Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 163-181

ArticleProcesses Prior to Outbursts of Vulcanian Eruption
at Showa Crater of Sakurajima Volcano
Akihiko YOKOO＊,＊＊, Masato IGUCHI＊＊, Takeshi TAMEGURI＊＊ and Keigo YAMAMOTO＊＊(Received September 27, 2011 ; Accepted December 26, 2012)
Showa crater of Sakurajima volcano became active in June 2006 after 58 years of quiescence. From multiparametric geophysical observations, we have identified the processes that typically occur prior to an explosive eruption
at the crater. A few hours prior to the onset of an eruption, magma starts to migrate and accumulates at a depth of about
1 km. This accumulation of magma can be clearly observed in strain change records as an inflation process. Several
tens of minutes prior to an eruption, the SO2 gas emission rate gradually decreases, indicating that a sealing process istaking place in the crater bottom as the eruption nears. During the same time period, the volcanoʼs inflation rate starts toaccelerate due to the formation of a plug above the conduit that prevents the gas from escaping, with the result that a gas
pocket forms beneath the crater. In nighttime events, a volcanic glow is also seen, which weakens and then disappears.
A few minutes prior to an eruption, a small tremor starts to occur. Its amplitude grows as the strain changes frominflation to deflation as the stored gas is released through new fractures within the plug that had been confining the gas
pocket, leading to a minor depressurization in the conduit. Then, an expansion process starts, that could explain
seismically the first motion of an explosion earthquake. This is probably when the effect of depressurization downward
from the crater bottom reaches the magma head and a sudden magma expansion with degassing starts. After a shortperiod (about half a second), this expanding magma rises and pushes the gas pocket upward, leading to a swelling of thecrater ground along with the radiation of the preceding phase of infrasound waves, and then a breakup occurs. After theplug fails due to deformation, the accumulated gasses and expanding magma are ejected together from the crater as thesurface eruption phenomena starts.
Key words : Vulcanian eruption, eruption process, Sakurajima volcano, Showa crater

1．Introduction
Recent interdisciplinary and multi-parametric observations are attempting to elicit in detail the volcanic eruption
processes at several active volcanoes including Stromboli,Italy (Calvari et al., 2008) and Soufrière Hills, Montserrat(Voight and Sparks, 2010).
In Japan, Iguchi et al. (2008a) discussed the eruption
process of the summit crater of Sakurajima volcano, basedon integrated geophysical observations, as follows. The
process commences with a gradual volcano inflation,
which is thought to be induced by a combination of theeffects of the ascent of magma and the accumulation of
volcanic gas being emitted from the magma. Ishihara(1990) argued that such a gas accumulation leads to theformation of a “gas pocket” beneath the crater bottom.
The term “gas pocket” corresponds to a shallow transitionzone characterized by complex mingling between vesicularand dense magma a few hundred meters thick, which was＊

Aso Volcanological Laboratory, Institute for Geothermal
Science, Graduate School of Science, Kyoto University,Minami-Aso, Kumamoto 869-1404, Japan.
＊＊
Sakurajima Volcano Research Center, Disaster, Prevention Research Institute, Kyoto University, 1722-19, Sakura

mentioned as occurring at the Soufrière Hills volcano(Burgisser et al., 2011). This zone is covered by a dense
and strongly degassed plug a few tens of meters thick.
When the accumulated pressure inside the gas pocket
exceeds the strength of the plug as gas accumulates, afailure of a tiny part of the plug, such as a fracture within
it, may occur (Lyons et al., 2012). Then, a minor amountof gas will start escaping into the atmosphere. This
leakage of gas induces a pressure decrease in the uppermost part of the conduit as well as within the gas
pocket. It has been recognized in the strain change 1.5
min before an eruption (Iguchi et al., 2008a). The depressurization in the conduit can then propagate deeper into thevolcano as an expansion wave (Turcotte et al., 1990). When
this happens, a sudden outgassing of water-saturated magma
occurs at a certain depth, which corresponds to the onset ofan explosion earthquake in the conduit around 2 km beneath Sakurajima (Ishihara, 1985; Tameguri et al., 2002).
jima Yokoyamacho, Kagoshima 891-1419, Japan.
Corresponding author: Akihiko Yokooe-mail: yokoo@aso.vgs.kyoto-u.ac.jp