Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 227-237

ArticleShallow Velocity Structure Beneath the Aira Caldera and Sakurajima Volcano
as Inferred from Refraction Analysis of the Seismic Experiment in 2008
Hiroki MIYAMACHI1）, Chisato TOMARI1）, Hiroshi YAKIWARA1）, Masato IGUCHI2）, Takeshi TAMEGURI2）,
Keigo YAMAMOTO2）, Takahiro OHKURA3）, Takashi ANDO3）, Kyosuke ONISHI4）, Hiroshi SHIMIZU5）,
Yusuke YAMASHITA5）, Haruhisa NAKAMICHI6）, Teruo YAMAWAKI7）, Jun OIKAWA8）, Sadato UEKI9）,
Tomoki TSUTSUI10）, Hitoshi MORI11）, Makoto NISHIDA12）, Hideyuki HIRAMATSU12）,
Tomoyuki KOEDA12）, Yoshirou MASUDA12）, Kouji KATOU12）,
Kengo HATAKEYAMA12）and Tetsuo KOBAYASHI1）(Received December 24, 2010 ; Accepted July 21, 2011)
We performed refraction analysis for the first P-wave arrival time data observed in the seismic experiment in 2008,
and estimated a shallow velocity model up to 3 km depth beneath the Aira caldera and Sakurajima volcano. We foundthat a basement layer with a velocity of 4.6-5.0 km/s, which corresponds to geologically the Shimanto Group, inclines
toward the central part of the Aira caldera. A low velocity zone with a velocity of 4.2-4.4 km/s is located in a depth
range 1.5-3 km in the central part of the caldera. This low velocity zone suggests high activity of the magma plumbing
system from the deep magma reservoir distributed beneath the caldera. It is found that the basement layer steeply falls
down from 1 km to 2.5 km in depth along the northwestern boundary of the Kagoshima graben. The velocity structurein Sakurajima volcano is characterized by a zone with a velocity of 3.6-3.7 km/s. Moreover, we present a possibility
that the underground structure strongly restricts an expanse of a focal region of each different type of the volcanic
earthquakes.
Key words : Sakurajima, Aira caldera, velocity structure

1．Introduction
The Kagoshima bay occupies the southern part ofKagoshima graben (Tsuyuki, 1969). The Aira caldera, approximately 20 km in diameter, is located at the northernend of the Kagoshima bay. As presented in Fig. 1, Sakurajima volcano, located at the southern rim of the caldera, iswell known to be one of the most active volcanoes inJapan .
Many geological investigations have been carried out inand around Sakurajima volcano. Kobayashi (1988a, b) described detailed geology of Sakurajima volcano, and published a geological map of the volcano. According toHayasaka et al. (1978) and Aramaki (1984), the Cretaceous
Shimanto Group, a basement composed of alternation ofstrata of sandstone and mudstone, steeply inclines along
the boundary of the Kagoshima graben and is considered
to be located at a depth more than 1 km beneath Sakurajima.
Yokoyama and Ohkawa (1986) carried out gravity
measurements in the Aira caldera and showed the regional
gravity anomaly distribution. Their result shows that thegravity basement beneath Sakurajima volcano is located at
a depth of about 2.5 km. Miyamachi et al. (2000) presented the detailed gravity anomaly distribution in Sakurajima island from the high dense gravity measurements andestimated the location of a vent of “Satsuma (P14, Sz-S)”,
which is the most voluminous pumice fall deposit from
Sakurajima volcano (Kobayashi, 1988a). Komazawa et al.

Graduate School of Science and Engineering, Kagoshima
University, Korimoto 1, Kagoshima 890-0064, Japan.
2）
Disaster Prevention Research Institute, Kyoto University.
3）
Graduate School of Science, Kyoto University.
4）
Graduate School of Engineering, Kyoto University.
5）
Graduate School of Science, Kyushu University.
6）
Graduate School of Environmental Studies, Nagoya University.
7）
Volcanic Fluid Research Center, Tokyo Institute of Technology.

Earthquake Research Institute, University of Tokyo.
Graduate School of Science, Tohoku University.
10）
Faculty of Engineering and Resource Science, Akita University.
11）
Graduate School of Science, Hokkaido University.
12）
Japan Meteorological Agency.

