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 threeprofiles. 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 (Iguchiet 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.
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Fujitsu Software Technologies Limited.
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Disaster Prevention Research Institute, Kyoto University.
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Graduate school of Engineering, Kyoto University.
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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.
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Earthquake Research Institute, University of Tokyo.
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Corresponding author: Tomoki Tsutsui
e-mail: tomoki@gipc.akita-u.ac.jp