Bull. Volcanol. Soc. Japan
Vol. 57 (2012) No. 4, pp. 227-234

LetterSeismic Velocity Structure of the Crust Beneath the Aira Caldera
in Southern Kyushu by Tomography of Travel Timesof Local Earthquake Data
Paul Karson ALANIS＊†, Hiroki MIYAMACHI＊, Hiroshi YAKIWARA＊, Kazuhiko GOTO＊,
Reiji KOBAYASHI＊, Takeshi TAMEGURI＊＊ and Masato IGUCHI＊＊(Received October 1, 2012 ; Accepted December 7, 2012）
We applied the tomography method to the P- and S-wave arrival times of 829 local earthquakes observed at 101
stations in central and southern Kyushu, and revealed the detailed three-dimensional seismic velocity structure of thecrust, especially the region beneath the Aira caldera. The structure obtained beneath the Aira caldera was characterized
by a compacted high Poisson’s ratio zone at about 20 km depth, suggesting fluid saturation such as partial melts relating
to volcanism. We also found that the low frequency earthquakes occurred so as to infill the lower crust between thehigh Poisson’s ratio zone and the moho discontinuity beneath the Aira caldera. It was also obvious that earthquakes
clearly concentrated in regions with low Vp/Vs (low Poisson’s ratio) in the upper crust of the whole of southern
Kyushu.
Key words : Aira caldera, Sakurajima, velocity structure

1．Introduction
Southern Kyushu (Fig. 1) is bordered by the N40° E
trending Nankai Trough where the Philippine Sea (PHS)plate subducts to a direction of N50° W beneath the Eurasian plate at the rate of 4-5 cm/yr (Seno et al., 1993).
Associated with this subduction a nearly straight chain of
active Quaternary volcanism runs almost parallel to thetrough in the central part of the island. In southern
Kyushu, there are four large calderas: the Kakuto, the Aira,
the Ata and the Kikai calderas. Large-scale pyroclastic
eruptions about 29,000 years before present in the calibrated age led to the formation of the Aira caldera(Aramaki, 1984; Okuno, 2002). Especially, Sakurajima
volcano in the Aira caldera has been very active.
Using a seismic velocity inversion technique, Sadeghi etal. (2000) obtained a three-dimensional (3-D) seismic
structure of east China and western Japan and found highvelocity zones (HVZs) existed beneath Kyushu due to thesubducting PHS plate, whereas low velocity zones (LVZs)
existed in western Kyushu, which they cited as evidence ofmantle upwelling and back-arc spreading. Yakiwara(2000) reported low P- and S-wave velocity zones beneath
Unzen, Aso, Kirishima and Sakurajima volcanoes and＊Graduate School of Science and Engineering, Kagoshima
University, Korimoto 1-21-35, Kagoshima 890-0065,
Japan.
＊＊
Sakurajima Volcano Research Center of DPRI, Kyoto
University, Sakurajima-Yokoyama, Kagoshima 891-1419,
Japan.

suggested the presence of partial melts. Meanwhile, Salahand Seno (2008), Tahara et al. (2008), Wang and Zhao
(2006) and Xia et al. (2008) modeled the mantle wedge
beneath Kyushu as low Poisson’ s ratio zones and concluded them to be due to fluid saturation from the dehydration of the subducting PHS plate. Wang and Zhao
(2006) also indicated widespread low velocity zones
beneath volcanoes in Kyushu. All these previous studies
pointed out that the low velocity zones revealed by their 3D tomography were closely related to volcanism in Kyushu.
In this study we focused on obtaining a detailed 3-Dseismic velocity structure up to 50 km depth underneathsouthern Kyushu, specifically the Aira caldera, by tomography of arrival times from local earthquake data withfiner grid configuration than that in the previous tomography studies. The obtained velocity structure should
reveal the velocity variation related to the volcanic andseismic activity in southern Kyushu.
2．Data and Methodology
In order to reveal the crustal structure in and around theAira caldera in southern Kyushu, we set up a wide study
area with a range of 30° N-32.25° N and 129.25° E-132.
†

present address : Philippine Institute of Volcanology andSeismology, Philippines
Corresponding author: Hiroki Miyamachi
e-mail: miya@sci.kagoshima-u.ac.jp

