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