during the period from January to March 2010. After May
2010, explosive activity declined -except for July 2010and the activity fell to a low level in October 2010, when
only 15 explosions occurred and the monthly weight of
volcanic ash decreased to 0.07×106 tons.
From the end of November 2010 explosive activity
increased again, exceeding 90 explosions per month during
the period from January to April 2011. Associated with
the increase in the number of explosions, the weight of
volcanic ash ejected from the crater increased to 0.4〜0.7
×106 ton/month. Eruptive activity from November 2010
was smaller than that from October 2009 in terms of thenumbers of explosions and the volume of volcanic ashejected, and eruptive activity declined again in June andJuly 2011.
Both the monthly number of explosions and the volume
of volcanic ash began to increase from August 2011, andthis tendency continued until the end of December 2011.
The intensity of infrasonic waves generated by the
explosions correlated well with the number of explosions.

The number of explosions gradually increased from July
2009, reaching a peak during the period from December
2009 to March 2010, and decreased toward October 2010.
The amplitude of the infrasonic waves exceeded 200 Pa for
8 explosions and 150 Pa for 39 explosions during theactive period. A correspondence between the large amplitudes of infrasonic waves and the frequent occurrence of
explosions appeared during the period from January to
May 2011 and in the last quarter of 2011. In contrast, theamplitudes of infrasonic waves were less than 40 Pa in
October 2010 and June 2011, when the monthly number of
explosions was only 15 and 27, respectively.
5．Long-term ground deformation in 2006-2011
The transience in eruptive activity at the Showa crater
correlates to ground deformation. In order to clarify theground deformation related to the eruptive activity during
the period from 2006 to 2011, we examined the baselines
measured by GPS as well as the changes in tilt and strain.
Characteristic ground deformation was observed associated with an increase and decrease in eruptive activity.
5-1 Temporal changes in ground deformation
Fig. 6 shows the temporal change in the horizontal
lengths of the baselines measured by GPS during the period
from 2006 to 2011. No significant co-eruptive ground deformation was detected with the resumption of eruptive
activity of Showa crater in June 2006, while a minimalchange continued until September 2009.
Significant ground deformation began in October 2009
when the number of explosions and the ejected weight of
volcanic ash increased. The horizontal distances of thebaselines in the north-south direction showed an extension
during the period from October 2009 to April 2010. When
more than 130 explosions occurred during the period from
December 2009 to March 2010, the extension of baseline

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011
Table 1.

121

Extension-contraction events during Showa crater activity.

Fig. 7. Temporal change in strain in the Harutayama
underground tunnel (HVOT). EX-R and EX-T indicate
strain changes in the radial direction from the craterand tangential to it, respectively. The strains arecompared with temporal change of monthly numbers(middle) and monthly weight of volcanic ash (bottom).

in the east-west direction became clear. The east-westbaseline showed an extension of 1 cm and the baseline in
the north-south direction was extended by 2 cm during theperiod from October 2009 to April 2010. The east-westbaseline showed a disturbance of up to 1 cm after April
2010, while the north-south baseline repeated a cycle ofcontractions and extensions. This turned into a contractionin May 2010 and showed a clear extension in November
2010. The extension turned into a contraction again in
April 2011, followed by a restart of an extension in August
2011. The change in baseline ranged from 1 to 2 cm forboth the extension and contraction episodes. When extension of the north-south baseline was observed during theperiods from November 2010 to April 2011 and from
August to October 2011, the monthly number of explosions and the ejection of volcanic ash showed a tendency
to increase or stayed at a high level (Fig. 6). In contrast,
contractions in the north-south baseline corresponded to adecrease in or low-level activity during the periods from
April to July 2010 and from April to July 2011.
An extension-contraction pattern similar to the change
in the baseline in the north-south direction was noted instrain records at Station HVOT, as shown in Fig. 7. It canbe seen that the extensions and contractions repeatedalternatively. The extension-contraction events are summarized in Table 1. The cycle of repeated extensions andcontractions lasted about 12 months. The extensioncontraction event during the period from September 2009
to October 2010 was the largest among the 5 events. Thechange in inflationary strain was 1.2×10−6 in the radial
component and 0.7×10−6 in the tangential component.
The second-largest event, from November 2010 to August
2011, had large contraction strains of −0.9×10−6 in theradial and tangential components. These two events
corresponded to the extension-contraction episodes in thenorth-south baselines measured by GPS. Rapid extension
strains were observed during the period from December
2009 to March 2010 when more than 130 explosions
occurred, while a rapid contraction strain from August to
October 2010 corresponded to a decrease in eruptive
activity.
In the rest of this section, we describe in more detail thelargest extension-contraction event, which took place during
the period September 2009 to October 2010. Fig. 8 shows
the tilt changes at Stations AVOT, HVOT, HAR and KOM
and the strain changes at Station HVOT. The trend components are reduced by using long-term records (Iguchi etal., 2011). An upward tilt of the crater side began on
September 12, 2009 (Inf1 in Fig. 8) and the rate of the
upward tilt became rapid at 08 : 00 on September 30, 2009
(Inf2 in Fig. 8). The rapid tilt change continued until theoccurrence of an explosion at the Minamidake crater at 16:
45 on October 3, 2009. The scale of the explosion was
evaluated as being rather large for explosions at the
Minamidake crater, because the volcanic ash cloud reached
3000 m above the crater and the amplitude of the air shock
was 837 Pa at Station HAR. However, only one explosion
at Minamidake occurred during the extension-contraction