228 Paul Karson ALANIS, Hiroki MIYAMACHI, Hiroshi YAKIWARA, Kazuhiko GOTO, Reiji KOBAYASHI, Takeshi TAMEGURI and Masato IGUCHI

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Fig. 1. A map showing the study region in Kyushu.
Solid triangles are active volcanoes: (1) Kuju, (2)Aso, (3) Unzen, (4) Kirishima, (5) Sakurajima, (6)Kaimon, (7) Kikai, (8) Kuchino-erabujima. Areas
enclosed by broken lines show the Aso, Kakuto,
Aira, Ata, and Kikai calderas, respectively. PHS is
the subducting Philippine Sea plate. A-B is a section
line in Fig. 5.

25° E in a depth range from 0 to 200 km. We assigned thehorizontal and vertical grid nodes with a spacing of 15 km
and 10 km, respectively up to 50 km depth to get good
spatial resolution in the study area. On the other hand, we
allocated nodes with a spacing of 25 km in the verticaldirection to the deeper regions with a depth range from 50
to 200 km.
We then selected 829 earthquakes, which were locatedup to 200 km depth in central and southern Kyushu, takenfrom the Japan Meteorological Agency (JMA) earthquake
catalogue from 1997 to 2006, as shown in Fig. 2. These
events were recorded by a total of 101 seismic stations
deployed in the study area, belonging to JMA, National
Research Institute for Earth Science and Disaster Prevention (HINET), Kagoshima University (Nansei Toko
Observatory for Earthquakes and Volcanoes: NOEV), andKyoto University (Sakurajima Volcano Observatory: SVO).
P- and S-wave arrival times at all stations were picked byvisual inspection and total numbers of P- and S-wave
arrival time data used for the 3-D tomography were 20,302
and 13,196 in this study.
First we estimated a one-dimensional (1-D) 6-layered Pwave velocity model up to 50 km depth, which suggested
the thickness of the crust at around 30-40 km, by using the1-D inverse method (Crosson, 1976) of simultaneous
damped least squares estimation of event location andvelocity. This 1-D model with a constant Vp/Vs of 1.73

Fig. 2. Distribution of 101 seismic stations and 829
hypocenters used in the 3-D tomography.

was used as the initial velocity model for the 3-D
tomography using the program package SIMULPS14(Evans et al., 1994; Thurber, 1983; Thurber, 1993; Um andThurber, 1987). In order to ensure reliability of resolution,
we adopted the derivative weight sum (DWS) at each node
as an indication of ray density (Thurber and EberhartPhillips, 1999). As shown in Fig. 3, a minimum DWS was
assigned to be 2000 or 1000 for P-waves, and 1500, 1000
or 500 for S-waves. We simply assumed that the tomography yielded reliable solutions only at nodes with DWS
values larger than the minimum DWS.
3．Results
Figure 3 shows the horizontal distributions of P- and Swave velocities, and Vp/Vs on each plane with relocatedhypocenters from 0 to 100 km depth obtained by the 3-D
inversion. In the shallower sections at 0 and 10 km depth(Fig. 3a and 3b), LVZs (Vp and Vs change less than -10 %from the initial model) are found in the eastern part of
Kyushu, where basement rocks are made up of uplifted
sedimentary marine terraces (Nakada et al., 2002). The
figures also show that LVZs are distributed along the
volcanic front between Kirishima volcano to Kaimon
volcano (Fig. 1). Meanwhile, the hypocenters occurred in
the inland area are obviously concentrated in regions witha relatively higher velocity and a low Poisson’s ratio at a
depth of 10 km. A possible reason for this concentration isthat these regions are less fluid-saturated (a low Poisson’sSeismic Velocity Structure of the Crust Beneath the Aira Caldera in Southern Kyushu by Tomography of Travel Times of Local Earthquake Data

