Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 239-250

ArticleStructure of Northeastern Sakurajima, South Kyushu, Japan,
Revealed by Seismic Reflection Survey
Tomoki TSUTSUI＊, Naofumi YAGI＊＊, Masato IGUCHI＊＊＊, Takeshi TAMEGURI＊＊＊,
Hitoshi MIKADA＊＊＊＊, Kyosuke ONISHI＊＊＊＊, (present ＊), Hiroki MIYAMACHI＊＊＊＊＊,
Takeshi NISHIMURA＊＊＊＊＊＊, Yuichi MORITA＊＊＊＊＊＊＊ and Atsushi WATANABE＊＊＊＊＊＊＊(Received June 27, 2011 ; Accepted September 3, 2012)
Seismic reflection structure in the northeast part of Sakurajima Volcano is discussed down to ca. 11 km with three
profiles. Data acquisition was conducted along intersecting two lines with 221 stations and eight shot points. The twolines covered the northeastern portion of the volcano. One of the lines was spread along NNW to SSE direction in theeast foot, and another line was oriented east to west in the northern flank of the volcano. Data processing were madethrough the conventional procedure of the reflection seismology, and depth migrated sections were obtained. Fourcontinuous reflection horizons appear in the profiles. One of the reflection horizons around 5.4 km depth disappears in
the northeast portion of the coverage area. The interruption of the horizon suggests existence of magma or magma
supply path beneath it. The inferred magma supply path locates to the west of that previously presented.
Key words : Sakurajima volcano, Seismic structure, Seismic exploration

1．Introduction
Sakurajima is an active volcano with an altitude of 1117
m located in southern Kyushu. In Sakurajima volcano,effusive eruptions were recorded in history (Fukuyama andKobayashi, 1981). In this half century, frequent explosions from the summit crater of Mt. Minamidake begun in1955 and continued afterwards until 1990ʼs. Recently, theShowa crater opened again at the eastern flank of the Mt.
Minamidake in June 2006, and explosive eruptions fromthe crater have been enhanced since February 2008 (Iguchi
et al. 2008).
Such progress of the activity has a close relation toaccumulation of the magma in the reservoir beneath Airacaldera in the depth of 10 km, and movement of the magma
to the deep part below Mt. Minamidake. According toground deformation analyses, two pressure sources arepresented in the depth of 10 km beneath Aira caldera, andat 5 km depth beneath Mt. Minamidake and are inferred asmagma chambers. For example, Eto et al. (1997) explainedthat subsiding ground deformation during 1974-1992 isdue to the major pressure source in the depth of 10 km
beneath Aira caldera and the additional pressure source＊

Graduate school of Resource Science and Engineering,
Akita University.
＊＊
Fujitsu Software Technologies Limited.
＊＊＊
Disaster Prevention Research Institute, Kyoto University.
＊＊＊＊
Graduate school of Engineering, Kyoto University.
＊＊＊＊＊
Graduate school of Science and Engineering, Kagoshima

beneath Mt. Minamidake. Hidayati et al. (2007) presented
a model that magma moves along an tensile crack from
northeast to the southwest through the foot of Mt. Minamidake, which was inferred from inflation around Aira caldera and the seismicity in northeastern Sakurajima (Fig. 1).
The subsurface structure of Sakurajima volcano has
been investigated by gravity survey and by electromagnetic sounding. Yokoyama and Ohkawa (1986) constructed
the first density model and presented the gravity basement
depth at 2.5 km beneath Sakurajima volcano and a grabenlike structure beneath Kagoshima Bay from the Bouguer
anomaly. Then Komazawa et al. (2008) performed thehigh density gravity survey, and presented detailed topography of the gravity basement around 1. 2 km b. s. l.,which is approximately northeastern dip and ridges eastward in the northeast Sakurajima. Furthermore, Kanda etal. (2008) performed the AMT electromagnetic sounding
in Sakurajima, and revealed that a high resistivity layer isdeeper in northeastern part than in other part in Sakurajima. The deeper high resistivity layer implies the basement and underlies the low resistivity layer which ispossibly contained by permeable materials.
University.
Graduate school of Science, Tohoku University.
＊＊＊＊＊＊＊
Earthquake Research Institute, University of Tokyo.
＊＊＊＊＊＊