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

Fig. 8. Temporal changes in tilt at Stations AVOT,
HVOT, HAR and KOM. The bottom graph shows
strain changes at HVOT. Extensions “-R” and “-T”
indicate tilt or strain changes in the radial direction to
the crater and tangential to it, respectively. (b) Enlargement of tilt and strain changes at Station AVOT
around Inf2 in (a). Radial tilt is indicated by “WT-R,”“EX-R” and “EX-T” show strain changes in the radial
direction from the crater and tangential to it, respectively. Downward arrows indicate explosions (black:
Minamidake event, grey: Showa crater event). Upward
arrow indicates start of inflation. Disturbance immediately before was caused by the earthquake at Samoa
Islands.

event, and the occurrence of the Minamidake event seemed
to have triggered the subsequent continuous upward tilt.
A chronology of tilt changes is presented below basedon tilt changes at Station AVOT. After the explosion, achange in upward tilt continued at a rate of 6 × 10−9

rad/day and the tilt rate increased to 9 × 10−9 rad/day
during the period from December 2009 to March 2010.
After a period of stable tilt from April to June 8, 2010 anda short-term rapid change in tilt upward from June 9 to 13,
2010, the tilt turned downward at the crater side on June15, 2010. The downward tilt was quite rapid from 21 : 00
to 8 : 00 on the next day and it attained −210×10−9 rad(Def1 in Fig. 8). From the middle of July 2010, the tilt
showed a long-term trend downward on the crater side
(Def2 in Fig. 8). The downward tilt rate was rapid (−51×10−9 rad/day) during the period from 17 to 29 July anddecreased stepwise (− 13 × 10−9 rad/day in the period
July 29 to September 17 and −5.4×10−9 rad/day in the
period September 29 to November 15).
Similar patterns were observed in the strain records at
Station HVOT and for tilt changes at Stations HVOT,
HAR and KOM (Fig. 8). The extension strain at Station
HVOT and the upward tilt of the crater side at Stations
HVOT, HAR and KOM started in September 2009, andthe contraction strain and the downward tilt finished in
October or November 2010, although the termination of
deflation was not clear due to the gradual decrease in the
deformation rates. A higher rate of deformation was notedin almost the same period as the tilt record at Station
AVOT. The strain rate was large (6.7×10−9 /day) from
December 2009 to March 2010 in the radial component at
Station HVOT. Short-term changes can be noted in all therecords, such as the inflation event from 08: 00 September
30, 2009 to 16 : 00 on October 3 followed by an explosion
at Minamidake and a rapid deflation event from 21 : 00 on
June 15, 2010 to 08 : 00 on June 16. The transition fromthe inflation to the deflation stage was a little different.
The downward tilt of the crater side became significant in
the middle of July 2010 at Station AVOT (Def2 in Fig. 8);
however, the contraction strain and the downward tilt
clearly started in the middle of June 2010 at Stations
HVOT, HAR and KOM (Def1 in Fig. 8), one month earlier
than at Station AVOT.
5-2 Location and volume change of pressure sources
To estimate the location of the pressure sources inducing
the largest ground deformation during the period from
September/October 2009 to October 2010, horizontal displacements as measured by GPS were analyzed. The spatial
distribution of the horizontal displacements in the inflation
stage is shown in Fig. 9. The displacements are based oncontinuous GPS observations and differences in the two
campaigns at the end of November 2009 (Iguchi et al.,2010) and early April 2010. The horizontal displacements
obtained are from Station ONEG (39 km southeast from
Minamidake) on the Osumi Peninsula. Takayama andYoshida (2007) estimated background maximum strain
rate in South Kyushu to be 8.1×10−8 strain/year to thedirection of N147° E. Taking relative locations of Station
ONEG and Sakurajima into account, influence of background extension is inferred to be 〜1 mm between the twoCharacteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011123

Fig. 9. Vectors of horizontal displacement around the Aira caldera (top) and at Sakurajima (bottom). Horizontal
displacement vectors are shown by arrows, relative to Station ONEG. Top: Central cones of Sakurajima are indicated by“S.” The dot indicates pressure source. Bottom: Black arrows are observed horizontal displacements and gray ones are
obtained by assuming two pressure source “A” and “B.” Letters “M,” “K” and “S” indicate craters Minamidake,
Kitadake and Showa, respectively.

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

campaigns. The horizontal displacement vectors show an
outward radial pattern from the center of the Aira caldera.
At Sakurajima, a characteristic pattern can be observed.
Displacements at the northern part are <0.5 cm and thedisplacements become larger at the southern part, showing
clear southward vectors. The movements in the eastern
and western parts show southeast the southwest directions,
respectively. The pattern of horizontal displacements canbe approximated by an inflation source in the Aira caldera
and beneath Sakurajima. A dual-source model was applied
to the inflation stage prior to activity peaking at Minamidake (Yoshikawa, 1961) and to the deflation stage in the
1980s (Eto, 1988). Assuming two sources of the Mogi
type (Mogi, 1958), the locations and changes in volume of
the two sources were determined by applying a grid searchmethod. The major source was obtained at a depth of 12
km beneath the center of the Aira caldera (Source A in Fig.
9) and the increase in volume of the source was estimatedto be 8×106 m3 during the period from November 2009 to
April 2010. The additional source with increase in volume
of 4×105 m3 was obtained at a depth of 5 km beneath thenorth flank of Kitadake, as indicated by Source B in Fig. 9.
The location of Source B as obtained by GPS observation is supported by the tilt vectors on the flanks of the
volcano. The tilt vectors at Stations AVOT, HAR and
KOM during the same inflation stage are shown in Fig. 10.
A water-tube tiltmeter in the underground tunnel (HVOT)
was not operated well during the period (Fig. 8). It can beseen that the tilt vectors intersect each other at the summit
area of the Minamidake crater (S in Fig. 10) and thenorthern flank of Kitadake (N). The intersect S coincides
the pressure source beneath Minamidake, which has beenpreviously known and correspond to a magma reservoir at
depths 3-6 km (Ishihara, 1988; 1990). Source B obtainedby GPS observation corresponds to the intersect N of thetilt vectors.
Amount of the tilt changes at these stations induced by
Source B are examined based on Mogi model (Mogi,
1958). According to Ishihara (1990), the tilt change (d) ata horizontal distance r caused by a spherical pressuresource a radius (a) at depth D is expressed as follows:
δ=k