1）

8）
9）

Corresponding author: Hiroki Miyamachi
e-mail: miya@sci.kagoshima-u.ac.jp

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MIYAMACHI, TOMARI, YAKIWARA, IGUCHI, TAMEGURI, YAMAMOTO, OHKURA, ANDO, ONISHI, SHIMIZU, YAMASHITA, NAKAMICHI,YAMAWAKI, OIKAWA, UEKI, TSUTSUI, MORI, NISHIDA, HIRAMATSU, KOEDA, MASUDA, KATOU, HATAKEYAMA and KOBAYASHI(2008) carried out gravity measurements in the remote areaof Sakurajima and reported that the gravity basement isshallower than 1 km. They also suggested that there ismagma chamber beneath the active crater of the Minamidake in the north or east area of Sakurajima island.
Ishihara (1988) presented from geophysical studies thata magma reservoir was located at a depth range deeper
than 3-4 km below sea level beneath the summit of
Sakurajima volcano. Recently, Hidayati et al. (2007)
studied volcanic and tectonic earthquakes and geodetic
data observed in and around Sakurajima volcano, andproposed a hypothetical model on the magma supply
system: two magma reservoirs are located at a depth of 5
km beneath Sakurajima volcano and 11 km at the center ofthe Aira caldera, respectively, and both the reservoirs areconnected through a tensile fault.
Velocity structures in and around the active volcanoes inJapan have been investigated by means of artificial
explosive sources, mainly by the National Project for thePrediction of Volcanic Eruption: for example, Usu volcano
by Onizawa et al. (2007), Suwanosejima volcano by
Yakiwara et al. (2009), Hokkaido Komagadake volcano
by Onizawa et al. (2009), and Asama volcano by Aoki etal. (2009).
As mentioned above, images of the understructure of the
Aira caldera and Sakurajima volcano have been presented
by means of geological, gravity and/or geodetic data.
However, it is necessary to increase spatial resolution ofthese models. In 2008, Iguchi et al. (2009) conducted aseismic experiment using artificial sources at Sakurajima
volcano and the Aira caldera, in order to achieve threeobjectives: the first is to reveal the basement structure of
the Aira caldera, the second is to detect magma supply
system of magma reservoirs beneath the Aira caldera and
Sakurajima volcano, and the third is to clarify magma pathfrom the magma reservoir beneath the Aira caldera to thatbeneath Sakurajima volcano. In this study, we focus toaccomplish the first objective, the shallow and basement
velocity structure beneath Sakurajima volcano and theAira caldera, by analyzing the first P-wave travel times
observed in the seismic experiment.
2．Seismic experiment and data acquisition
A seismic experiment, which was designed to obtain theseismic data for the 2-D refraction, 2-D reflection, and 3D tomography analyses, was performed in and around
Sakurajima volcano in 2008 (Iguchi et al., 2009). As
shown in Fig. 1, 8 large explosive shots with more than
200 kg charges for the refraction profiles in and around theAira caldera and 7 small explosive shots with 20 kg
charges for the reflection profiles in Sakurajima Island
were located. In the refraction profiles, 426 temporary
seismic stations with a 2 Hz vertical seismometer in theland area and 32 Ocean Bottom Seismographs with a 4.5
Hz three-component seismometer in the sea area were

Fig. 1. A map showing configuration of the shots andtemporary seismic stations of the seismic experimentin 2008. (a) Configuration of refraction profiles from
Profile-I to Profile-IV in and around the Aira caldera
and Sakurajima Island. Stars are the shot locations.
Solid and open circles are temporary seismic stations.
Stations marked by solid circles are available for our
refraction analysis in this study. Shaded rectangularareas on Profile-II and Profile-III indicate low velocity
zones with a velocity of 4.2-4.4 km/s and 3.6-3.7 km/s
revealed by this study, respectively. The rectangle
enclosed by solid lines in the figure describes an areain (b). (b) Configuration of reflection profiles, ProfileNS and Profile-EW in Sakurajima Island.

