Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 1-18

Review

Magma Movement from the Deep to Shallow Sakurajima Volcano
as Revealed by Geophysical Observations
Masato IGUCHI＊(Received September 20, 2011 ; Accepted October 8, 2012)
Studies on the supply system of magma and its behavior based on seismic and geodetic observations are reviewed
for recent volcanic activity of the Sakurajima volcano, where three characteristic types of eruptions occurred in the 20th
century: the 1914 gigantic flank eruption; the 1946 minor flank eruption; and frequent vulcanian eruptions at the
Minamidake summit crater, continuing from 1955 for more than 50 years. The magma plumbing system of Sakurajima
is approximated by a deep (〜10 km) magma reservoir beneath the Aira caldera, a shallow reservoir (3-6 km) beneaththe central cones, and a conduit connecting the shallow reservoir to the summit crater. From the inflation-deflation
pattern and intensity of the vertical displacement, it is estimated that magma has been supplied to the deep reservoir at a73
rate of 10 m /year. The shallow reservoir is inferred from vertical displacements, a tilt vector showing the crater side
up before the vulcanian eruptions, and an anomalously attenuated zone of seismic waves. The magma migrated fromthe deep reservoir to the shallow reservoir before the increase in vulcanian eruptivity at the Minamidake crater, as
demonstrated by the relocation of the upheaval center of the ground deformation. The increase in seismicity of A-type
earthquakes and the migration of their hypocenters toward the shallow part also support the migration of magma. The
hypocenters of B-type earthquakes and explosion earthquakes with volumetric sources are concentrated beneath thecrater. The hypocenters of A-type earthquakes generated by shear fractures surround the origins of B-type earthquakes
and explosion earthquakes. The separation of the hypocenters indicates the existence of a volcanic conduit connecting
the shallow reservoir to the bottom of the summit crater. The intrusion of magma into the conduit and smooth ascent up
to the crater bottom induce strombolian eruptions and swarms of B-type earthquakes. The magma at the uppermost partof the conduit becomes a cap rock of the conduit against the following intrusive magma, which generates vulcanianeruptions. Upward tilt of the crater side and extensional strain are observed prior to both strombolian activities and thevulcanian activities that follow and turn into downward and contraction strain, respectively, associated with theeruptions. The volatility of the magma has an important role in vulcanian eruptions, in the formation of a gas pocket atthe uppermost part of the conduit, and in sudden outgassing triggered by a pressure decrease in the conduit due to gas
leakage.
Key words : Sakurajima, Aira caldera, Minamidake, magma reservoir, volcanic conduit

1．Introduction
Sakurajima is an andesitic volcano located at thesouthern edge of the Aira caldera and is formed by thecentral cones of the Kitadake (northern peak) and theMinamidake (southern peak) and a number of parasitic
cones on the flanks (Fig. 1). Historically, the eruptive
activity of the Sakurajima volcano is characterized bylarge flank eruptions and vulcanian eruptions at the
Minamidake. The flank eruptions occurred at both sidesof the central cones during the periods of 1471-1476 and1779-1780.
In the 20th century, the Sakurajima volcano was quite
active, and three types/scales of eruptive activities wererecorded. The 1914 eruption was a flank eruption at twosides of the volcano and was the largest in Japan according＊

Sakurajima Volcano Research Center of DPRI, Kyoto Univ.,
Sakurajima-Yokoyama, Kagoshima 891-1419, Japan.

to modern volcano observations, which has a history ofnearly a century. Fortunately, seismological observation
was begun using a Milne-type seismometer installed at
Kagoshima Weather Station in 1888 (Yamasato, 2005).
This was the first seismometer to be installed near a
volcano in Japan. The seismometer recorded precursory
earthquakes to the 1914 eruption and the following
destructive earthquake (M 7.1; Abe, 1981), which occurred
8 hours after the start of the eruption. Also fortunate forresearchers is that the first leveling survey was conductedby the Imperial Military during the period from 1891 to1895 in south Kyushu. A large amount of ground subsidence around south Kyushu was detected, as indicated by
the height difference of the second precise levelingconducted after the eruption (Omori, 1916).
Corresponding author: Masato Iguchi
e-mail: iguchi@svo.dpri.kyoto-u.ac.jp

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Fig. 1. Map of Sakurajima and the Aira caldera. The dots
indicate important benchmarks referred to in the text,
and the inverted triangles indicate tide gauges. The
central cones and craters are K: Kitadake, M: Minamidake, and S: Showa crater. Representative parasitic
cones are indicated by triangles (H: Hikinohira, G:
Gongen-yama, and Kb: Kabano). Cross represents apressure source location obtained by Mogi (1958).

After the 1914 eruptions, Sakurajima was dormant forsome time, but the eruptive activity resumed at the
Minamidake summit crater in 1935 and then shifted to avent at the eastern flank in October 1939. Pyroclastic
flows frequently occurred at the eastern crater (Tsuya andMinakami, 1940). Vulcanian eruptions reoccurred at thecrater after 1941. In January 1946, explosive eruptions
became frequent and lava began to flow out from thesingle crater on March 9. A number of observations on
post-activity were performed by the Earthquake ResearchInstitute of University of Tokyo. For example, thecharacteristics of volcanic tremors (Nagata et al., 1948)
and the physical properties of lava flow (Hagiwara et al.,1948) were investigated. At present, the crater is referred
to as the Showa crater.
The most recent eruptive activity was the frequent
occurrence of vulcanian eruptions at the summit crater ofthe Minamidake, which commenced on October 13, 1955
and continued for more than 50 years, producing 7,900
vulcanian eruptions (Fig. 2). The Sakurajima VolcanologicalObservatory (SVO) was established as an affiliated facilityof the Disaster Prevention Research Institute of Kyoto
University in 1960 to study the forecast of volcanic
eruptions. The magma supply system has been investigated through a combination of seismic and geodetic
methods. The National Project of Prediction of Volcanic
Eruption, which began in 1974, has promoted the development of an observation system and collaboration among