3Dr
52 ,
 p 2+r 2

(1)

where the parameter k is written in terms of Lameʼ sconstants, λ and μ, and the pressure change Δp,k=

λ+2μa 3 Δp.
2μλ+μ(2)Based on the Mogi model, volume increase of Source B
by 4×105 m3 induces tilt changes of 1.1, 0.4 and 1.0×
10−6 rad at Stations KOM, AVOT and HAR, respectively.
Observed tilt change at Station KOM during the period(0.9×10−6 rad) is almost same to our estimation while theobserved tilt changes at Stations AVOT and HAR, 1.1 andFig. 10. Tilt vectors in the deflation period from June toNovember 2010. “N” and “S” are points where the tilt
vectors intersect each other.

0.5×10−6 rad are twice and half of the calculated values,
respectively. The difference at Station AVOT may beaffected by another pressure source at the intersect S in
Fig. 10.
6．Ground deformation associated with individual explosions
Each explosion at the Showa crater was associated withstrain changes and sometimes with tilt changes, similar to
the explosions at Minamidake, which showed an upwardtilt of the crater side and extensions in both radial andtangential strain 10 min to 7 h prior to the start of the
explosions, and the deflationary tilt and strain associatedwith them (Kamo and Ishihara, 1989). However, the grounddeformations associated with the explosions at the Showa
crater are different from those of Minamidakeʼs origin in
their time series and source locations. Similarities to anddifferences from the ground deformation associated with
the Minamidake events are useful in extracting characteristics of eruptive activity at the Showa crater in addition to
long-term changes.
Fig. 11 shows an example of the records of tilt and strainchanges at Station AVOT on December 20, 2009. The
Sakurajima volcano is surrounded by the sea, and thus thetilt and strain records are strongly affected by ocean tides.
At the largest, the strain changes caused by the tidal levelchange attain a value of 7×10−7 in the radial component
at Station AVOT. We corrected the strain recordsby removing the effects of ocean tide by using the hybrid
method of BAYTAP-G (Tamura et al., 1991) and the observed tidal level (Fig. 11). The hybrid method is effective
in removing unexpected changes in tidal levels, such as
secondary undulation, tsunami, and high tidal levels
resulting from typhoons.

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011125

Fig. 11. An example of tilt and strain records associated with explosions at the Showa crater. Top: Amplitude of air shock toindicate occurrence of explosions. Bottom: Tilt and strain changes at Station AVOT. Radial tilt is indicated by “WT-R,”“EX-R” and “EX-T” show strain changes in the radial direction from the crater and tangential to it, respectively.

6-1 Typical inflation
Prior to an explosion, the contraction and extension
strains are recorded in the radial and tangential components of the extensometers, respectively (Fig. 11). Whenan explosion occurs, the changes in strain are extensions
and contractions in the radial and tangential components,
respectively. Among 578 explosions in 2009 at the Showa
crater, radial strain changes with amplitude of larger than
10×10−9 were detected prior to 340 explosions at Station
AVOT. Such strain changes prior to the explosions lasted
for less than 1 h for 209 events, and durations of 1-2 h
were recorded for 71 events and 2-3 h for 27 events. On
the other hand, precursory tilt changes with amplitudes
larger than 20 × 10−9 rad were detected only for 31
explosions including the last event in Fig. 11.
The inverted polarities between the radial and tangential
components of strain and the smaller change in the tilt arelikely caused by the shallowness of the pressure source.
Based on Mogiʼs model (Mogi, 1958), the radial strain (εr )and the tangential strain (εt ) at the horizontal distance r,
caused by the pressure change (Δp) of a small sphere witha radius (a) at depth D are written as follows (Ishihara,
1990):
p 2−2r 2εr=k 2 2 52p +r εt=k1
32
p 2+r 2

.

(3)

As estimated from Eqs. (1) to (3), the radial andtangential strains induced by a source at depth zero are−2kr 3 and kr 3, respectively, otherwise the tilt change
becomes zero. Defining β as the ratio of the radial totangential strain, it is written in terms of horizontal

Fig. 12. Deflation volumes and depths of pressure sources.
Gray and white circles indicate explosions at the Showa
and Minamidake craters, respectively.

distance and source depth as:εr p 2−2r 2β≡ = 2 2 .
εt
p +r

(4)

Then, the source depth is obtained as follows:
r.
 2+β
1−β

p=

(5)

Applying Eq. (5) to the strain changes associated with
340 explosions at the Showa crater in 2009, possible
source depths and volume changes derived from theparameter k are obtained as shown in Fig. 12. The source