Corresponding author: Tomoki Tsutsui
e-mail: tomoki@gipc.akita-u.ac.jp

240

Tomoki TSUTSUI, Naofumi YAGI, Masato IGUCHI, Takeshi TAMEGURI, Hitoshi MIKADA, Kyosuke ONISHI,
Hiroki MIYAMACHI, Takeshi NISHIMURA, Yuichi MORITA and Atsushi WATANABE

Thus the northeastern Sakurajima is the key location inorder to discuss movement of magma to Sakurajima. In
the 2008 Project of Artificial Explosion Experiment at
Sakurajima Volcano (Iguchi et al., 2009), seismic reflection survey was carried out at the northeast portion in
Sakurajima, expecting any evidence of magma supply
paths. This paper discusses the underground structurefrom the depth of 2 km down to ca. 11 km beneath the
Sakurajima volcano through the 2008ʼs reflection survey.
2．Observation and data
The 2008 Project of Artificial Explosion Experiment
was carried out on the Sakurajima volcano and its surroundings in November 2008 as a part of the 7th national
project for prediction of volcanic eruption. Reflection
survey was concentrated at east to north flank of Sakurajima. In this survey, two seismic lines which consist of
221 temporary seismic stations and eight shot points were
deployed as shown in Fig. 1 (Iguchi et al., 2009). The
field parameters are listed in Table 1. The line NS consistsof 126 stations X001A - X125A, and X192A which are
deployed in the east foot of the volcano, and the line EW

Fig. 1. Seismic lines. Stars present the shot points andsolid circles are temporary seismic stations. Thick line
represents the tensile crack which presented by
Hidayati et al. (2007). The solid triangles mark major
summits with the following codes, such as KDK: Kitadake, MDK: Minami-dake.
Table 1.

consists of 95 stations X001B - X094B, and X101B which
are deployed on the north flank. Average separations ofstations were 47 m along the line NS and 72 m along theline EW. In each seismic station, the vertical component
seismometer (natural frequency: 4.5 Hz), was installed andthe seismic signal was recorded in the recorder LS8200SD (Hakusan industry corp.) with sampling intervalof 2 ms.
Along the line NS, four shot points, S09, S10, S11, andS12, are aligned from the south. As for the line EW, five
shot points, S15, S14, S13, S11, and S06, are aligned fromthe west. The shot point S11 is located at the intersectionof the lines NS and EW.
3．Data processing
The flow of the main analyses is presented in Fig. 2.
The software package Seismic Un*x (Cohen and Stockwell,2008) was used for data processing, which installed inMacintosh system (Yagi and Tsutsui, 2009). Purposes andtheory of each step are described in Yilmaz (2001).
In the static correction, the datum was assigned as 372 m
above sea level and the static-correction velocity of 1.1
km/s is applied with referring to Tsutsui et al. (2011).
The sorting parameters are shown in Table. 2. Geometry of the profiles are shown in Fig. 3. CMP spacing is
the half of the mean station separation in each line. Theline EW was split at the shot S11 into two portions, theprofiles EW(W) and EW(E) because of its bending geometry. Local coordinate systems are used along eachprofile, orienting x-axis to the north for the line NS or to
the east in the line EW, respectively. The both origins are
assigned at the shot point S11. Major analyses and theparameters are described as follows.
Fig. 4 shows examples of the shot records in the profiles
NS and EW(W). Fig. 4 correspond to the labeled step <1>
in Fig. 2. AGC (Automatic Gain Control) was applied on
the seismograms with 0.6 s in gate time. Artificial quiescence appears before the first arrival. Parasitic later
arrivals with parallel to the first arrival are dominant andlikely mask deeper reflections out in the sections.
The filtered shot records are shown in Fig. 5. Fig. 5
corresponds to the labeled step <2> in Fig. 2. The arrows
mark enhanced arrivals with high apparent velocity. The
three-point mixing, the band-pass filtering (1-8 Hz), theThe field parameters.