Fig. 2. Annual numbers of vulcanian eruptions andweights of volcanic ash ejected from the Minamidake
crater during the period from 1955 to 2005. Observation of weight of volcanic ash started in 1978 by the
Kagoshima Prefectural Government.

universities and institutes. Continuous observations haverevealed precursory phenomena to the vulcanian eruptions
and their mechanisms.
Studies on the magma plumbing system of the Sakurajima volcano, which provides adequate information as aresult of the frequent occurrence of volcanic eruptions, aresummarized based on observations of the volcano.
2．Magma reservoir beneath the Aira caldera
On January 12, 1914, an eruption occurred at thewestern and eastern flanks (Fig. 1). The eruption was thelargest in the 20th century in Japan, and 1.34 km3 of lava
and 0. 52 km3 of pumice and ash were ejected from thecraters (Ishihara et al., 1981). The first leveling survey insouth Kyushu was conducted in 1891. After the 1914
eruption, a second leveling survey was conducted andsignificant ground subsidence was detected all over south
Kyushu (Omori, 1916), as shown in Fig. 3. For example,
BM2474 (Fig. 1) at the western rim of the Aira caldera
subsided by 0. 9 m (Omori, 1916). The center of theground subsidence is not located beneath Sakurajima but
rather near the center of the Aira caldera. Mogi (1958)
applied a model of the pressure source of a small sphereembedded in a homogeneous half space to the spatial
distribution of the vertical displacement, instead of theblock movement hypothesis discussed by Tsuboi (1929)
and Miyabe (1934). The elastic model applied by Mogi
(1958) was first proposed by Yamakawa (1955). The
deflation pressure source was estimated to be located at a
depth of 10 km near the center of the Aira caldera. This
indicates that the magma reservoir, as the origin of themagma product ejected by the 1914 eruption, was not
located beneath Sakurajima, but rather beneath the Airacaldera.
Associated with the 1914 eruption, a large amount ofMagma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

3

Fig. 3. Spatial distribution of height difference in south Kyushu before and after the 1914 eruption of Sakurajima (modified
from Omori, 1916). Benchmarks are indicated by dots with numbers in brackets, and contours lines indicate isobaths inmillimeters.

magma intruded into Sakurajima as observed by uplift near
the center of the volcano and extension in the north-south
direction. A trigonometric point was uplifted by 0.14 m
and the peaks of Hikinohira and Gongen-yama (Fig. 1)
were uplifted by 9.7 m and 10.4 m, respectively. Horizontal
displacements were quite large and ranged from 1 to 4.5 m(Omori, 1916). Yokoyama (1986) proposed a model composed of two P (cos θ) sources at depths of approximately 2 km (beneath the volcano) and 8 km, which is anoblique pressure source resulting in an asymmetric depression around the Aira caldera. Hashimoto and Tada (1988)
proposed a composite model of a deflation source beneath
the Aira caldera and a tensile fault in Sakurajima.
The ground around the Aira caldera has been upliftedsince 1916 (Fig. 4). As a result of the eruption at theShowa crater located at the eastern flank of the Minamidake (Fig. 1), 0.18 km3 of lava was effused (Ishihara et al.,1981). The ground around the Aira caldera is estimated tohave subsided by approximately 10 cm (Sassa, 1956).
However, uplift of the ground resumed after the 1946
eruption.
Moreover, the ground deformation is closely related toeruptive activity at the Minamidake crater during theperiod from 1955 to 2009. Fig. 2 shows the annual
numbers of explosive eruptions and the annual amounts of
volcanic ash ejected from the crater. Explosive eruptions
are defined by the SVO according to the intensity of the
explosion (≥10μm) and the air-shock (≥10 Pa) at Harutayama, which is located 2.7 km from the crater (Fig. 1).
The amounts of volcanic ash are estimated from the4Masato IGUCHI

monthly weights of ash deposit at 58 sampling points insouth Kyushu (Ishikawa et al., 1981). Before 1973, theannual number of explosive eruptions was less than 200,
except for 1960, and the ground around the Aira caldera
continued to be uplifted (Figs. 4 and 5). Eruptive activity
significantly increased after 1974. The annual number of
explosive eruptions ranged from 200 to 400, and theFig. 4. Secular change in vertical displacement after the1914 eruption (Sassa, 1956) I: BM2480, II: BM2474.
See the locations of the benchmarks in Figs. 1 and 3.
The dashed curves indicate estimations, and eruptions
are indicated by arrows.

annual weight of volcanic ash ranged from 1×107 to 3×
107 tons during the period from 1974 to 1992. During theperiod of enhanced activity, uplift of the ground around theAira caldera suspended or changed to subsidence (Fig. 5).
Analyzing the vertical displacement during the period oflower eruptive activity in 1957 and 1958, Yoshikawa(1961) found an inflation source at a depth of 10 km
beneath the center of the Aira caldera, where the depression source was located for the 1914 eruption (Mogi,
1958). Similarly, a deflation pressure source occupied thesame position during the highly eruptive period startingfrom 1974 (Eto and Nakamura, 1986). Note that the mainpressure source stably exists at a depth of 10 km beneath
the Aira caldera, and the inflation and deflation of thepressure source is closely related to the eruptive activity, i.
e., inflation for the case of low eruptivity and deflation forthe case of high eruptivity. Therefore, it is inferred thatthe pressure source corresponds to the main magma
reservoir beneath the Aira caldera. The existence of themagma reservoir is supported by strongly attenuatedseismic waves, which propagate under the Aira caldera(Ono et al., 1978).
The explosive activity of Sakurajima declined after the
peak in 1985, when the annual number of explosive
eruptions reached 452 (Fig. 2). Corresponding to a gradualdecrease in eruptivity, the subsidence ground deformation
changed to uplifting around 1993 (Fig. 5; Eto et al., 1997;
Eto et al., 1998a). The ground at the northern part of

