Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 115-135

ArticleCharacteristics of Volcanic Activity at Sakurajima VolcanoʼsShowa Crater During the Period 2006 to 2011
Masato IGUCHI, Takeshi TAMEGURI＊, Yusaku OHTA＊＊, Sadato UEKI＊＊ and Shigeru NAKAO＊＊＊(Received September 27, 2011 ; Accepted January 8, 2013)
Eruptive activity at the Showa crater of the Sakurajima volcano has steadily increased since it resumed in June
2006, and 2718 vulcanian eruptions occurred during the period from 2008 to 2011. In this paper, we clarify thecharacteristics of vulcanian eruptions at the Showa crater based mainly on ground deformation. A long-term extensionof the ground of the Aira caldera and Sakurajima, repeating quasi-annual cycles of minor inflation-deflation, wereobtained by GPS, tilt and strain observations. The inflation event that started in October 2009 was the largest. Majorpressure source was estimated to be located at a depth of 12 km beneath the Aira caldera for the inflation and a minor
source was obtained a depth of 5 km at the northern flank of Kitadake in addition to a source beneath Minamidakewhich has been know by previous studies. The magma plumbing system is composed of a major magma reservoir at a
depth of ≈10 km beneath the Aira caldera and additional magma reservoirs at depths of around 5 km beneath thesummit area from the north flank of Kitadake to Minamidake. Strain changes which indicate inflation were detectedprior to explosions and the inflation strain lasted mostly 1 h. The strain changes were caused by a shallow pressuresource less than 1.5 km. The inflation occasionally continued for more than 7 h with an addition of inflation of a deep
source (4 km), which corresponds to the magma reservoir beneath Minamidake. The conduit to the Showa crater may
be branched from the magma reservoir beneath Minamidake or from the major conduit connected to it. When
inflationary ground deformation progressed at a high rate, the eruptive activity reached a peak from December 2009 toMarch 2010. This suggests that the accumulation of magma beneath the central cones of the Sakurajima volcano
progressed simultaneously to a discharge of magma. The simultaneous progress of the accumulation and discharge ofmagma and the frequent occurrence of small vulcanian eruptions may be related to the small open conduit.
Key words : Sakurajima volcano, Showa crater, Minamidake crater, ground deformation, magma supply, vulcanianeruption

1．Introduction
Sakurajima is a post-caldera volcano formed at thesouthern rim of the Aira caldera. The central cones areKitadake, Nakadake and Minamidake. Kitadake is theoldest cone. All the eruptions since the 8th century haveoccurred at the summit area of Minamidake and its flanks,including the sea area.
Eruptive activity has taken place continuously at theMinamidake crater since 1955. The eruptive activity ischaracterized by frequent activities of a vulcanian type,
and the crater was quite active during the period from 1972
to 1992, with 4919 vulcanian eruptions and 2.4×108 tonsof volcanic ash being emitted. Of particular note, 498
vulcanian eruptions occurred in 1974, 415 in 1983 and 452in 1985. The mechanisms of the vulcanian eruptions havebeen studied mainly by seismological and geodetic obser＊

Sakurajima Volcano Research Center, DPRI, Kyoto Univ.,
1722-19, Sakurajima-Yokoyama, Kagoshima 891-1419,Japan.
＊＊
GraduateSchool of Science, TohokuUniv., 6-3 Aramakiaza Aoba, Aoba-ku, Sendai 980-8578, Japan.

vations. Tameguri et al. (2002) analyzed the explosion
earthquakes associated with the vulcanian eruptions andproposed a four-source model with moment tensors toexplain the waveforms of the explosion earthquakes.
However, this seismological method cannot cover theentire process of vulcanian eruptions due to the insufficientfrequency range of even the most broadband seismometer.
Ishihara (1990) detected inflation of the ground prior to thevulcanian eruptions and the deflation associated with themby using water-tube tiltmeters and extensometers sited in
an underground tunnel, and interpreted the inflation anddeflation as being caused by magma intrusion at a depth of
2-6 km and by ejection of the magma product, respectively. Iguchi et al. (2008c) proposed a model for theprocess of vulcanian eruptions in which an explosiveeruption is triggered by minor deflation, which is induced＊＊＊

GraduateSchool of Science and Engineering, Kagoshima
Univ., 1-21-24, Korimoto, Kagoshima 890-8580, Japan.
Corresponding author: Masato Iguchi
e-mail: iguchi@svo.dpri.kyoto-u.ac.jp