Shallow Velocity Structure Beneath the Aira Caldera and Sakurajima Volcano
as Inferred from Refraction Analysis of the Seismic Experiment in 2008

installed. The station intervals were set to about 200 m in
Sakurajima Island, about 450 m outside Sakurajima Island,
and about 1 km in the sea area. Details of the experimentare given in Iguchi et al. (2009).
We manually picked the first P-wave arrivals of seismic
waves digitally recorded by a 250 Hz sampling. Next, weclassified the first arrivals into five ranks termed as A, B,
C, L, and X, according to the accuracy of the picking,where A, B, C, and L refer to the ones with picking errorsof less than 0. 01, 0. 01-0. 03, 0. 03-0. 1, and 0. 1-0. 2s,
respectively, and X refers to the ones which were unable tobe picked as first arrivals within 0.2s.
As shown in Fig. 1, we selected four profiles, Profile-I(S1-S5), Profile-II (S1-S8), Profile-III (S1-S2-S3), andProfile-IV (S4-S6), among many combinations of all large
shots for our refraction analysis, because these profiles
have sufficient amount of P-wave travel time data withgood quality and the station distribution on each profile isalmost linear. We noted that a profile connected between
S5 and S6 was excluded from our refraction analysis,
because the travel time data were obscure due to strongly
attenuated seismic waves. Fig. 2 shows examples of therecord sections of seismograms on the profiles. In addition, we used also the travel time data along two reflection
profiles, Profile-NS and Profile-EW, as shown in Fig. 1(b),for our refraction analyses. A dense station distribution
with an interval of 50-60 m along the reflection profiles is
expected to provide information of the upper-most subsurface velocity structure.
3．Travel time analysis
In order to construct a simple two-dimensional P-wave
velocity model along each profile, we carried out refraction analysis of the travel time data by a forward modelingtechnique following the three steps;
STEP 1:
We applied the classical travel time analysis “a layerstripping approach” (Ivandic et al., 2008) to the travel time
data in each profile. In Fig. 3(a), we showed examples of
the travel time curves along Profile-I (S1-S5) and ProfileII (S1-S8). From the travel time curves, we assumed a
two-layered model for Profile-I and a three-layered modelfor Profile-II. An apparent P-wave velocity for the upper
layer along each profile was estimated by the travel time
data observed at the stations located adjacent to the shots.
It is found that the apparent velocities in the upper layers
for all profiles take various values. We also distinguishedthe refracted phases from all data. It is noted that there is
no reverse data for the phases refracted through the thirdlayer in Profile-II.
STEP 2:
We estimated the least squares velocity for the refracted
layer and the time-terms at each stations along eachprofile, by applying the time-term method (Berry andWest, 1966; Mereu, 1966) to the travel time data of the229

refracted phases certified in STEP 1, as shown in Fig. 3(b).
We also showed the thickness of the upper layer converted
from the time-terms obtained, assuming that the mean
velocity for the upper layer near S1 is 2.5 km/s, in Fig. 3(c). In consideration of these results and the various
velocities for the upper layer, we constructed the initialrough model for the ray tracing analysis for STEP 3.
STEP 3:
We calculated the theoretical travel times for the initialmodel obtained in STEP 2 by the ray tracing computer
program `RAYINVRʼ designed by Zelt and Smith (1992).
We adjusted the model parameters, i.e. velocity values at
the assigned points and the depth distribution of thevelocity boundary in the model, by a trial-and-error
procedure so as to reduce the difference between thetheoretical and observed travel times.
4．P-wave velocity models
Fig. 4 shows a surface geological map in the inland
study area describing major rocks and geologic structure
modified from Uto et al. (1997). It is obvious that thesurface in the inland area is widely covered by Quaternary
pyroclastic flows and volcanic rocks. Under the surface
pyroclastic flows, Pleistocene sediments, tuff and tuff
breccia can be found. In the eastern part of the study area,
Late Cretaceous Shimanto Group (Kamae Subgroup) is
widely distributed. In geology, the Shimanto Group consisting of alternation of strata of sandstone and mudstone isregarded as a basement in south Kyushu. It is also
confirmed that the Shimanto Group goes underground in
the western and northern parts. There is an outcrop of the
Middle Miocene igneous rocks, penetrating the Shimanto
Group, in the southeastern part.
We show the final velocity model, ray diagrams for twoor three shots, and a reduced travel time graph obtained
along each profile in Figs. 5 to 10.
Profile-NS and Profile-EW
Figs 5 and 6 show the subsurface velocity models along
the Profile-NS and Profile-EW, respectively. A subsurface with a thickness of 100-200 m consists of twolayers; an upper layer with a velocity of 0.8-1.0 km/s in
Profile-EW and 1.0-1.2 km/s in Profile-NS and a lower
layer with a velocity of 1.1-1.3 km/s in Profile-EW and 1.
3-1.5 km/s in Profile-NS (Figs. 5(a) and 6(a)). It is also
found that the lower layer between S9 to S10 on ProfileNS significantly thickens to 400 m. Based on the geology
around the area, we think that the upper layer is composed
mainly of a lava bed and unconsolidated volcaniclastic
material, and that the lower layer is composed of indurated
tuff and tuff breccia. A bottom layer below the subsurface
usually has a velocity of 2.5-2.8 km/s in Profile-EW and2.4-2.7 km/s in Profile-NS.
Profile-I (S1-S5)
This profile is located outside of the northern edge of the
Aira caldera. As shown in Fig. 7, the model simply