Fig. 5. Secular changes in the relative heights of BM.2474 referring to BM.2469 and the volcanic activity at Sakurajima(Eto et al., 1997). The vertical displacement after 1960 is enlarged in a square in the lower-right corner. The dashed lines
indicate an estimation based mainly on Sassa (1956). The volumes of the lava flows effused by the 1914 and 1946
eruptions are 1.34 km3 and 0.18 km3, respectively.

Magma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

5

precise leveling to estimate the average magma supply rate
to the magma reservoir beneath the Aira caldera after the1914 eruption. The average rate was obtained as 1×107
m3 /year. Continuous GPS observation since 1995 hasrevealed a temporal change in the magma supply rate. The
magma supply rate increased to 2×107 m3/year during theperiods from November 1997 to October 1999 andOctober 2004 to February 2005. However, the averagerate was 1×107 m3 /year (Iguchi, 2006). It is estimatedthat 1. 4 × 108 m3 of magma has been newly supplied
during the period from 1993 to 2011.

Fig. 6. Horizontal displacement vectors during the period
from December 1996 to December 2007 (Iguchi et al.,2008a). The solid and gray arrows show theobservation and calculated vectors, respectively. The
gray circle indicates the location of the Mogi pressuresource. Station ONEG is used for reference.

Sakurajima was uplifted by 12 cm during the period from1993 to 2009 (Yamamoto et al., 2010). Similarly, an
EDM survey revealed the start of extension of a baseline
across the Aira caldera around 1992-1993 (Eto et al.,1998b). Clear extension of the ground around the Airacaldera was confirmed by GPS measurement, revealingradially outward displacements from the center of thecaldera (Fig. 6; Iguchi et al., 2008a; Kriswati and Iguchi,
2003). Analyzing the horizontal displacements from 1996
to 2007, a Mogi type pressure source (Mogi, 1958) is
estimated to be located at a depth of 11 km beneath thecenter of the Aira caldera and the volume increase isestimated to be 8×107 m3 (Iguchi et al., 2008a). Thisimplies that the magma reservoir is located beneath theAira caldera and that the volume of magma has increased
since 1993. The amount of magma continues to grow in
2011.
The magma supply rate is estimated from the addition of
volume change of the pressure source inducing the grounddeformation and the magma volume equivalent to thevolcanic product ejected from the crater. Ishihara (1981)
analyzed the vertical displacements measured by repeated

3．Magma reservoir beneath central cones
The SVO has conducted leveling surveys in the Sakurajima area since the first stage of eruptive activity at the
Minamidake from 1955. A dual-pressure source model was
proposed by adding a minor source beneath Sakurajima to
the major source beneath the Aira caldera. Minor pressuresources are estimated to be located at a depth of 3 km in the
inflation stage (Yoshikawa, 1961), and the depth isdetermined to be in the range of from 2 to 6 km in the
deflation stage after 1974 (Fig. 7; Eto and Nakamura,1986). Inflations and deflations of the major pressuresource are closely related to the eruptive activity, and thepressure source corresponds to the magma reservoir
beneath the Aira caldera. Therefore, the minor pressuresource corresponds to a supplementary magma reservoir
beneath the Minamidake crater.
The existence of a supplementary magma reservoir isconfirmed by tilt and strain changes associated with
explosive eruptions at the Minamidake crater and strong
attenuation of seismic waves propagating below the
Minamidake. The crater-side-up tilt and the extension
strain are observed 10 minutes to several hours before theexplosive eruptions and changed into crater-side-down tilt
and contraction strain, respectively, after the start of the
explosions (Kamo and Ishihara, 1989). The inflation anddeflation tilt vectors are oriented approximately in the
direction of the Minamidake crater, and the inflation anddeflation sources are located at depths of from 2 to 6 km
(Ishihara, 1990).
S-waves propagating at depths of 3 to 6 km beneath thecentral cones experienced strong attenuation (Kamo et al.,1980). The amplitudes of P-wave first motion comingfrom the direction of the crater are significantly smaller, by
one to two orders of magnitude (Suzuki, 1999). Such ahighly attenuated zone has an extent of 2 to 3 km. In
addition, the region of 3 to 6 km in depth is an aseismiczone of the A-type earthquake (Fig. 8).
4．Migration of magma from the Aira caldera toward
Sakurajima
Volcanic earthquakes at the Sakurajima volcano areclassified as A-type and B-type earthquakes, explosion
earthquakes, and tremors, based on Minakamiʼs classifi-

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Fig. 7. Vertical displacement fitted by the theoretical value of sources A and B (Eto and Nakamura, 1986). The depths ofsources A and B are 10 km and 3 km, respectively. The dashed curves ΔhA and ΔhB indicate the displacements induced
by individual sources A and B, respectively, and the solid curves are the sums of ΔhA and ΔhB on the line passing
through sources A and B. The dots, triangles, and open circles indicate the vertical displacements at the east and northrims of the Aira caldera, at the south and west parts of Sakurajima, and at the east and north parts of Sakurajima,
respectively. The locations of the benchmarks are shown on the map, although some are omitted.

