Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 153-162

ArticleAbsolute Gravity Variation at Sakurajima Volcano from April 2009
through January 2011 and its Relevance tothe Eruptive Activity of Showa Crater
Shuhei OKUBO＊, Takahito KAZAMA＊,＊＊, Keigo YAMAMOTO＊＊＊, Masato IGUCHI＊＊＊,Yoshiyuki TANAKA＊, Takayuki SUGANO＊, Yuichi IMANISHI＊, Wenke SUN＊,＊＊＊＊,
Mamoru SAKA＊, Atsushi WATANABE＊ and Shigeo MATSUMOTO＊＊＊＊＊(Received September 30, 2011 ; Accepted February 27, 2012)
We describe absolute gravity measurements performed from April 2009 through January 2011, and present
technical suggestions for carrying out continuous observations in a volcanic area. The results clearly show significant
gravity variations of as large as 30μgal during the observation period. Hydrological simulations reveal that about halfof the gravity change is attributable to groundwater disturbance. After correcting for this disturbance, the observed
variations in gravity can be divided into 5 separate phases. Phase I is a period with few eruptions, which extends fromApril to late June 2009 when an abrupt 10μgal gravity decrease was observed. During the succeeding phase II, fromJuly 2009 to May 2010, gravity oscillated about a mean value with an amplitude of 5μgal, while the monthly number ofexplosions at Showa crater dramatically increased from 50 to about 150. In phase III, which was a transient quiescentperiod, gravity increased by as much as 10μgal in a single month. This was followed by phase IV, during which there
was a steady gravity decrease until November 2010. During the final phase V, gravity remained almost constant until atleast January 2011. These five phases are closely linked to the eruptive activity at Showa crater. In fact, excellent
correlations are found among the records of absolute gravity, ejected weight of volcanic ash, ground tilt, and infrasoundair shock amplitude. The gravity data are transformed into changes in magma head height using a simplified line massmodel.
Key words : absolute gravity, magma head, open conduit, Sakurajima volcano, Showa crater

1．Introduction
Sakurajima volcano woke from a decade-long period ofdormancy in 2006. Many Vulcanian eruptions andpyroclastic flows from Showa crater have been recorded(Iguchi et al., 2010). The volcanic activity since 2006, andin particular since 2009, exhibited the following features:(1) Most of the eruptions occurred at Showa crater with
only a few exceptions at Minamidake crater, which had
been the major vent for a 50 year period since 1955.
(2) The annual number of explosive eruptions in 2010
marked the highest ever recorded in Sakurajima volcano
since 1955. Roughly 500 eruptions in 2009, 900 in 2010,
600 in 2011 as of September 1, 2011, are classified asexplosions with distinct infrasound pressure pulses.
Previously, only in 1960, 1974, 1983 and 1985 did thenumber exceed 400 although it never reached 500.
＊Earthquake Research Institute, University of Tokyo, 1-1-1,Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
＊＊
Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan.
＊＊＊
Sakurajima Volcano Research Center of Disaster Prevention Research Institute, Kyoto University, 1722-19,
Sakurajima-Yokoyama, Kagoshima 891-1419, Japan.

(3) Several parameters characterizing the intensity ofvolcanic activity since 2008 were smaller by a factor of 3
to 10 than those during the last active period from 1974 to1985 (Iguchi et al., 2010). For example, the total weightof ejected volcanic ash in 2009 is estimated to be only 3.2
million tons while in the 1980ʼs it was 10 to 30 million tons
every year. In addition, the volumetric changes of theinflation/deflation sources for individual explosions since2008 were only 102 to 104 m3 while those in the 1980ʼswere 103 to 105 m3. Furthermore, the overall seismicity
since 2008 was lower than that before 2007; it was not
unusual to count more than 2,000 earthquakes per month
before 2007 whereas this decreased to less than 800 since2008.
To summarize, although there have been more frequent
explosions since 2008, both the degree of seismicity and＊＊＊＊

Key Laboratory of Computational Geodynamics, Graduate
University of Chinese Academy of Sciences, 19A Yuquanlu,
Beijing 100049, China.
＊＊＊＊＊
Sumiko Consultant Corp., 2-9-7, Ikenohata, Taito-ku, Tokyo110-0008, Japan.
Corresponding author: Shuhei Okubo
e-mail: okubo@eri.u-tokyo.ac.jp