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MIYAMACHI, TOMARI, YAKIWARA, IGUCHI, TAMEGURI, YAMAMOTO, OHKURA, ANDO, ONISHI, SHIMIZU, YAMASHITA, NAKAMICHI,YAMAWAKI, OIKAWA, UEKI, TSUTSUI, MORI, NISHIDA, HIRAMATSU, KOEDA, MASUDA, KATOU, HATAKEYAMA and KOBAYASHI

Fig. 2. Record sections for (a) shot S5 along Profile-I, (b) shot S1 along Profile-II, (c) shot S1 along Profile-III, and (d) shotS4 along Profile-IV. Amplitude of each trace is normalized by its maximum and travel times are reduced by 5 km/s.

Shallow Velocity Structure Beneath the Aira Caldera and Sakurajima Volcano
as Inferred from Refraction Analysis of the Seismic Experiment in 2008

231

Fig. 3. Examples of the results in Profile-I and Profile-II obtained by classical travel time analysis. (a) the travel time curves(open circles and triangles) and those (solid circles and triangles) modified by “a layer-stripping approach” (Ivandic etal., 2008). Apparent velocities for the upper layers are also shown in the figures. (b) the obtained distribution of thetime-terms (solid circles). The travel time data in the range indicated by a hatched line are applied to the time-term
method (Berry and West, 1966; Mereu, 1966). The estimated least squares velocity (L.S.V.) is also shown in the figures.
(c) the depth distribution of the boundary (solid circles) between the upper and lower layers. The velocity of 2.5 km/s forthe upper layer is assumed. Open triangles indicate the station heights.

consists of two layers: the first layer with a velocity of
2.3-2.8 km/s is a sedimentary surface and the second layer
with a velocity of 4.9-5.2 km/s is the basement. The boundary between these two layers gradually inclines to a depthof 1-1.4 km toward west (Fig. 7(a)). Kobayashi and Yano
(2007) compiled the boring core data in south Kyushu, andpresented a depth distribution of the geological basement
layer. According their result, an upper boundary of the
Shimanto Group is inferred to be located at about 1 km
depth in the central part of the profile. Threfore, we
recognize that the first layer is composed mainly of Middle
Pleistocene indurated tuff and tuff breccia, and that thesecond layer corresponds to the Shimanto Group.
Profile-II (S1-S8)
This profile comes across the central part of the Airacaldera. Fig. 8 indicates that the second layer with a velocity of 2.5-2.8 km/s is about 1 km thick in the northwest
and central parts of the Aira caldera, however it abruptly
gets thin in the southeastern part of the profile. Comparing
with the velocity model beneath S1 in Profile-I, we thinkthat the third layer corresponds to the Shimanto Group. As
shown in Fig. 4, an outcrop of the Shimanto Group can beseen near S8. The layer is also found to have a strong
lateral velocity variation: a velocity of 4.2-4.4 km/s in thecentral part of the profile is significantly lower than that of
4.7-4.9 km/s in both sides of the profile. It is difficult toconfirm the boundary of the low velocity zone, due to lackof shots in the sea area. However, as shown in Fig. 2(b), it
is clear that seismic waves passing through the central areain the caldera are especially attenuated due to an effect ofboth of the low velocity zone and the thick unconsolidated
sedimentary layer in the sea area. Because of a sparse ray
distribution, a velocity of the fourth layer and a depth
distribution of the upper boundary of the layer can not be
explicitly determined.
Profile-III (S1-S2-S3)
At some parts of the profile, a sedimentary layer with avelocity of 1.8-2.1 km/s is the first layer, as shown in Fig.
9. A velocity of the second layer near S1 is about 2.7-3.0
km/s, whereas it decreases to 2.2-2.8 in Sakurajima volcano. The second layer in the Sakurajima area is composed mainly of lava flow and pyroclastic deposits, whereas
in other areas it is composed mainly of indurated tuff andtuff breccia with some amount of lava flows.
The most important feature is a difference of velocities
in the third layer between inside and outside of the
volcano. The third layer in a zone between S1 and S2,
considered to be composed of the Shimanto Group, isfound to have a velocity of more than 4. 6-4. 7 km/s atabout 1 km depth. Moreover, it is certain that this 4.6-4.7
km/s layer abruptly deepens under Sakurajima volcano.
On the other hand, in a zone just beneath the volcano