cation criteria (Minakami, 1974). A-type earthquakes belong
to the same category as volcano-tectonic earthquakes
having distinct P- and S-phases. B-type earthquakes arecharacterized by the dominance of lower-frequency content
and ambiguous phases. B-type earthquakes are categorized
as BH-type and BL-type earthquakes according to thedominant frequency (BH-type: 4-7 Hz and BL-type: 1-3
Hz; Iguchi, 1989). Explosion earthquakes are associatedwith vulcanian eruptions and have larger amplitudes andlower-frequency content than B-type earthquakes. Volcanic
tremors are classified as spasmodic tremors with continuous
volcanic ash emission and harmonic tremors, which arereferred to as C-type earthquakes, at Sakurajima (Kamo etal., 1977).
Transitions of volcanic earthquake types suggest theascent or migration of magma. In March 1976, an A-type
earthquake occurred at a depth of grater than 10 km SSW
of Sakurajima, and the hypocenter became shallower andapproached the summit (Fig. 8). The seismicity changed
to swarms of B-type earthquakes in May, and the activityof the vulcanian eruptions then increased. The migrationof hypocenters of A-type earthquakes was interpreted tocorrespond to the upward migration of magma (Kamo,1978).
The spatial distribution of the vertical ground deformation revealed the relocation of a pressure source, related tothe eruptive activity. Ishihara and Eto (1978) revealed thetransition of the spatial pattern of the vertical displacement, as shown in Fig. 9. The pressure source remained at
the center of the Aira caldera, and Sakurajima exhibited an
upward tilt of the north side in the first period. The pattern
may continue from the beginning of the eruptive activity at
the Minamidake crater in 1955. The second period
corresponds to transition of activity from low eruptivity tohigher eruptivity from September 1972. The entirety of
Sakurajima was uplifted, and the center of uplift migrated
to the northern part of the island. Either magma migrated
to Sakurajima from the center of the Aira caldera or asecondary magma reservoir was formed beneath Sakurajima. When eruptive activity was maintained at a highlevel after 1974, the ground in and around Sakurajima
exhibited minor depression due to excess discharge ofmagma product.
Considering migrations of hypocenters of A-type earthquakes SSW of Sakurajima and pressure source at thenorthern part of the island, Kamo (1988) proposed a dualpath model for magma supply to Sakurajima based on bothseismological and geodetic observation (Fig. 10c). The
magma reservoirs are inferred to exist at a depth of 10 km
beneath the Aira caldera and at a depth of 5 km beneath the
Minamidake. The main path of the magma to the reservoir
beneath the Minamidake runs from the reservoir beneathMagma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

7

Fig. 8. Hypocenter distribution of A-type earthquakes during the period from 1976 to 2004. The numbered white circles
represent the hypocenter location as determined by Kamo (1978) and indicate the order of earthquake occurrence prior
to the increase in eruptive activity in May 1976. The numbers 6 through 9 are omitted in order to avoid congestion.
These hypocenters are located at depths of less than those of events 5 and 10 beneath the Minamidake.

the Aira caldera. An additional path from the SW part
directly to the reservoir beneath the Minamidake wasoccasionally active. However, Hidayati et al. (2007) proposed a single-path model (Figs. 10a and 10b). The magma
stored in the reservoir beneath the Aira caldera moved
laterally southwestward through a tensile fault striking
from NE to SW below the Minamidake, and the seismicityof A-type earthquakes SSW of Sakurajima is interpreted tobe due to the concentration of strain at the tip of the tensile
crack. The hypocenter distribution SSW of Sakurajima
did not exhibit migration in November 2003 and wasconcentrated at depths of from 6 to 9 km. The focal
mechanisms of the events are normal fault typemechanisms, striking NNE to SSW as suggested by thetensile fault direction. No significant ground deformation
was detected at the SSW part of Sakurajima during thehigh-seismicity period. The model proposed by Hidayati
et al. (2007) may simply explain both seismicity andground deformation. However, the application of thedual-source model of the tensile fault and the Mogi source
to the deformation in the inflation stage after 1993 has notbeen confirmed. The Mogi source was assumed to belocated northeast of Sakurajima in the model, based on thevertical displacement in the deflation stage (e.g., Eto andNakamura, 1986). However, the inflationary Mogi source
after 1993 is obtained north of Sakurajima by both vertical(Eto et al., 1998a; Yamamoto et al., 2008) and horizontal
displacements (Iguchi et al., 2008a).
5．Conduit beneath the Minamidake crater
The hypocenter distribution of the various types ofearthquakes provides an image of the structure of the
conduit connecting the magma reservoir beneath thecentral cone with the summit crater. Fig. 11 shows the
hypocenter distribution of A-type, BH-type, and BL-type
earthquakes and explosion earthquakes (Iguchi, 1994).
The hypocenters of the BH-type and BL-type earthquakes
and the explosion earthquakes are concentrated within aradius of 200 m beneath the crater at depths of 1 to 3 km.
In contrast, the hypocenters of the A-type earthquakes are
not located within the zone of the BH-type and BL-type
earthquakes and explosion earthquakes, but rather surround
it. A-type earthquakes are generated by shear faulting ofrock. Therefore, the surrounding zone is brittle. On theother hand, the moment tensors of the source for B-type
earthquakes and explosion earthquakes exhibit a volumetricincrease that is dominated by a non-double couple component (Iguchi, 1994; Tameguri et al., 2002). In addition,

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Fig. 9. Distribution of vertical displacement in three
periods (Ishihara and Eto, 1978). The displacement isin reference to BM.2469 (Fig. 1), and the elevation at
Sakurajima is connected by the tide difference between
two ports: Kagoshima and Hakamagashi in Sakurajima(see Fig. 1). Top: the vertical displacements at thenorthern part are maximal. Middle: the entirety of
Sakurajima is uplifted higher than Kagoshima.
Bottom: the number of explosive eruptions increased.
Ground deformation revealed no uplift or subsidence,
particularly at the NE part.