Structure of Northeastern Sakurajima, South Kyushu, Japan, Revealed by Seismic Reflection Survey

predictive deconvolution have been applied on the seismograms in order. The optimum parameters were determinedthrough the tuning; The prediction distances were ranging
between 0.8 to 1.2s and the operation length was around
0.3 s, depending on each waveform. Instrumental compensation was not applied through the processing.
We used combined CMP gathers in the velocity analysis
because of poor fold number for each single CMP gather.
We choose eight sets of the combined gathers for the line
NS, two sets for the profile EW(W), and one set for the
profile EW(E) for the analysis as taking into accounts of
their fold number. Fig. 6 shows an example of velocity
spectra with the final velocity function. Fig. 6 corresponds
to the labeled step <3> in Fig. 2. We worked on the picking with referring to the corresponding CMP gather at the
points in order to take an exact peak in the spectrum.
Because of the peak broadening, accuracy of velocity
picking becomes poorer with increasing travel time.
Though the peak width keeps within 500 m/s until 3 s oftravel time, it broadens in later travel time.
The results of the velocity analysis are shown in Fig. 7.
The symbols in Fig. 7a are obtained RMS P-wave velocity

Fig. 2.

through the velocity analysis. There looks no clear spatial
dependence among the pick-up values so that a single
velocity function is appropriate over all the profiles. The
final velocity was determined from the medians of thepick-up value in the analysis down to 3. 4 s, and wasimported from that of Ono et al. (1978) at 4.8 s because of
the broadening of deeper peaks. The final stacking velocity function is shown by the solid line with solid circles.
Fig. 7b shows an interval velocity with the solid line
which derived from the final stacking velocity in Fig. 7a.
A broken line represents the model in Osumi peninsula by
Ono et al. (1978). Our interval velocity down to 2400 m
shows slower velocity than that of Onoʼs model by 1 km/s.
This is consistent with the graben-like structure thatYokoyama and Ohkawa (1986) presented.
Fig. 8 shows time sections with their folds and offset
range of corresponding gathers, which correspond to thelabeled step <4> in Fig. 2. Some continuous reflections
appear in these sections and marked with arrows. Their
two way time are consistent with those pointed in Fig. 5.
Appearance of the sections matches among each other at
the intersection. Short period fluctuation of the fold num-

The data processing. Conventional processing is included. The labels 1 to 4 are described in the text.

Table 2.

241

The sorting parameters.

242

Tomoki TSUTSUI, Naofumi YAGI, Masato IGUCHI, Takeshi TAMEGURI, Hitoshi MIKADA, Kyosuke ONISHI,
Hiroki MIYAMACHI, Takeshi NISHIMURA, Yuichi MORITA and Atsushi WATANABE

ber and the offset range may cause contamination over the
profiles with high frequency noise.
Then Kirchhoff depth migration (Geoltrain and Brac,
1993) was applied in order to obtain depth sections. It
migrates the reflections in the apparent position to theircorrect location resulting improvement of spatial resolution. The simple horizontal layer model that is presented
in Fig. 7b is applied. The migration aperture was 12
degree on the basis of Fresnel radius at 5.4 km in the depthof the model. The migrated sections are shown in Fig. 9.

Fig. 3. Distribution of the profiles. Thick lines with themarkers describe three profiles, NS, EW(W), EW(E), respectively. Numbers along the profiles are
CMP location code. The crosses and stars indicate
stations and shot points, respectively. The origin of
the local coordinate system is the shot point S11.

4．The final seismic sections and its interpretations
The profiles in Fig. 9 contain reflective shallow part
down to ca. 5 km and less reflective deeper part. Suchappearance of the profiles in Fig. 9 are similar as theseismic reflection profile by Mikada (1996) in Kirishima
Volcanoes, the northern neighboring volcanoes of
Sakurajima. Mikada (1996) presented a reflective upper
layer above 5 km in the depth with underlying transparent
layer, and a reflection horizon in the depth of 10 km.
Fig. 10 represents an interpretation of the migrated
depth sections in Fig. 9. Major reflection horizons aremarked as A to D. The shallower horizon A appears along

Fig. 4. Record sections. (a) for the shot S09, (b) for the shot S10, (c) for the shot S11, and (d) for the shot S12 are acquiredalong the profile NS. (e) for the shot S15, (f) for the shot S14, (g) for the shot S13, and (h) for the shot S06 are acquiredalong the profile EW(W). Horizontal axis is the location of a station. AGC (Automatic gain control) is applied on the all
seismograms with 0.6s of gate time.