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MIYAMACHI, TOMARI, YAKIWARA, IGUCHI, TAMEGURI, YAMAMOTO, OHKURA, ANDO, ONISHI, SHIMIZU, YAMASHITA, NAKAMICHI,YAMAWAKI, OIKAWA, UEKI, TSUTSUI, MORI, NISHIDA, HIRAMATSU, KOEDA, MASUDA, KATOU, HATAKEYAMA and KOBAYASHI

Fig. 5. P-wave velocity model for Profile-NS. (a) The
final P-wave velocity model. Numerical values in the
figure indicate velocities in km/s. (b) Ray diagrams forS10 (blue) and S12 (red) for the final velocity model.
(c) Observed (black bars) and calculated (circles)
travel times. Travel times obtained for S9, S10, S11
and S12 are used for the refraction analysis. The ray
diagrams for S10 and S12 are only presented in (b).

Fig. 4. Outline of a distribution of major rocks and geologic structure in the study area (modified after Uto etal., 1997). Stars and solid circles indicate the shot andstation locations along the refraction profiles, respectively. A shaded ellipsoid area is the Wakamiko caldera.

between S2 and S3, the velocity of the third layer isestimated to be 3.6-3.7 km/s at 1 km depth. These facts
are consistent with a schematic geological image of the
Kagoshima graben showing a step-like depression structure of the Shimanto Group toward the center of the graben
presented by Hayasaka (1982). However, the thickness ofthe 3.6-3.7 km/s layer remains obscure due to sparse ray
distribution as shown in the ray diagram in Fig. 9(b). The
low velocity layer underneath Sakurajima volcano may becomposed of caldera ponded deposits such as Middle
Pleistocene brecciated tuff, tuff breccia, and lava. We also
suppose that the boundary between the 4.6-4.7 km/s and 3.
6-3.7 km/s layers may correspond to the western margin of
the Kagoshima graben in topography.

Profile-IV (S4-S6)
This profile, running across the southern boundary of
the Aira caldera, is characterized by an existence of theinclined layer (the fourth layer) of high velocity (5.2-5.4km/s), as described in Fig. 10. This layer is distributed atabout 0.3 km depth near S4. It gradually descends toward
the northern part of the profile and reaches to a depth ofabout 2 km near S6. We also found that the second andthird layers, with a velocity of 2.4-2.7 and 4.3-4.5 km/s,
thicken from south to north, though the velocity of 4. 3
km/s in the third layer is not well constrained due to theinsufficient travel time data. The second layer is supposed
to be composed of volcaniclastic deposits. Geological outcrop near S4 also suggests that the third and fourth layers
may consist of the Shimanto Group and granitic rocks.
5．Discussion and Summary
We compared our velocity models with regional gravity
anomalies by Komazawa et al. (2008). As shown in Fig.
11, a positive high gravity anomaly with more than+30
mgal can be seen in the southern part of Profile-IV (S4S6), and the value of the anomaly decreases toward thecenter of the Aira caldera, and becomes negative near S6.
This gravity anomaly distribution is in accordance with thedepth distribution of the high velocity layer with a velocity
of more than 5.2 km/s deepening toward S6 from S4 in Fig.
10. In Profile-II, an extent of the negative gravity anomaly
in the sea area of the Aira caldera may be caused by the
thick sedimentary layer with a velocity of 2.5-2.8 kms/ inFig. 8. In particular, a location of the strongly negativeShallow Velocity Structure Beneath the Aira Caldera and Sakurajima Volcano
as Inferred from Refraction Analysis of the Seismic Experiment in 2008