BL-type earthquakes and explosion earthquakes are associated with strombolian and vulcanian eruptions, respectively. Therefore, it is inferred that both BL-type earthquakes and explosion earthquakes occur in the fluid-filled
conduit. The separation of the hypocentral zone suggeststhat the magma-filled conduit is distinguishable from thesolid rock (Iguchi, 1994). The radius of the conduit isestimated to be 200 m at a depth of from 2 to 3 km.
However, the radius of the upper part of the conduit is 40
to 60 m (Ishihara, 1990). The radius of conduit decreases
as the shallowness increases.
B-type earthquakes at the Sakurajima volcano sometimes
swarm from one to a few days before the number ofvulcanian eruptions increases (e.g., Nishi, 1984). However,
the occurrences of the BH-type and BL-type earthquakes
correspond to different stages of magma intrusion into the
conduit. The hypocenters of the BL-type earthquakes are
distributed at depths of from 1 to 3 km below the crater,which is similar to the hypocenters of the explosion
earthquakes. The hypocenters of the BH-type earthquakes
are located at greater depths of 2 to 3.5 km (Fig. 11). The
ground deformations and surface phenomena at the crater
during the occurrence of swarms of BH-type and BL-type
earthquakes exhibit significant contrast. When BL-type
earthquakes swarm, crater-side-down tilt and contraction
strains associated with eruptive activity of strombolian
eruption or andesitic fountain at the crater were observed(Fig. 12a; Ishihara and Iguchi, 1989). The deflationary
ground deformation is caused by the emission of volcanic
ash and gas from the crater. Successive occurrence of BHtype earthquakes is associated with crater-side-up tilt (Fig.
12b) and extensional strain, but is accompanied by only
weak fumarolic activity. BH-type earthquakes are believed
to have been generated by the intrusion of magma into thelower part of the conduit. Moreover, BL-type earthquakes
are associated with the ascent of magma to the bottom of
the crater, and effusion of the magma is associated withsignificant degassing.
Tateo and Iguchi (2009) detected upward tilt of thecrater side and extensional strain before the occurrence ofBL-type earthquake swarms. Inflations begin 2 to 20
hours before the commencement of BL-type swarms. The
durations of these inflations are longer and the deformation
rates are smaller than those of the ground deformation
immediately before the occurrence of vulcanian eruptions(Fig. 13). The inflation source is located at a depth of 4
km below the Minamidake crater. This indicates that
inflation before BL-type earthquake swarms is a process ofmagma intrusion into the upper part of the conduit and theascent of the magma head approaching the ground surface.
The rate of inflation associated with a BH-type earthquake
swarm is as large as the maximum level of the inflation
rate before BL-type earthquake swarms. The intrusion ofmagma at a high rate induces a BH-type earthquake
swarm, which then changes to a BL-type earthquake

Magma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

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Fig. 10. Models of magma migration toward the Sakurajima volcano. A hypothetical model of the magma supply system at
the Sakurajima volcano. (a) Horizontal plane and (b) vertical cross-section in the SW-NE direction (Hidayati et al.,2007). The numbers correspond to volcanic processes during the period from 1999 to 2005. The open circles indicate VT
earthquakes. The open circles with “A” and “B” are pressure sources beneath the Aira caldera and Sakurajima,
respectively. The tensile fault is indicated by the rectangular dashed line, and the active zone from 2000 to the middle of2004 is shown in gray. The inferred magma migration is indicated by arrows. The solid circle with M 7 indicates thelocation of the largest earthquakes for the 1914 eruption as determined by Abe (1981). The lines with branches indicate
geological faults, most of which are normal faults. (c) Previous model modified from Kamo (1988). The VT earthquakesthat occurred during the period from 1976 to 1980 are plotted.

swarm when the magma reaches the crater bottom.
Based on the lower seismicity of A-type earthquakes(Hidayati et al., 2007) and the concentration of hypocenters of explosion earthquakes (Ishihara, 1988) at a
depth of 2 km, the conduit is partially enlarged at thisdepth. The focal mechanism of A-type earthquakes at
depths of greater than 2 km exhibit constant strike-slip
fault and may be affected by the regional tectonic stress of
the N-S extension (Nishi, 1978; Hidayati et al., 2007). In
contrast, a focal mechanism at a depth of less than 2 km iseither a normal or reverse fault mechanism. Typically, thenormal fault mechanism is dominant during an increase in
explosive activity, and so the mechanism changes to areverse or strike-slip mechanism when the explosivity
decreases (Nishi, 1978). An increase in pressure at a depthof 2 km may cause a tensional stress field above the fault,
generating normal fault earthquakes, and a decrease inpressure induces a contraction stress field corresponding toreverse fault mechanism earthquakes (Hidayati et al.,2007).