Structure of Northeastern Sakurajima, South Kyushu, Japan, Revealed by Seismic Reflection Survey

the line NS. The horizon A decreases its depth northward.
However, the horizon A is not clear in the profiles EW(W)and EW(E). The horizon A is inferred as the horizon III of
Tsutsui et al. (2011) and a intermediate interface in thepost caldera fill on the basis of its depth and velocity
around the interface (Fig. 7).
The horizon B which underlies the horizon A appears
around 1800 m below the datum. Corresponding arrivals
appear around 2 s in Fig. 5. The horizon B looks a panlike interface along the line NS. The horizon appears at
1800 m below the datum at CMP 40. The horizon
increases its depth gently northward. The horizon can beinferred as the “basement” in Tsutsui et al. (2011) on thebasis of its depth. The depth of the horizon is also
identical with gravity basement by Komazawa et al.
(2008). Hence the steep rise of the horizon is acceptable at
CMP 140. The horizon B is interpreted as a part of thebottom of Aira caldera for its depth and the velocity
contrast (Fig. 7).
The horizon C appears flat around 5.4 km depth in all
profile. The horizon C looks a boundary of reflection
pattern between the continuous horizontal overlays andless continuous and undulated underlays. The horizon C
can be the interface which is commonly presented in the243

upper crust of south Kyushu (Ando et al., 2002; Kakuta,
1982; Ono et al., 1978). However, the horizon is not
uniform beneath Sakurajima. The horizon C and its underlying disappear at the discontinuity H in the eastern portionof the line EW(W)(Fig. 10b).
The deepest horizon D appears on 11 km below thedatum along the profiles NS and EW(W). The horizon
dips northward along the profile NS. The horizon can bealso the common reflector in this area because Mikada(1996) presented a reflection horizon in the same depth
beneath Kirishima Volcanoes.
5．Discussions
Resolution of the profiles
The migrated profiles include some artifacts whicharose from heterogeneity in the folds and the offset range
for small fold number analysis. We should recognize themand avoid misleading our interpretations. Following markers
are shown in Fig. 10; The minimum fold boundaries aremarked with thick broken lines and the offset range
discontinuity are thin broken lines. Apparent weakening
and apparent discontinuities of the reflection align on thesebroken lines.
Fresnel radius controls horizontal resolution of seismic

Fig. 5. Filtered, Deconvolved and muted records. The panel order is the same as the previous figure. The parameters of thedeconvolution are around 0.1s as the prediction distance and 0.32s as the operation length. Arrows mark reflection
arrivals and some remarkable phases. Refraction arrivals and their preceding noises are muted.

244

Tomoki TSUTSUI, Naofumi YAGI, Masato IGUCHI, Takeshi TAMEGURI, Hitoshi MIKADA, Kyosuke ONISHI,
Hiroki MIYAMACHI, Takeshi NISHIMURA, Yuichi MORITA and Atsushi WATANABE

Fig. 6. An example of the velocity spectra. The spectrum
is obtained on the profile NS in 2.3 to 3.6 km south of
the shot S11. Gray scale represents intensity of thespectrum. A thick line represents the final velocity
function. Sensitivity of velocity becomes worth withincreasing two way time due to the peak broadening.