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depths are obtained at a range from 0 to 1.5 km and thevalues of the parameter k are less than 105 m3. Although
the estimated values contain large uncertainties due tounknown source size compared with the source depth andcomplexity of volcanic topography, especially near thecrater, the contraction in the radial strain and extension in
the tangential strain indicate a shallow depth for thepressure source associated with explosions at the Showa
crater. For inconsistent polarities between the radial andtangential components, the source depth should be shallower than  2 r (approximately 3 km for the threshold at
Station AVOT), considering Eq. (3) under the assumptionof a sufficiently small source. In many explosions, nosignificant changes were detected by the sensors at Station
HVOT at a further distance (3.4 km) from the crater. The
pressure sources associated with explosions at Minamidake were obtained at depths of 2-6 km (Ishihara, 1990)
and 4 km (Tateo and Iguchi, 2009) beneath the crater,where consistent-polarity strain changes in both the radial
and tangential components were observed (Ishihara, 1990).
In addition, significant tilt changes were observed prior to
explosions at Minamidake (Ishihara, 1990). However,
only trivial tilt changes are detected prior to the mostevents at the Showa crater. These facts indicate a shallower depth for the pressure source associated with
explosions at the Showa crater than those at the Minamidake crater.
6-2 Longer and larger inflation
Inflation deformation prior to the explosions at theShowa crater sometimes continued longer than Minamidakeʼs explosions, which were preceded by inflation with aduration of up to 7 h (Kamo and Ishihara, 1989). The
explosions in May 2008 had precursory inflations withdurations from several to 20 h (Fig. 13a). Despite small
number (9 events among 340 in 2009) of explosions,inflations with a long duration of >7 h were followed bylarger explosions than usual ones at the Showa crater. Tilt
and strain changes are also different from the usual ones:
long duration makes a large strain changes, as theamplitude of Station AVOT attained more than 40×10−9.
In addition, upward tilts of the crater side were observed at
Station AVOT, and tilt and strain changes were also
detected at Station HVOT. Fig. 14 shows a typical example of tilt and strain records associated with such an
explosion with long duration and large amplitude. The
explosion started at 15 : 29 on April 9, 2009, producing apyroclastic flow up to distance of 1.1 km, and an ash cloudrose 4000 m above the crater. The radial tilt at Station
AVOT began to show a crater-side-up motion at 10 : 00 on
April 8, 29 hours prior to the explosion, and contractionand extension strains started simultaneously in the radial
and tangential components, respectively. The inverted
polarities between the radial and tangential components
indicate that the inflation was induced at a shallow depth
beneath the crater. The tiltmeters showed a crater side up

Fig. 13. Tilt and strain records associated with explosions
at the Showa crater. The records were obtained at
Station AVOT. Dashed vertical lines indicate occurrences of explosive eruptions. Temporal change inradial tilt is indicated by “WT-R.” “EX-R” and “EXT” show strain changes in the radial direction from thecrater and tangential to it, respectively.

motion, and extensions appeared in both the radial andtangential strains simultaneously at Station HVOT. However, the consistent extensional polarity between the radial
and tangential stains cannot be explained by the shallow
source. The source generating consistent polarities ofradial and tangential strains must be deeper than  2 r from
Eq. (3). It was inferred that the strains recorded at Station
AVOT were caused by a shallow source, and that the
deformations at Station HVOT and the tilt at Station
AVOT were mainly induced by a source deeper than  2 r .
This indicates that the ground deformation prior to such alarge explosion was induced by the shallow and deep
sources. The smaller change in the radial strain thantangential one at AVOT may be due to an addition of adeep extension source to a shallow contraction one.
Otherwise, tangential strain was enhanced by an additionof extensions at both a deep and a shallow depth.
Next, a dual-Mogi source was applied to the tilt andCharacteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011

127

Fig. 14. Tilt and strain records before and after the explosion at 15: 29 on April 9, 2009. Top: AVOT, bottom: HVOT.
Radial tilt is indicated by “WT-R.” “EX-R” and “EX-T” show strain changes in the radial direction from the crater andtangential to it, respectively.

strain data at Station AVOT and Station HVOT shown in
Fig. 14 to obtain the temporal changes of parameters k of
the two sources. It is difficult to adequately constrain thedepths of the sources in each time interval. Hence, we
fixed a deep source at a depth of 4.8 km from the ratio of
the areal strain to tilt changes observed at Station HVOT
from the beginning of inflation to the explosion, and ashallow one at a depth of 1. 0 km from the records at
Station AVOT from Eq. (5). Hourly changes of the source
volumes were estimated by minimizing the residuals
between the theoretical tilt and strain changes calculated
from the dual-Mogi sources and the observed values of 6components: 2 tilts, 2 radial, and 2 tangential strains at
Stations AVOT and HVOT. The results are shown in Fig.
15. An increase in volume at the deep source started at
10 : 00 to 11 : 00, and the volume increase in the shallow
source started 3 h later. However, the rate declined on
April 9. In contrast, the increase in the volume rate for the
deep source increased stepwise. In particular, the rateincreased rapidly immediately prior to the explosion. The
volume change for the modeled deep source was 40 times
larger than that for the shallow source. Small explosions
are related to only shallow sources, however, larger
explosions are contributed to by deep sources with large
volume changes, in addition to shallow ones.
6-3 Time-series patterns of inflation
The time-series patterns of inflation associated with
explosions at Showa crater are different from those at the
Minamidake crater, where the inflation rates are nearly
constant (Ishihara, 1990; Tateo and Iguchi, 2009). The
inflation rate of explosions at the Showa crater seems to

Fig. 15. Temporal change in estimated volume of deepand shallow sources. Calculated hourly tilt and strainchanges are compared with observation (bottom).