233

Fig. 6. P-wave velocity model for Profile-EW. Explanation is the same as in Fig. 5. Travel times obtained for
S11, S13, S14 and S15 are used for the refraction
analysis. The ray diagrams for S11 and S15 are only
presented in (b).
Fig. 8. P-wave velocity model for Profile-II. Explanationis the same as in Fig. 5.

Fig. 7. P-wave velocity model for Profile-I. Explanation is
the same as in Fig. 5.

gravity anomaly with more than −20 mgal in the central
part of the Aira caldera is consistent with the distributionof the low velocity zone (4.2-4.4 km/s). It is found in thesea area near S2 in Profile-III that the gradient of thegravity anomaly is steep. This suggests that the underground structure in the area is complicated and varies
abruptly. The steep inclination of the basement layer with
a 4.6 km/s velocity toward Sakurajima volcano in Fig. 9
may reflect the complexity of the structure.
Komazawa et al. (2008) also estimated the depth distribution of the gravity basement in Sakurajima. According
to their result, the depth of the gravity basement in thestrongly negative gravity anomaly area in the northeastern

Fig. 9. P-wave velocity model for Profile-III. Explanationis the same as in Fig. 5. Travel times obtained for S1,S2 and S3 are used for the refraction analysis. A part of
the ray diagrams for S2 is neglected in (b).

sea area of Sakurajima is about 2 km below sea level,
which agrees with the depth distribution of the upperboundary of the high velocity layer with more than 5 km/s
in Profile-II. In the northwestern and western regions in
Sakurajima, Komazawa et al. (2008) estimated depth of
the gravity basement to be at about 0-1 km below sea
level. This depth corresponds to the upper boundary of the234

MIYAMACHI, TOMARI, YAKIWARA, IGUCHI, TAMEGURI, YAMAMOTO, OHKURA, ANDO, ONISHI, SHIMIZU, YAMASHITA, NAKAMICHI,YAMAWAKI, OIKAWA, UEKI, TSUTSUI, MORI, NISHIDA, HIRAMATSU, KOEDA, MASUDA, KATOU, HATAKEYAMA and KOBAYASHI

layer with a velocity of 3.6-3.7 km/s in Profile-III. On theother hand, Yokoyama and Ohkawa (1986) showed thegravity basement located at about 2. 5 km in depth justbeneath Sakurajima volcano, which is consistent with thedepth of the Shimanto Group with a velocity of 4.6 km/s
estimated in this study.
Fig. 12 illustrates a schematic shallow velocity image in
Sakurajima volcano and the Aira caldera presumed by thisstudy. Within Sakurajima volcano, the surface layer witha low velocity (1. 5-1. 8 km/s), composed of lava andvolcanic ash of Sakurajima volcano, is distributed andusually has a thickness of less than 100 meters. The

Fig. 10. P-wave velocity model for Profile-IV. Explanation is the same as in Fig. 5.

second layer with a velocity of 2.3-2.8 km/s is considered
to be composed of Pleistocene sedimentary rocks, Quaternary pyroclastic flow deposit, and volcanic rocks. The
velocity distribution of the third layer, which is considered
to be the basement in this study, is more complicated: inthe edge of the Aira caldera, a layer with a velocity of
4.6-5.0 km/s, considered to be composed of the Shimanto
Group, is widely distributed at 1-3 km depth, and in theFig. 11. Bouguer gravity anomaly distribution revealed by
Komazawa et al. (2008). Numbers in the figure
indicate the gravity anomaly in mgal. Stars and solid
circles indicate the shots and temporary seismic
stations used in this study, respectively. The figure ismodified after Komazawa et al. (2008).