images (Yilmaz, 2001). Fig. 10d shows Fresnel radiuses
and their envelope in the same scale with the profiles.
Because of poor fold number, foot-print of individual
Fresnel zones which are centered at each shot point may
not be cancelled out and migration noise arises at themargin of the zones. For example, some migration noisealigns along the envelope of Fresnel zone in the left end of
Fig. 10a.
However the horizon D is detected in this study, it isdifficult to distinguish its exact geometry, such as a point
or a plane, because its extension is the same as Fresnel
radius at the depth.
The discontinuities F and H
There are two discontinuities F and H in Figs. 9 and 10.
The discontinuity F spreads out around CMP 95 to 110 in
the line NS (Figs. 9a and 10a). The discontinuity H
appears around CMP 110 at the line EW(W) (Figs. 9b and
10b).
The discontinuity F is possibly artifact. The boundary
overlays the reversed region f in 2 to 3 km below thedatum. The gap e locates on the apex of the discontinuity
F. The gap e arose from a lack of a shot point, and top partof stacked seismograms beneath the gap e are distorted
through the stretching in NMO processing (Fig. 8a).
Therefore we concluded that the discontinuity F is a
migration noise and the region f is not actual structure.
On the other hand, the discontinuity H is not an artifact.
The discontinuity H looks convex face with its apex ataround 5.4 km depth in the east end of the profile EW(W).
Its dip angle differs from those of Fresnel envelopes andthe offset range discontinuities. Hence we concluded thatthe discontinuity H represents a certain boundary in theactual structure.
The discontinuity H spreads out to west up to CMP 80of the profile EW(W), and can be recognized northwardfrom CMP160 of the profile NS. However, the discontinuity itself or any similar pattern disappear in the restof the profiles EW(W) and NS. The discontinuity is notclear in the profile EW(E). The discontinuity H is inferredto spread out to north in the east end of the profile EW(W).
Inferred distribution is mapped in Fig. 11.
The region h beneath the discontinuity H possibly
represent low impedance region against the background.
The low impedance is attributed to low velocity and/or low
density in the region. A rapid decrease in elastic velocity
is presented around the melting point of igneous rocks byMurase and McBirny (1973). And Abseth et al. (2005)
describes impedance decrease with substitution of porous
liquid into gases. One of the possible source of heat andgases is magma which is supplied from the deep part ofAira caldera and moves to Sakurajima.
The region h locates an intermediate place between thepressure source in depth at the center of caldera (Eto et al.,1997) and the craters An, KDK and MDK. The depth ofthe discontinuity H is consistent with that of the crack topof Hidayatiʼs model (Hidayati et al., 2007).
Moreover, other geophysical studies have provided theevidences of magma beneath this area. The largest verticaldisplacement has been observed above the region h both in
the deflation phases during 1964 to 1975 and 1988 to 1991
and in the inflation phases during 1974 to 1988 and 1991
to 1996 through the leveling surveys in the north shore of
Sakurajima (Eto et al. 1997). Thus such the vertical displacement pattern strongly supports that magma exists in
the region h.
From the above discussion, our interpretation is that theregion h directly associates with magma, and that some ofactual magma supply paths can locate more west than that
presented by Hidayati et al. (2007). A chimney structurev is a possible magma supply path, which penetrates theregion h. If we can have more shot points along the lines
or more folds at each CMP, direct imaging of magma will
be more successful.
6．SummaryThree seismic reflection profiles were obtained in thenortheastern part of Sakurajima volcano. Four reflectionStructure of Northeastern Sakurajima, South Kyushu, Japan, Revealed by Seismic Reflection Survey

245

Fig. 7. Velocity structures. (a) Picked RMS velocities and the final stacking velocity function. The symbols denote pickedvelocities and the solid line describes the final stacking velocity. (b) Derived interval velocity. Thick solid line
represents the final interval velocity derived from the final stacking velocity and broken line is the velocity model after
Ono et al. (1978).

horizons in different depths are recognized in the depthmigrated sections. These are the horizon A around 1 km,
the horizon B in 1.8 km, the horizon C around 5.4 km, andthe horizon D at 11 km below the datum. The horizon A isinferred to be a reflector in post-caldera fill. The horizon
B looks like a pan along the profile NS and decreases its
depth westward along the profile EW(W). The horizon B
corresponds with the gravity basement and inferred to be apart of floor of Aira caldera. The horizon C is the common
interface in the upper crust in this area and its polarity is
not uniform. The discontinuity H is found and interrupts
the horizon C in the east end of the profile EW(W) and thenorth end of the profile NS. The depth of the discontinuity
H is consistent with a top end of the preceding Hidayatiʼsmodel. The underlain region h of the discontinuity H isinferred to be a low impedance which represents hightemperature. The low impedance region h is interpreted to
be strongly affected by magma because the largest verticaldisplacement has been detected above the region h. Thus
it is concluded that magma supply path of Sakurajima
volcano locate in more west than expected through thepreceding study.