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Fig. 11. Hypocenter distribution of A-type, BH-type, andBL-type earthquakes and explosion earthquakes during
the period from 1987 to 1988 (Iguchi, 1994). Top:
epicenter distribution. Bottom: hypocenter distribution
on the cross-section in the east-west direction. The
shaded area indicates the hypocentral zone of the
explosion earthquakes (Ishihara, 1990)

6．Magma movement associated with vulcanian explosion
6-1 Preparation process
We believe that a swarm of BL-type earthquakes is aprecursor to an increase in explosive eruptions. Uhira andUeda (1988) investigated the time interval between BLtype earthquake swarms and subsequent explosive erup-

tions. More than 40 % of time intervals were more than 10days in 1980-1983, whereas 37 % of time intervals wereless than one day in 1984-1988. BL-type earthquakes
associated with ascent of magma to the crater bottom oreffusion of magma from the vent occur. A swarm of BLtype earthquakes, indicating a magma-filled conduit(Iguchi, 1994) might be a precursor to explosive activity.
However, it is difficult to explain difference in the timeinterval between 1980-1983 and 1984-1988, as shown by
Uhira and Ueda (1988).
The extension strain and upward tilt of the crater side
are detected immediately before the occurrence of
explosive eruptions (Kamo and Ishihara, 1989), and thisinflation indicates magma intrusion into the lower part ofthe conduit and the reservoir at depths of from 2 to 6 km
(Ishihara, 1990). Nogami et al. (2006) analyzed the watersoluble content-attached volcanic gas and found that themolar ratios of HCl/SO4 of volcanic ash ejected by
explosive eruptions were lower than those by strombolian
eruptivity in the case of a BL-type swarm. This indicatesthat the explosive eruptions ejected degassed magma.
Based on these considerations, two requirements must be
satisfied for the explosive eruptions to occur under an
open-conduit system during the period of the Minamidake
activity. The first requirement is that the upper part of the
conduit is plugged by degassed magma. Fresh magma isdriven up to the crater bottom, and the magma is degassed
by strombolian activity with a BL-type swarm. A lava
dome at the bottom of the crater and a gas pocket areformed by the degassed magma before the occurrence ofan explosive eruption (Ishihara, 1985). The second requirement is a subsequent intrusion of new magma into thelower part of the conduit. The difference in the time interval from BL-type earthquake swarms to the subsequent
explosive eruptions between 1980-1983 and 1984-1988(Uhira and Ueda, 1988) may be related to the time intervalof the intrusion of magma. Magma intrusion frequently
occurs in shorter time intervals in 1984-1988, as compared
to 1980-1983. As a result, a greater number of BL-type
earthquake swarms and explosive eruptions occurred in
1984-1988. When the upper conduit is not occupied bydegassed magma, new magma smoothly ascends to thecrater bottom, followed by strombolian eruptions and aBL-type earthquake swarm. Successive occurrence of aBL-type earthquake swarm and explosive activity is
explained schematically by the interaction of old degassed
magma and newly intruded magma (Fig. 14; Tateo andIguchi, 2009).
The formation of the gas pocket sometimes induces
harmonic tremor. The spectrum of the harmonic tremor at
Sakurajima is characterized by sharp peaks at frequencies
of multiple integers of the fundamental frequency (Kamoet al., 1977; Maryanto et al., 2008). Nishi (1984) demonstrated that the occurrence of the harmonic tremor after theBL-type earthquake swarm is a more reliable precursor toMagma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

11

Fig. 12. Tilt changes associated with swarms of BL-type earthquakes (a) and BH-type earthquakes (b) (modified from
Ishihara and Iguchi, 1989). Tilts were observed at the underground tunnel at Harutayama (Fig. 1). (a) The seismicenergy at station HIK (Fig. 1), which is 1.7 km from the summit crater, is shown in the lower part of the figure, and thetemporal change of the eruptive activity associated with the BL-type earthquake swarm is shown in the upper part of the
figure. No significant changes in surface phenomena were detected in the BH-type earthquake swarm.

Fig. 13. Examples of tilt change associated with a BL-type earthquake swarm (upper) and vulcanian eruption (lower).
Eruptive activity continued during the downward tilt of the crater side (Tateo and Iguchi, 2009). Tilts were observed at
the underground tunnel at Harutayama (Fig. 1). The upward tilt of the crater side changes to downward tilt after a shortperiod of deadlock, as indicated by the suspension process.

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Masato IGUCHI
Surface1phenomena Strombolian

Emissionof magma2

3

4

Gas emission Gas and ash stop

Vulcanian

A

A

Voout

5
Gas emission

6

7

Ash emission

Strombolian

BB

lave domeAA
cooledanddegassedBL swarmgaschamber

B
A

A

A
A

BL
swarm

B
B

B

B

B
Supply of
new magmaVin

Grounddeformation

Vin << Vout

Vin > Vout

Vin >> Vout

Vin << Vout

Vin > VoutVin ≈ Vout

Vin << Vout

Deflation

Gradual inflation

Rapidly inflation

Deflation

Gradual inflation

Suspension

Deflation

extension, up

1

≈0
3

4

5

6

7

2

contraction, down

Fig. 14. Schematic diagram of the internal change of the conduit inferred from ground deformation and surface phenomena(Tateo and Iguchi, 2009). Vin indicates the volume of magma supplied from the magma reservoir, and Vout indicates thevolume of magma emitted as volcanic ash and gas. The symbols “A” and “B” represent magma that was supplied to the
conduit in different stages. The inward and outward arrows indicate deflation and inflation of the deep part of the
conduit, respectively. Larger volume changes are indicated by larger arrows. A schematic diagram of the grounddeformation of tilt and strain changes is shown at the bottom of the figure.