229

Fig. 3. Horizontal distributions of P-wave velocity (Vp), S-wave velocity (Vs), and Vp/Vs at a depth of (a) 0 km, (b) 10 km,(c) 20 km, (d) 30 km, (e) 40 km, (f) 50 km, (g) 75 km, and (h) 100 km. Solid circles, gray triangles and crosses indicate
relocated hypocenters, active volcanoes and grid nodes in the 3-D model, respectively. Focal depth range of the
hypocenters and the minimum value of DWS are also indicated in each section.

230 Paul Karson ALANIS, Hiroki MIYAMACHI, Hiroshi YAKIWARA, Kazuhiko GOTO, Reiji KOBAYASHI, Takeshi TAMEGURI and Masato IGUCHI

Fig. 3

continued.

Seismic Velocity Structure of the Crust Beneath the Aira Caldera in Southern Kyushu by Tomography of Travel Times of Local Earthquake Data

231

Fig. 4. An enlarged map showing the horizontal distribution of Vp/Vs around the Aira and the Ata calderas at a depthof (a) 10 km, (b) 20 km, and (c) 30 km. Open circles are the low frequency earthquakes occurred in and around theAira caldera, which are detected routinely by NOEV. A gray triangles are Kirishima, Sakurajima and Kaimon
volcanoes, respectively.

ratio is indicative of low fluid content) and thus more
prone to brittle deformation.
In order to detail the particulars of the main target, theAira caldera, we present an enlarged map showing thehorizontal Vp/Vs distributions in Fig. 4. At 20 km depth(Fig. 4b), the most interesting feature is that a distinctly
high Poisson’s ratio zone is located exactly beneath theAira caldera. This zone extends southward and connects
the Aira caldera with the Ata caldera. As shown in Fig. 3c,
it is found that large areas of low velocities exist beneath
Kirishima and Sakurajima volcanoes. In contrast, there isan unusually high velocity zone beneath Kaimon volcano
located at the edge of our model, and thus may not be
reliable. It is noted that the earthquake distribution in theinland area is fairly sparse, as shown in Fig. 3c.
At 30 km depth in Fig. 4c, a small zone with highPoisson’s ratio appears to remain beneath the Aira caldera.
This small zone is assumed to be a portion of the distinctly
high zone observed at 20 km depth. In Fig. 3d, thevelocity distribution in the western region (Satsuma
Peninsula) is quite different from that in the eastern region(Ohsumi Peninsula) : the western region is characterizedby high velocities of P- and S-waves, while the easternregion is characterized by low P- and S-wave velocities.
At the deeper sections seen in Figs. 3e to 3 g, these low
velocity anomaly zones in the eastern region progressively
shift westward due to the subducting PHS plate.
Hypocenters at 30 km depth are distributed along acoastal line with a NNE-SSW orientation in the easternregion, and at deeper ranges from 40 to 75 km the hypo-centers appear to concentrate along a boundary between
the high and low velocity zones. We interpret that these
HVZs with low Poisson’ s ratios are related to the subducting PHS plate.
4．Discussion and Summary
Figure 5 shows a vertical sections of the P- and S-wave
velocities, and Vp/Vs distributions along the A-B line
across the Aira caldera in Fig. 1, and hypocenters of low
frequency (LF) earthquakes, which occurred in and aroundthe Aira caldera, detected routinely by NOEV are alsoplotted. As described in the previous section, there is ananomaly with a very high Poisson’s ratio at about 20 km
depth beneath the Aira caldera, possibly suggesting thepresence of partial melts and the source of volcanism in thearea. It is also found that LF earthquakes occur in thelower crust. A deeper part of the focal zone of these LF
earthquakes appears to overlap the high Poisson’ s ratiozone observed at 30 km depth. These facts lead us topostulate that magma penetrating into the crust from theupper-most mantle may construct the high Poisson’s ratiozone at about 30 km depth, and move upwards through the
LF focal zone, and finally be stored at about 20 km depth.
Previous tomography studies such as Sadeghi et al.
(2000), Salah and Seno (2008), Tahara et al. (2008),
Wang and Zhao (2006), Xia et al. (2008) and Yakiwara(2000) have revealed the 3-D velocity structure of thesouthern part as well as the whole Kyushu region, and haveshown that the low velocity zone is widely distributed in
the crust and the upper-most mantle. Therefore, it is quite