Acknowledgement
The data used by the present study is obtained by nicecooperation of participant (82 persons) of the 7th national
project for prediction of volcanic eruption (Iguchi et al.,2009) and the personnel of the Volcano Research Center,
Kyoto University. Moreover, in the execution of the 2008
Project of Artificial Explosion Experiment at Sakurajima
Volcano, we obtained the cooperation by local residents
and organizations concerned. The colleagues of the repititive seismic survey 2009 to 2011, Dr. Hiromitsu Oshima,
Dr. Sadato Ueki, Dr. Kenji Nogami, Dr. Jun Oikawa, Dr.
Takao Ohminato, Dr. Haruhisa Nakamichi, Dr. Takahiro
Ohkura, Dr. Hiroshi Shimizu, and Dr. Hiroshi Yakiwara,
kindly permit us to stack some part of their data at thesame shot points. Moreover, the discussions with Dr.
Tadashi Nishitani and Dr. Shinʼ ya Sakanaka in Appliedgeophysics laboratory, Akita-University and also with thegraduates and the undergraduates were useful to compose
and complete this research. Two anonymous referees gave
us large amount of helpful comment to improve thismanuscript. Moreover, patience of editor also encourage
us. We describe all of them here and express gratitude forall of them.

246

Tomoki TSUTSUI, Naofumi YAGI, Masato IGUCHI, Takeshi TAMEGURI, Hitoshi MIKADA, Kyosuke ONISHI,
Hiroki MIYAMACHI, Takeshi NISHIMURA, Yuichi MORITA and Atsushi WATANABE

Fig. 8. Time sections. (a) The line NS, (b) the line EW(W), and (c) the line EW(E). Horizontal axes are the CMP number.
The top panel represents the fold number and the middle panel is the offset range along each profile. The time section isthe bottom panel. The vertical axes are the normal two way time from the datum.

Structure of Northeastern Sakurajima, South Kyushu, Japan, Revealed by Seismic Reflection Survey

247

Fig. 9. Migrated depth sections. (a)The profile NS, (b) the profile EW(W), and (c) the profile EW(E). Horizontal axes are
CMP number. Vertical axes are depth from the datum. Reddish color represents positive polarity and bluish color is
negative polarity reflection and those densities represent intensity.

248

Tomoki TSUTSUI, Naofumi YAGI, Masato IGUCHI, Takeshi TAMEGURI, Hitoshi MIKADA, Kyosuke ONISHI,
Hiroki MIYAMACHI, Takeshi NISHIMURA, Yuichi MORITA and Atsushi WATANABE

Fig. 10. Interpretation of the depth sections. (a)The profile NS, (b) the profile EW(W), (c) the profile EW(E), and (d) the
corresponding Fresnel radius in depth. The zones e correspond to gaps in the time sections. The region f is considered to
be artifact by the gap. Thick broken lines are boundaries of the single fold zone. Thin broken lines mark disruptions in
the offset range curve (Fig. 8).

Structure of Northeastern Sakurajima, South Kyushu, Japan, Revealed by Seismic Reflection Survey

249

Fig. 11. Distribution of the region h. The gray zone describes a possible magma supply path which inferred through this study.
The broken line represents the trace of Hidayatiʼs model (Hidayati et al., 2007). Major craters are described with hatchedcurves after Kobayashi (1982). “An” labels the craters of 1779-80(An-ei)ʼs eruption.

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(Editorial handling Yosuke Aoki)

桜島火山北東部の地震反射構造
筒井智樹・八木直史・井口正人・為栗 健・三ヶ田 均・
尾西恭亮・宮町宏樹・西村太志・森田裕一・渡邊篤志

桜島火山で反射法地震探査を行い深さ 11 km までの 3 つの反射断面を得た．2 本の地震探査測線は 221 点の臨時観測点と 8 点の発振点で構成された．1 本の測線は桜島の東麓に展開され，もう 1 本の測線は桜島の北山腹に展開された．これらのデータに通常の反射法地震探査の処理を行い，深度マイグレーションを施して深度断面を得た．得られた反射断面には 4 つの連続した反射面が認められる．このうち深さ 5.4 km 付近の反射面は桜島北東部で消失する．この反射面の不連続の下にはマグマかその供給経路が存在する可能性が高い．桜島のマグマ供給経路は先行研究のモデルより西側に位置している可能性がある．