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 ata 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, KagoshimaUniversity, 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