explosive activity. Harmonic tremor is generated by theresonance of bubbly magma (Maryanto et al., 2008) at a
depth of from 300 to 500 m (Tameguri et al., 2007), whichindicates that the formation of a gas pocket composed ofbubbly magma has an important role in promoting explosive eruption.
6-2 Process of vulcanian eruption
The extension strain and upward tilt of the crater side
are detected by extensometers and water-tube tiltmeters
immediately before the occurrence of explosive eruptions(Kamo and Ishihara, 1989; Ishihara, 1990). An exampleof tilt and strain records is shown in Fig. 15. Since theprecursory time is only 10 minutes to several hours, theinflation of ground, indicating intrusion of magma, isdirectly related to subsequent explosive eruptions.
Iguchi et al. (2008b) summarized the process of explosive eruption. As mentioned in the previous section, thetop of the conduit is plugged by a lava dome, and a gas
pocket is formed immediately below the plug. The
pressure in the conduit increases due to the intrusion ofmagma (first stage in Fig. 16). As the pressure approaches
a threshold of the strength of the plug, the gas in the pocket
starts to leak (second stage). The second stage is inferredfrom the minor contraction of strain. The third stage ismarked by a transient increase in the volume and pressureof magma as a result of abrupt outgassing of the watersaturated magma due to a decrease in pressure following
the second stage. The duration of the second stage thus
extends from the beginning of the pressure decrease to thestart of outgassing in the water-saturated magma is one totwo minutes at Sakurajima. The fourth stage involves anincrease in volume and pressure of the gas pocket at the
top of the conduit associated with explosive eruptions
starting at the bottom of the crater. The plug confining thegas pocket is destroyed by the expansion of the gas pocket,
resulting in an air shock (propagation velocity: M 1.1-1.4;
Ishihara, 1985; Yokoo and Ishihara, 2007) and the ejectionof volcanic bombs (initial velocity: approximately 200m/s; Iguchi et al., 1983). In the fifth stage, the deflation of
the volcanic conduit occurs as a result of a pressure andvolume decrease due to the ejection of volcanic gas andash. The deflation stage is separated into sudden
contraction at the shallow part and gradual deflation at the
deeper part, as shown in 5A and 5B in Fig. 16,
respectively. After destroying the plug confining the gas
pocket, the volcanic gas and ash are rapidly ejected, andMagma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

Fig. 15. (a) Tilt and (b) strain changes associated withvulcanian eruption (Ishihara, 1990). (a) “WT-R” and“WT-T” indicate the radial component from the craterand component tangential to the crater, respectively, as
recorded by the water-tube tiltmeter in an undergroundtunnel at Harutayama (Fig. 1). “BT-R” is the radial
component of the borehole tiltmeter at a depth of 290
m beneath Harutayama. (b) Extensometer records.
EX-1: radial, EX-2: tangential, and EX-3: obliquecomponents in the underground tunnel. The upward tiltof the crater side and the extension strain change
started at 8 h.

the gas pocket is collapsed. The deflation graduallyproceeds as shown by contraction strain and downward tilt
in Fig. 15 at the magma reservoir at a depth of from 2 to 6
km (Ishihara, 1990).
The third through fifth stages appear as characteristicphases in the explosion earthquake. The third stage isidentified as the onset of explosion earthquakes, the13Fig. 17. An example of velocity seismograms of an
explosion earthquake (Tameguri et al., 2002). The
explosion occurred at 04h55 m, September 3, 1999.
“V” is the vertical component of the broadband
seismometer, and “R” and “T” represent the horizontal
components radial from the crater and tangential to thecrater, respectively. “P”, “D”, and “LP” indicate thecharacteristic phases of the explosion earthquake. “P”
refers to the P-wave first motion with compression
onset. “D” refers to the subsequent dilatational Pwave. “LP” refers to the long-period Rayleigh wave.

hypocenters of which are located at depths of from 2 to 3
km immediately beneath the Minamidake crater (Nishi,
1976; Yamasato, 1987; Iguchi, 1989; 1994 Tameguri etal., 2002). An example of an explosion earthquake isshown in Fig. 17. The P-phase is a compressional P-wave
first-motion-generating isotropic expansion, and thesubsequent D-phase is a dilatational P-wave that inducescylindrical contraction (Tameguri et al., 2002). Moreover,the cylindrical contraction is detected by a longer-period
wave (Uhira and Takeo, 1994). The contraction isinterpreted to be caused by the upward flow of gas in the
conduit. The expansion at a depth of 0.5 km in the fourthstage in Fig. 16 corresponds to an isotropic expansion at
the depth, generating the first half of the LP phasecomposed of a Rayleigh wave (Tameguri et al., 2002).
More specifically, the cap rock swells as a result of the
expansion and is destroyed at the moment of the greatest
seismic moment rate of expansion (Yokoo et al., 2009).
The collapse of the gas pocket at a depth of approximately

14

Masato IGUCHI

Fig. 16. Process of explosive eruption based on observations at Sakurajima, Semeru, and Suwanosejima volcanoes (Iguchi
et al., 2008b). Inferred pressure changes are shown at the bottom. “Shallow” and “Deep” indicate pressure changes at
the upper part (less than 0.5 km) and the deep part (approximately 2 km) of the conduit. The difference is recognized at
the Sakurajima volcano, and only changes at the shallow parts are detected at Suwanosejima and Semeru volcanoes. (1)Ascent of magma and the accumulation of volcanic gas in the conduit below a confining cap (plug). Pressure andvolume increases are observed as inflation-related tilt/strain and upward displacement. (2) Release of gas as the gaspressure exceeds the strength of the cap. The resultant minor contraction is detected as deflation tilt and downward
displacement. (3) Pressure decrease in the conduit starting from the gas pocket, inducing sudden outgassing of watersaturated magma deep in the conduit. Expansion is detected by upward P-wave first motion of the explosion earthquake.
(4) Expansion process destroying the cap at the top of the conduit. Stages (3) and (4) could not be distinguished at the
Suwanosejima and Semeru volcanoes. (5A) Failure of the cap at the top of the conduit. The gas pocket at the upper partof the conduit collapses, inducing a contraction ground deformation source at the shallow part. The deformation isrevealed by the step of the crater-side-down tilt and the step of the tensional strain, as well as downward grounddeformation. (5B) Ash and gas emission from the conduit. Pressure and volume decreases are observed as deflationrelated tilt/strain and downward displacement, respectively.

