Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 183-189

ArticleSeparate Quantification of Volcanic Gas Fluxes from Showa andMinamidake Craters at Sakurajima Volcano, Japan
Ryunosuke KAZAHAYA＊,＊＊, Toshiya MORI＊ and Keigo YAMAMOTO＊＊＊(Received October 11, 2011 ; Accepted December 26, 2012)
We conducted SO2 flux measurements at Sakurajima volcano, Japan during 2007-2010 to ascertain differences inthe degassing activity at two craters of the volcano: Minamidake and Showa craters. Recent developments in SO2visualization techniques using a UV camera observation system enabled us to quantify the SO2 flux from each crater.
SO2 flux reflects conditions of the volcanic conduit. Therefore, a separate estimate of the gas fluxes from different
vents is useful to gain insight into conditions within the volcano. Sulfur dioxide flux from Showa crater ranged from afew hundred to several thousand ton/day. In contrast, sulfur dioxide flux of 100-500 ton/day from Minamidake crater
remained at a lower level. These results suggest that degassing conditions (i.e. permeability of the conduit or theamount of degassing magma) of Minamidake crater have remained stable. In contrast, the degassing activity of Showa
crater is probably variable and sensitive to volcanic activities in the crater. The difference implies that drastic variationsof Sakurajima volcanoʼs SO2 flux data, observed using a conventional traverse method with a UV spectrometer, resulted
from the Showa craterʼs SO2 flux, not the Minamidake craterʼs SO2 flux.
Key words : volcanic gas, UV camera, SO2 flux, remote sensing

1．Introduction
Volatile components are dissolved in magma underpressure. As magma ascends to the shallow region of avolcanic edifice, the magma begins to decompress, thereby
causing exsolution of volatiles, which provides a driving
force for explosive eruptions. Therefore, measurements ofvolcanic gas emissions provide reliable information related
to the state of the magma and contribute to the monitoringof volcanic activity.
Sulfur dioxide (SO2), a major volcanic gas constituent
emitted from active volcanoes, can be detected using ultraviolet (UV) light absorption. Sulfur dioxide flux observations are used to monitor the variation of volcanic activity.
The instrument used most widely to measure SO2 flux is acorrelation spectrometer (COSPEC; Stoiber et al., 1983).
At the beginning of the 21st century, cheaper and morecompact UV spectrometer systems (e. g., mini-DOAS;
Galle et al., 2002; COMPUSS; Mori et al., 2007) weredeveloped to replace COSPEC. These advancements havefacilitated data collection and have increased the numberof observations. Although these developments have been
innovative, the production of high temporal resolution SO2flux measurements remains challenging as UV spectrometers scan volcanic plumes. Recently developed UV＊The University of Tokyo, Tokyo, Japan
Now at Institute of Seismology and Volcanology, Facultyof Sciences, Kyushu University.
＊＊
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Kagoshimacamera observation systems (Bluth et al., 2007; Mori andBurton, 2006) can provide images of the SO2 column
amount and measure SO2 flux at high frequencies (up to 2Hz). Recent studies using this new tool have revealed
links between volcanic gas emissions and seismic signals(e.g., Kazahaya et al., 2011; Nadeau et al., 2011).
Sakurajima volcano is an active volcano in southern
Kyushu, Japan. This stratovolcano consists of two adjoining edifices: Kitadake and Minamidake. Previously,
the main active crater had been Minamidake crater, which
generated numerous eruptions since 1955. After the 1990s,
the volcanic activity subsided to moderate levels. Showacrater, located east of Minamidake crater, awakened fromabout 60 year dormancy in June 2006. The main volcanic
activity shifted from Minamidake crater to Showa crater.
In contrast to the now active Showa crater, few explosions
have occurred at Minamidake crater since 2006 (JMA,2007, 2008, 2009, 2010).
The two craters are the major degassing source of Sakurajima volcano since June 2006. Sulfur dioxide flux is controlled by the several degassing parameters. One candidate is permeability of a volcanic conduit (e.g. Edmonds etal., 2003). Because these two craters have different eruptive behaviors, as described above (e.g. number of erupJapan.
Corresponding author: Ryunosuke Kazahayae-mail: kazahaya@sevo.kyushu-u.ac.jp