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

Fig. 16. Strain records associated with explosions at theShowa crater. The records were obtained at Station
AVOT. Beginning of explosion (a) and eruption (b) areindicated by “E.” “MC” indicates the start of minor
contraction prior to the eruption. “EX-R” and “EX-T”
show strain changes in the radial direction from thecrater and tangential to it, respectively.

Fig. 17. Tilt and strain records before and after the explosion at 11: 25 on February 6, 2008. Top: AVOT,
bottom: HVOT. Radial tilt is indicated by “WT-R.”“EX-R” and “EX-T” show strain changes in the radial
direction from the crater and tangential to it, respectively.

decrease near the onset of an explosion, and inflation
sometimes ceases as shown in Fig. 13b. Among 340 explosions in 2009, 209 events were preceded by inflation
with such decreasing rates. Otherwise, deformations ofconstant inflation rates were observed for 124 events andaccelerating patterns were only 16. The decreasing patternof the inflation rate or the suspension of inflation and alonger duration of precursory inflation are similar to thoseprior to non-explosive eruptions at the Minamidake crater(Ishihara, 1990) or inflation prior to swarms of BL-type
earthquakes (Tateo and Iguchi, 2009).
Iguchi et al. (2008c) detected minor deflations immediately prior to the occurrence of explosions at Sakurajima,
Suwanosejima and Semeru volcanoes. A minor deflation
appears as a contraction in the strain record at Station
HVOT 1-2 minutes prior to an explosion at the Minamidake crater. Some explosions at the Showa crater also occur
immediately after minor contractions. Among 209 explosions of which precursory inflation rate decreased approaching onset of explosions, 143 events were precededby minor contraction 1 to 15 minutes before. Fig. 16a showsan enlargement of strain records prior to the occurrence of
two explosions at the Showa crater. An extension of strain
started at 17 : 00 on November 17, 2009 and an explosion
occurred at 21 : 14. The tangential strain began to show

contraction at 20 : 59, 15 minutes prior to the explosion.
The radial strain showed a slight extension later. This
indicates that the deflation source immediately prior to the
explosion is located at a shallow depth (maybe <1 km). A
second example is shown in Fig. 16b. Although the eventat 04 : 23 on November 13, 2009 is not defined as an
explosion based on the definition in this study because theamplitude of the infrasonic wave was only 6 Pa, it was
accompanied by fairly large ground deformation. The
extensional strain became a minor contraction at 04: 12, 11
minutes prior to the occurrence of the eruption, and thestrain changes were <5×10−9. The duration of minor
contraction was longer than that for Minamidake (1-2 min;Iguchi et al., 2008c).
6-4 Upward tilt after explosion
Some explosions show an upward tilt of the crater side
immediately after their onset. Among 340 explosions in
2009, 135 events were followed by such upward tilts. Fig.
17 shows tilt and strain records before and after an
explosion at 11 : 25 on February 6, 2008. The explosion
accompanied a pyroclastic flow that flowed downhill
eastward, reaching a distance of 1.3 km. Prior to the occurrence of the explosion, records at Station AVOT show
an upward tilt of the crater side, a contraction strain in theradial component, and an extension strain in the tangential

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011

component. An upward tilt prior to the explosion also was
observed at Station HVOT. After the beginning of the
explosion, the upward tilt accelerated, in contrast to thedownward tilt associated with explosions at the Minamidake crater (Ishihara, 1990). The upward tilt turned downward 10 minutes after the beginning of the explosion. The
observed simultaneous extension in radial strain and thecontraction in tangential strain after the beginning of the
explosion were caused by a deflation of a Mogi source in ashallow area beneath the crater. However, the upward tiltof the crater side was induced by inflation alone - independent of source depth.
The inconsistent ground deformation between tilt andstrain can be explained by a dual-Mogi source; that is, it iscomposed of shallow deflation and deeper inflation sources.
Shallow source contributes to strain mainly, whereas adeeper source influences tilt, as inferred from Eqs. (1)-(3).
A dual source possibly is applied to the tilts and strain dataat Stations HVOT and AVOT. Shallow deflation and deep
inflation sources could be obtained at depths of 60 m and
640 m, respectively, for the explosion on February 6, 2008.
The shallow source is constrained at shallower than 200 m,
however the deep source has a larger range of ambiguity
from 150 m to 1650 m.
7．Discussion
Associated with eruptive activity at the Show crater
since 2006, quasi-annual cycles of inflation-deflation wereobserved. The inflation ground deformation during theperiod from October 2009 to March 2010, the largest
event, is caused by a dual-source located at the center ofthe Aira caldera and the northern flank of Kitadake.
Ground deformation associated with individual explosions
is caused by shallow sources. Durations of inflations prior
to the explosions are mostly less than 1 h, but sometimes
are much longer than those of Minamidake event.
Temporal changes of inflation strain show decelerating
patterns with time and inflationary tilt immediately after
explosions. We will discuss the characteristics of thelong-term and short-term deformations with relation to themagma supply system and eruptive activity.
7-1 Magma plumbing system from the Aira caldera
to Sakurajima
Previous studies mainly based on seismic and grounddeformation observations (Mogi, 1958; Yoshikawa, 1961;
Kamo, 1988; Ishihara, 1988; 1990; Eto et al., 1997;
Hidayati et al., 2007) show that magma plumbing systemof the Sakurajima mainly consists of the main magma
reservoir located at a depth of 10 km beneath the center ofthe Aira caldera, the minor magma reservoir located at a
depth of from 3 to 6 km beneath Minamidake and the
conduit connecting the minor reservoir to the crater bottomof Minamidake.
The inflation that started in September/October 2009
was the largest ground deformation since the beginning of