500 m (stage 5A) is confirmed by the strain step and thestep of the crater-side-down tilt (Ishihara, 1990) and thesecond half of the LP phase generated by the contractive
horizontal dipoles (Tameguri et al., 2002). This stage ismodeled by Kanamori et al. (1984) and involves an
implosion source and a downward single force acting at
the crater, although the two sources are not welldiscriminated from each other at Sakurajima (Uhira et al.,1995). Harmonic tremors sometimes follow explosion
earthquakes one to eight minutes after the onset of
explosions, and these tremors are interpreted to be causedby the resonance of bubbly magma in the uppermost partof the conduit. In this case, the conduit opened partially(Maryanto et al., 2008).

7．Conclusions and future considerations
At the Sakurajima volcano, where three representative
eruptions occurred in the 20th century, a magma plumbing
system has been extensively investigated. The main
magma reservoir is located at a depth of 10 km beneath thecenter of the Aira caldera, and the minor magma reservoir
located at a depth of from 3 to 6 km beneath the central
cone of Sakurajima is directly related to the eruptive
activity of the Minamidake crater. The minor reservoir isconnected to the crater bottom of the Minamidake by the
conduit. Magma supply systems inferred from seismic
and geodetic observations are schematically summarized
in Fig. 18. Magma is supplied to the main reservoir at a
rate of 107 m3/year. High and low eruptivities correspondto deflation and inflation of the reservoir, respectively.
The ascent of magma through the conduit and theMagma Movement from the Deep to Shallow Sakurajima Volcano as Revealed by Geophysical Observations

15

Fig. 18. Image of magma supply beneath the Sakurajima volcano. (a) 1974-1992 (modified from Kamo, 1988) whenvulcanian eruptions reoccurred at the Minamidake crater. (b) after 1993 (modified from Hidayati et al., 2007) when theeruptive activity decreased.

subsequent intrusion induces a sequence of BL-type
earthquake swarms and explosive eruptions. The cap rockand the gas pocket immediately beneath the cap rock at the
top of the conduit increases the pressure in the conduit andtriggers an explosion by minor pressure decrease to startsudden formation of gas bubbles.
The magma path remains an unresolved problem. The
major pressure source remains near the center of the Airacaldera. However, the inflation source is located significantly to the west of the deflation source, considering thehigh density of the leveling benchmarks in northern
Sakurajima. In addition, the inferred magma path at thenorth and northeastern parts of Sakurajima is the lowest
seismic zone in and around Sakurajima.
In June 2006, the eruptive activity at the Showa crater
resumed after 58 years of dormancy, and the eruptive
activity has gradually increased. However, the seismicity
of volcanic earthquakes and ground deformation remained
at a lower level compared with the previous period treatedin this review. More precise and dense seismic andgeodetic observations, a survey on the internal structure of
the volcano and its temporary change, and quasi-real-timemonitoring of eruptive material are required to prepare for
future eruption.
Acknowledgements
We would like to thank Professors Kenzo Sassa, thefounder of the SVO, Keizo Yoshikawa, Kosuke Kamo, andKazuhiro Ishihara, all of Kyoto University; as well as thestaff members of the SVO for performing 50 years of
volcano observations.

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(Editorial handling Takeshi Nishimura)18

Masato IGUCHI

地球物理学的観測によって明らかになった桜島火山の深部から浅部へのマグマの動き
井口正人

地震観測と地盤変動観測に基づいて 1914 年，1946 年，1955 年以降と 3 つの特徴的な噴火が発生した桜島
火山におけるマグマ供給系とマグマの動きに関する研究について総説する．桜島のマグマ供給系は桜島の北方，姶良カルデラの地下約 10 km にあると推定される深部マグマ溜り，中央火口丘下の深さ 3〜6 km の浅部マグマ溜りおよび南岳山頂火口につながる火道から構成される．上下変動の空間分布と時間変化から，年7 3間 10 m のマグマが深部溜りに供給されてきた．浅部溜りは，地盤の上下変動，爆発的噴火直前の火口方向の隆起の傾斜変化や地震波の異常減衰域の存在から推定される．姶良カルデラの隆起に続く桜島への隆起
中心の移動と火山性地震の震源の深部から南岳直下の浅部へ移動は中央火口丘下へのマグマの移動・上昇に対応する．中央火口丘下のマグマ溜まりと火口底をつなぐ火道の存在とその大きさをせん断破壊により発生する A 型地震と体積膨張に伴う B 型・爆発地震の震源位置の違いから推定できる．火道内をマグマがス
ムーズに上昇して火口底に達した時は，BL 型地震を伴うストロンボリ式噴火が発生し，脱ガスしたマグマが火道上部に蓋とガス溜まりを形成している状態でマグマが貫入してきたときはブルカノ式噴火が発生す
る．いずれの場合も火口周辺の地盤の隆起と伸長が観測され，噴火の発生とともに沈降，収縮へ反転する．
マグマの揮発性成分は火道上部のガス溜りの形成と火山ガスの漏洩に伴う圧力低下によって引き起こされるマグマの急激な発泡に重要な役割を果たす．