232 Paul Karson ALANIS, Hiroki MIYAMACHI, Hiroshi YAKIWARA, Kazuhiko GOTO, Reiji KOBAYASHI, Takeshi TAMEGURI and Masato IGUCHI

Fig. 5. Cross sections of a distribution of (a) P-wave velocity, (b) DWS for P-wave, (c) S-wave velocity, (d) DWS for
S-wave, and (d) Vp/Vs, along the A-B line in Fig.1, respectively. Solid circles, a solid triangle and crosses indicate
relocated hypocenters, Sakurajima volcano and grid nodes, respectively. Open circles show the low frequency
earthquakes occurred in and around the Aira caldera. Thick broken line outlines the upper boundary of the
subducting PHS plate (Yakiwara, 2000).

difficult to recognize the relation between the low velocity
zone and the volcanism in southern Kyushu. However, thedistribution of the Poisson’s ratio obtained by using the finegrid configuration in this study can specify the extent of
the high Poisson’s ratio zone, which suggests possibility ofthe existence of the magma conduit and reservoir in the crust.
Ishihara (1990) described the magma supply system of
Sakurajima volcano wherein one magma reservoir islocated at about 4 km depth just beneath Sakurajima
volcano and another at 8 to 10 km depth beneath the Airacaldera. Although space resolution in our tomography isinsufficient to distinguish these magma reservoirs in theupper-most crust, our result puts forward a possibility ofthe deeper magma reservoir supplying two shallow magma
reservoirs previously found.

Seismic Velocity Structure of the Crust Beneath the Aira Caldera in Southern Kyushu by Tomography of Travel Times of Local Earthquake Data

Acknowledgements
We thank the Japan Meteorological Agency and National
Research Institute for Earth Science and Disaster Prevention for providing the earthquake data used in thisstudy. Figures in this paper were made using Generic
Mapping Tools software (Wessel and Smith, 1998). P. K.
Alanis would also like to thank the Japan International
Cooperation Agency for funding a Long-term Training
Program (2005/2008) to conduct this research at Kagoshima University. We are also very grateful to the editor
Dr. T. Hashimoto and two anonymous reviewers for theirvaluable comments and suggestions.
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(Editional handling Takeshi Hashimoto)234 Paul Karson ALANIS, Hiroki MIYAMACHI, Hiroshi YAKIWARA, Kazuhiko GOTO, Reiji KOBAYASHI, Takeshi TAMEGURI and Masato IGUCHI

走時データのトモグラフィーによる姶良カルデラの地殻速度構造
ポール・カルソン・アラニス・宮町宏樹・八木原寛・
後藤和彦・小林励司・為栗健・井口正人九州の中部および南部に展開している 101 カ所の地震観測点と 829 個の自然地震から得られた P 波と S
波の走時データにトモグラフィー法を適用し，南九州，特に，姶良カルデラ下の地殻速度構造を明らかにした．姶良カルデラの深さ約 20 km に Vp/Vs が高い小さな領域が存在し，この領域は火山活動に関連する部
分溶融領域であることを示唆している．さらに，この高 Vp/Vs 領域とモホ面を接続するように低周波地震
が下部地殻中に発生していることがわかった．また，南九州の上部地殻で発生する地震は低 Vp/Vs 領域に集中していることを明らかにした．