129

eruptive activity at Showa crater in June 2006. Horizontal
displacements detected by GPS observations showed thatthe deformation was mainly caused by inflation of thepressure source at a depth of 〜10 km beneath the Airacaldera. The pressure source occupies almost the same
location as those during the inflation stage of the Airacaldera (Yoshikawa, 1961), during the pre-eruptive periodof the violent activity at the Minamidake crater before
1974 (Ishihara and Eto, 1978) and before the eruptive
activity beginning from 2006 at the Showa crater (Eto etal., 1997; 1998a; Iguchi et al., 2008b; Yamamoto et al.,2008). This consistency suggests that the magma has been
accumulated at the same position for the period when the
Minamidake was active and after the eruptive activity thatstarted at the Showa crater.
An additional pressure source in the northern part of
Sakurajima was obtained by the GPS observation (Fig. 9),
which is supported by tilt vectors (Fig. 10). Inflation
produced by a similar source in the northern part was
detected by precise leveling and tide gauge observation
prior to the increase in explosive activity at the Minamidake crater in 1974 during which 489 explosions occurred(Ishihara and Eto, 1978). Although the source was
supported only by the leveling survey near the north coastat the time of Ishihara and Eto (1978), the inflation sourceswere detected at the northern part prior to the increase inviolent eruptive activity at Minamidake and associatedwith increase in the eruptivity during the period from
October 2009 to March 2010. Kitadake and its surroundings are known to be anomalous zones of attenuation of seismic waves in addition to the Minamidake
area (Kamo et al., 1980; Suzuki, 1999). It is possible thatthe northern part of Sakurajima serves as a temporarystorage area for magma when the magma migrates fromthe Aira caldera to Sakurajima. The magma supply systemfrom the Aira caldera to a site beneath the central cones
has not, to date, been changed by the current eruptive
activity at the Showa crater. Therefore, two magma
storages beneath the central cones -Kitadake and Minamidake- are inferred.
7-2 Reservoir-conduit system for eruptions at Showa
crater
The deformation associated with explosions at the Showa
crater can be explained by a dual source: a shallow source
at a depth <1.5 km, and a deep one at a depth of around 4
km. The sources are related with a reservoir-conduit
model beneath the Minamidake summit crater proposed by
Ishihara (1988; 1990). The magma reservoir occupies aregion at depths of 3-6 km and the reservoir is connectedto the crater bottom of Minamidake by the conduit with thediameter of 0.3-0.5 km at a depth of 2 km and 40-60 m inthe uppermost part. The depths of the deep sources
inducing larger pre-eruptive deformation of the Showa
crater are almost the same as the depths of pressure sources
inducing ground deformation associated with explosions at

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

Fig. 18. Schematic figure of the conduit system for eruptive activity of the Showa crater. a) Eruptive period at the
Minamidake crater, b) Eruptive period at the Showa crater since 2006. Hypocenters of A-type earthquakes are indicatedby asterisks, BH-type by dots, BL-type by open circles and the zone of explosion earthquake by the dashed rectangle.

the Minamidake crater (Ishihara, 1990; Tateo and Iguchi,2009). This indicates that a deep reservoir is used for the
eruptive activities at both the Minamidake and the Showa
craters. It is inferred that the conduit that connects to theShowa crater extends from a reservoir independently of the
major conduit to Minamidake, or it may branch from the
major conduit at a depth. The shallow source for grounddeformation related to explosions at the Showa crater may
be located in the branched conduit. The change in strain at
Station AVOT induced by the shallow source is 4 times
larger than that at Station HVOT. Considering thedifference in distances (HVOT: 3. 2 km, AVOT: 2. 1 km
from the Showa crater, HVOT: 2. 7 km, AVOT: 2. 4 km
from the Minamidake crater), it is more reasonable to inferthat the shallow source is located in the branched conduit
beneath the Showa crater rather than in the major conduit
below Minamidake. The hypocenter of the explosion

earthquake on February 3, 2008 was located 2 km beneaththe Minamidake crater. It is, therefore, inferred that the
conduit connected to the Showa crater branches at a depthof 1-2 km from the major conduit to Minamidake (Fig.
18).
7-3 Magma intrusion rate and a smaller conduit toShowa crater
If magma migrated to the Showa crater by the same
supply system during the eruptive activities at Minamidake -from the Aira caldera to Minamidake throughKitadake- a more frequent occurrence of smaller explosions at the Showa crater might result from the branched
conduit from the magma reservoir beneath the Minamidake crater or in the middle of the major conduit to thecrater. The difference of the branched conduit to theShowa crater from the major conduit to the Minamidake
crater can be discussed in the relation of long-term groundCharacteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011131

Fig. 19. Relation of monthly change in volume of pressure source to volcanic ash ejected. Top: monthly change in volume
of pressure source estimated from radial strain change at HVOT. Middle: monthly weight in volcanic ash ejected fromthe crater. Bottom: monthly volume of magma supply.