Fig. 12. A schematic shallow velocity image in the Sakurajima volcano and the Aira caldera. Numbers in the figure indicate
the P-wave velocities in km/s. The upper enclosed area describes the focal regions of the shallow volcanic-tectonic(VT1), A-type (A1) and explosion (EX) earthquakes. The circular areas in the enclosed area correspond to the main
focal region of BL-type and BH-type earthquakes, respectively. The lower enclosed area is the focal region of the deep
volcanic-tectonic (VT2) and A-type (A2) earthquakes. Classification of volcanic earthquakes is referred to Iguchi (1994)
and Hidayati et al. (2007).

Shallow Velocity Structure Beneath the Aira Caldera and Sakurajima Volcano
as Inferred from Refraction Analysis of the Seismic Experiment in 2008

central part of the Aira caldera the low velocity zone with avelocity of 4.2-4.4 km/s is located. On the other hand, alayer with a velocity of 3.6-3.7 km/s and a thickness of
1-2 km is located just only beneath Sakurajima volcano.
As shown in Fig. 8 (Profile-II) and Fig. 10 (Profile-IV), it
is noted that the 3.6-3.7 km/s layer is not detected beneaththe eastern side of Sakurajima volcano. Therefore, we
deduce that the layer is locally distributed in the westernpart of the volcano. This is consistent with the geologicalimage of the Kagoshima graben by Hayasaka (1982),
showing that the Shimanto Group in the western area of
Sakurajima is notably deeper than that in the eastern area.
The fourth layer, the deepest layer revealed in this study,
generally has a high velocity of 5.2-5.4 km/s. This highvelocity layer deepens towards the center of the Aira caldera.
Previous studies for the velocity structures in the volcanic regions such as Unzen volcano (Nishi, 2002),
Hokkaido Komagadake volcano (Onizawa et al., 2009),
and Asama volcano (Aoki et al., 2009) indicate a common
feature that the high velocity basements uplift beneath thevolcanoes. However, our model shows no uplift of thebasement: the Shimanto Group with a velocity of 4.6-5.0
km/s is inclined toward the center of the Aira caldera. We
suppose that the inclination of the basement layer is causedby the predominant normal fault-type structure in the
Kagoshima graben. Geological studies such as Aramaki(1984) and Hayasaka et al. (1978) showed that theKagoshima graben was formed 3 million years ago andwas accumulated and filled up by the thick marine deposit
composed of tuffaceous sandstone and tuff breccia. About
29,000 years ago (Okuno, 2002), a series of large-scale
pyroclastic eruptions occurred at the northern end of the
Kagoshima bay and formed vast pyroclastic plateau
around the Aira caldera. After 3000 years dormant period,
Sakurajima started to erupt as a submarine volcano(Okuno, 2002). Therefore, we suppose that the magma of
Sakurajima volcano had penetrated through the thickmarine deposit in the Kagoshima graben. This may be acause of an existence of the low velocity zone with avelocity of 3.6-3.7 km/s just beneath Sakurajima volcano,as shown in our model.
Hidayati et al. (2007) and Iguchi et al. (2008) showed
the locations of two magma reservoirs: one reservoir islocated at a depth of 5 km beneath Sakurajima volcano andthe other is at the 11 km depth beneath the Aira caldera. It
is found that the low velocity zone revealed in the central
part of the Aira caldera is located over the deep magma
reservoir. As shown in Fig. 2(b), it is obvious that theseismic waves observed in the sea area are strongly
attenuated. In addition, it is also well known that there is acontinuing vigorous submarine fumarolic activity at the
Wakamiko caldera in the Aira caldera (Kobayashi, 1988a).
Though our refraction analysis can clarify the velocity
structure only up to 3 km depth, it is suggested that thehigh activity of the magma plumbing system connected to235