deformation.
After the beginning of the eruptive activity in 2006, thestrain data show quasi-annual cycles of extension andcontraction (Table 1 and Fig. 7). Although annual cycles
of strain changes have been discussed with precipitationand temperature change in underground tunnels (Teraishi
et al., 2009) and atmospheric pressure and ocean tides
could affect crustal deformation (Ohtake and Nakahara,
1999; Tsuruoka et al., 1995), the inflation-deflation processduring the period from October 2009 to October 2010 was
inferred to be directly caused by the intrusion anddischarge of magma for the following reasons: (1) The
strain change was larger than the previous annual cycles
and attained a sufficiently high level to be detected by thebaseline changes measured by GPS (Fig. 6). (2) After thestart of extension strain in October 2009, the number of
explosions and the weight of volcanic ash emitted
increased (Fig. 7).
In order to discuss the relationship of ground deformation with eruptive activity more quantitatively, the monthly
change in the volume of the pressure source was comparedwith the monthly amount of volcanic ash ejected from
Sakurajima. The volume change at the ground surface(ΔV) was obtained by integrating the vertical displacement for distance from zero to infinity. Using the expression of radial strain in Eq. (1), it is described as follows:
52

ΔV=

2πεrp 2+r 2
.
p 2−2r 2(6)

Considering the relation of volume change between the
ground surface and at the source (Delaney and McTigue,1994), the volume change of the pressure source isrepresented by:
52

ΔV p=

πεrp 2+r 2,1−νp 2−2r 2

(7)

where ν is Poissonʼs ratio. Here, a pressure source isassumed to be located at a depth of 4 km beneath Minamidake. In addition, we estimate the intrusion volume ofmagma in the reservoir by adding the eruption volume ofmagma to the inflation volume of the pressure source atthat depth. It is necessary to consider the volatility of themagma in order to estimate the volume of erupted magma;
however it is difficult to see the contribution of this volatility and estimate the influence on the bulk modulus ofinternal magma. Therefore, we suppose that the weight of
volcanic ash is directly related to the magma volume at thedepth of the reservoir by a DRE (Dense Rock Equivalent)density of 2500 kg/m3. Obtained monthly volumes of
intrusion magma are shown in Fig. 19 with volume of thepressure source and amount of volcanic ash.
It was observed that eruptive activity and pressuresource volume increased when the magma intruded at ahigher rate, and the eruptive activity and source volume
decreased when the intrusion rate became low or negative.
The monthly changes in volume became positive (2×105
m3 /month) in October 2009 and the positive volume
changes continued until May 2010 except November 2009.

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Masato IGUCHI, Takeshi TAMEGURI, Yusaku OHTA, Sadato UEKI and Shigeru NAKAO

When the volume changes were positive values, volcanic
ash of more than 5×105 ton/month was ejected. During
the high eruptivity and inflation period, the monthly
intrusion rate increased, exceeding 6×105 m3/month (December 2009 to March 2010). In contrast, the monthly
volume change became negative during the period from
July to November 2010, and this deflation period
corresponded to the ejection of smaller amounts of
volcanic ash (<5×105 ton/month̶except for July 2010).
The negative volume change implies a decrease in themagma in the reservoir. Considering the flux balance inthe reservoir between intrusion and discharge, the discharge overcomes the intrusion. The intrusion rate became negative during the period from July to October2010. The monthly weight of volcanic ash as an indicatorof magma discharge was smaller during the deflation
period. This indicates that the intrusion rates in the
deflation period were much less than those in the inflation
period̶possibly almost stopped̶and that the magma
accumulated in the inflation stage was ejected.
Accumulations and discharges of the magma simultaneously progressed beneath Sakurajima during the inflation
stages from October 2009 to May 2010, December 2010 to
February 2011 and after November 2011. Such a simultaneous occurrence of an accumulation of magma and adischarge may be caused by a small open conduit connected to the Showa crater and a slightly larger intrusion ofmagma. Intruded magma is mostly accumulated inreservoirs due to the completely closed conduit during thedormant phase with inflation of volcanoes; however a partof the magma that intruded into the reservoir is discharged
by eruptions at volcanoes with partially open conduits.
Aperture ratio for the reservoir in the eruptive activity at
the Showa crater may be much smaller than that at the
Minamidake crater. During the high eruptivity of Minamidake from 1972 to 1992, the northern part of Sakurajima
continued to subside, resulting in a deflation of the volcano(Eto et al., 1998a). The vulcanian eruptions at Minamidake were large, and 2.4×108 tons of volcanic ash wereejected during the period. The diameter of the major
conduit connected to the bottom of the Minamidake crateris estimated to be 0.3-0.5 km at a depth of 2 km, and it is40-60 m in the uppermost part from the size of the red-hot
portion of a lava dome at the bottom of the crater (Ishihara,
1990). A conduit of this size could discharge a large
amount of volcanic ash. On the other hand, the size of theuppermost part of the conduit connected to the Showa
crater is estimated to be only 5-10 m, based on aerial
photographs taken on May 31, 2011 by the Kagoshima
Local Meteorological Observatory of the Japan Meteorological Agency. Conduit size may control the balance of
intrusion and discharge of magma. The smaller size of the
conduit of the Showa crater is related to the characteristics
of ground deformation differently than from the explosions
at Minamidake. One is a slow, long-duration inflation

prior to explosions, while the inflation duration prior to
explosions at Minamidake is less than 7 h (Kamo andIshihara, 1989). Inflation strain prior to explosions at theShowa crater sometimes continues for more than 20 hours.
The explosion on April 9, 2009 was preceded by inflationfor 29 hours (Fig. 14). The tilt and areal strain rates prior
to the explosion were only 4×10−9 rad/h and 2×10−9 /h
at Station HVOT, respectively, which is much smaller than
the tilt rate (20-90 × 10−9 rad/h) and areal strain rates(10-50 × 10−9 /h) at the same station prior to the
explosions at Minamidake (Tateo and Iguchi, 2009). The
longer durations of minor contraction immediately prior to
explosions are noted, as shown in Fig. 16. Some explosions at the Showa crater are accompanied by a temporary
upward tilt of the crater side immediately after the onset ofthe explosions, as shown in Fig. 17. The explosions at
Minamidake have never resulted in a temporary upwardtilt after the onset of the explosions, and they are accompanied by a downward tilt only. To explain this, theinflation source needs to be located at a depth that isdeeper than the deflation source, as mentioned in Section
6-4. A pressure decrease caused by the collapse of a gas
pocket in the uppermost conduit induces a sudden degassing in the magma below, and this sudden degassing
causes a volume increase at depths in the conduit. The
volume increase in a large conduit may be observed as an
instantaneous expansion, such as an isotropic expansion of
2 km deep generating P-wave first motions of explosion
earthquakes at Minamidake (Tameguri et al., 2002), andthe expansion is rapidly eliminated by an upward movingof fluid through the large conduit. In the smaller conduit
connected to the Showa crater, it takes a longer time torelease the temporary expansion.
8．Conclusions and remarks
The major magma reservoir beneath the center of the
Aira caldera has continued to inflate since 1993. Eruptive
activity at the Showa crater of the Sakurajima volcano
resumed in June 2006 during a caldera-inflation stage.
The magma moves from the Aira caldera, passing throughan initial storage space below Kitadake and its northernflank, and reaches the magma reservoir beneath Minamidake. The Showa crater is a vent of a small conduit
branched from the reservoir or from the major conduit to
Minamidake. The ground deformation associated with
explosions at the Showa crater is mainly induced by ashallow source in the branched conduit in addition to a
deep source corresponding to the magma reservoir below
Minamidake.
Tilt and strain showed a rapid inflation in September
2009, exceeding the previous annual cycles, and the inflation, which continued until May 2010, was accompanied by the increase in eruptive activity at the crater. An
accumulation of magma beneath the central cones of the
Sakurajima progressed simultaneously with the discharge

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011of magma. The simultaneous progress of accumulationand discharge of magma and the frequent occurrence ofsmall explosions may be related to the small open conduit,
which is inferred from the characteristic patterns of grounddeformation.
However, the conclusions mentioned above are only
relevant factors during the period from 2006 to 2011, andmay not be essential to the overall activity of the Showa
crater, which can only be evaluated after termination of theactivity, because the activity is still ongoing and has not
culminated yet, suggesting that a drastic change of activity
in the near future is possible.
Acknowledgements
Volcano observations at Sakurajima volcano are maintained by staffers of SVRC, DPRI of Kyoto University.
The study is supported by the Project of Earthquakes andVolcanic Eruption Prediction from MEXT. The seismic,
geodetic and infrasonic data at Station AVOT are provided
by the Osumi Office of River and National Highway,
Kyushu Regional Development Bureau, MLIT. The
comments from two anonymous reviewers and Prof.
Nishimura as a guest editor for this Special Issue were very
helpful in revising the manuscript.
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(Editorial handling Takeshi Nishimura)

Characteristics of Volcanic Activity at Sakurajima Volcanoʼs Showa Crater During the Period 2006 to 2011

桜島昭和火口噴火活動期における火山活動の特徴について─ 2006 年〜2011 年─
井口正人・為栗健・太田雄策・植木貞人・中尾茂

2006 年 6 月に再開した桜島の昭和火口における噴火活動は，年を追うごとに活動的となってきており，
2008 年から 2011 年までに 2718 回のブルカノ式噴火が発生した．本稿では主に地盤変動観測に基づいて，
約 5 年間の活動の推移と個々の爆発的噴火の特徴について述べる．GPS 観測により姶良カルデラおよび桜島の地盤は長期的な膨張トレンドを示していること，伸縮計によるひずみ観測により，約 1 年の周期で膨張と収縮を繰り返していることがわかった．2009 年 10 月に始まった隆起・膨張は，2006 年以降の最大の変動であり，2010 年 6 月頃に沈降・収縮に反転した後，2010 年 10 月に沈降・収縮が停止した．2 つの球状圧力源からなるモデルを適用したところ，姶良カルデラ下の深さ 12 km と北岳北麓の深さ 5 km にその位置が求まった．従来から南岳下にマグマ溜りの存在が指摘されており，桜島のマグマ供給系は，姶良カルデラ下の深さ 10 km 付近の主溜りと北岳から南岳にかけての中央火口丘下の深さ 5 km 付近の副溜り群から構成されることがわかった．昭和火口のブルカノ式噴火に先行してひずみ変化が検出され，先行変動継続時間は多くの場合，1 時間以下である．ひずみ変化の特性から圧力源の深さは 1.5 km 以浅と推定される．規模の大きい噴火では先行変動継続時間が 7 時間以上に及ぶことがあるが，その場合は，浅部圧力源に加え，南岳下のマ
グマ溜りに位置する深さ 4 km の圧力源も膨張する．昭和火口に繋がる火道は，南岳下のマグマ溜り，もしくは南岳に繋がる主火道から枝分かれしたものである．地盤変動が急速に進む時期に，噴火活動は活発化した（例えば，2009 年 12 月から 2010 年 3 月）が，このことは桜島の中央火口丘下ではマグマの蓄積と放出が同時に進行していることを意味する．マグマの蓄積と放出の同時進行と小規模ブルカノ式噴火の頻発は，細い開口火道と関係するのであろう．

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