deep magma may be related to the low velocity zone
revealed in the central part of the Aira caldera.
Iguchi (1994) classified shallow volcanic earthquakes
occurred beneath the summit region of Sakurajima volcano
into four groups, A-type, BH-type (a high frequency Btype), BL-type (a low frequency B-type) and explosion
earthquakes, and revealed a precise distribution of theseearthquakes. According to the results in Iguchi (1994), Atype earthquakes seem to occur in two depth ranges: 0-1.5
km for shallow A-type and 2.4-3.4 km for deep A-type
earthquakes. BL-type and BH-type earthquakes are mainly
located in a depth range of 0-1. 2 km and 1. 3-2. 4 km,
respectively. A focal depth of explosion earthquakes is
0-2 km. Comparing these focal depth distributions with
our model, we found that the shallow A-type earthquakes
occur in the 2.3-2.8 km/s and 3.6-3.7 km/s layers, and the
deep A-type earthquakes in the 5. 2-5. 4 km/s layer, as
shown in Fig. 12. BL-type and BH-type earthquakes seemto be mainly distributed in the 2.3-2.8 km/s and the 3.63.7 km/s layers, respectively. The explosion earthquakes
are located in the 2.3-2.8 km/s and 3.6-3.7 km/s layers.
Ishihara (1988) and Hidayati et al. (2007) also showed aprecise distribution of volcano-tectonic (VT) earthquakes
beneath the summit region of Sakurajima volcano, andpointed out a seismic gap at about 2 km depth between theshallow and deep focal regions of VT earthquakes. Theyinterpreted the seismic gap as a small magma pocket. As
shown in Fig. 12, the shallow VT earthquakes are concentrated inside the low velocity zone with a velocity of 3.63.7 km/s. On the other hand, the deep VT earthquakes
mainly occur in the area with a velocity of 5.2-5.4 km/s. It
is obviously found that the seismic gap is located in thelayer with a velocity of 4.6-5.0 km/s directly below the
3.6-3.7 km/s zone. Because our seismic profile does notcross the summit region, it is difficult to directly estimate
the velocity in the seismic gap. However, it is expectedthat a velocity in the small magma pocket may be lower
than 4.6-5.0 km/s in the surrounding zone. These facts
indicate a possibility that the underground structurestrongly restricts an expanse of focal region of eachdifferent type of the volcanic earthquakes.
Finally, the first arrival time data of the refracted waves
can provide the shallow velocity model beneath the Airacaldera and Sakurajima volcano. In order to describe explicitly the shallow magma reservoir located at a depth of 5
km and the magma plumbing system in the velocity model,
it is necessary to analyze the later phases such as reflected
waves in the further study.
Acknowledgements
We wish to express our thanks to the participants of the
Sakurajima seismic experiment in 2008. We also thanktwo reviewers for their advice to improve greatly themanuscript. The seismic experiment in 2008 was supported by the National Project for the Prediction of Vol-

236

MIYAMACHI, TOMARI, YAKIWARA, IGUCHI, TAMEGURI, YAMAMOTO, OHKURA, ANDO, ONISHI, SHIMIZU, YAMASHITA, NAKAMICHI,YAMAWAKI, OIKAWA, UEKI, TSUTSUI, MORI, NISHIDA, HIRAMATSU, KOEDA, MASUDA, KATOU, HATAKEYAMA and KOBAYASHI

canic Eruption in Japan. Some figures are drawn by theGeneric Mapping Tools (Wessel and Smith, 1998).
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(Editional handling Takeshi Hashimoto)

屈折法地震探査による姶良カルデラと桜島火山の浅部速度構造
宮町宏樹・泊知里・八木原寛・井口正人・為栗健・山本圭吾・大倉敬宏・安藤隆志・

尾西恭亮・清水洋・山下裕亮・中道治久・山脇輝夫・及川森誠・平松秀行・小枝智幸・増田与志郎・加藤幸司・畠山謙吾・小林哲夫

済・西田純・植木貞人・筒井智樹・2008 年に実施された屈折法地震探査によって得られた P 波初動走時により，姶良カルデラおよび桜島火
山の深さ 3 km までの速度構造を推定した．本研究地域の基盤層である四万十層群は 4.6-5.0 km/s の P 波速度を持ち，姶良カルデラの中央部に向け傾斜している．姶良カルデラの中央部には，4.2-4.4 km/s の低速度域が深さ 1.5-3 km に存在している．そして，この低速度域はカルデラ下に存在する深部マグマ溜まりからのマグマ供給系が活発であることを示唆している．また，基盤層は鹿児島地溝帯の北西域の境界に沿って深さ 1 km から 2.5 km に急激に落ち込んでいることがわかった．桜島火山の速度構造は 3.6-3.7 km/s の領域が存在することで特徴づけられる．桜島火山の山頂直下で発生している火山性地震の震源域と速度構造の比較から，地下構造が種々の火山性地震の震源域の広がりに強い影響を与えていることを示した．

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