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Earth Planets Space, Vol. 65 (No. 6), pp. 563-571, 2013
doi:10.5047/eps.2013.03.014

Gravity variation around Shinmoe-dake volcano from February 2011 through March 2012—Results of continuous absolute gravity observation and repeated hybrid gravity measurements

Shuhei Okubo1, Yoshiyuki Tanaka1, Sadato Ueki2, Hiromitsu Oshima3, Tokumitsu Maekawa3, and Yuichi Imanishi1

1Earthquake Research Institute, The University of Tokyo, Tokyo 113-0032, Japan
2Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
3Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

(Received October 27, 2012; Revised February 24, 2013; Accepted March 22, 2013; Online published July 8, 2013)

Abstract: We report here on continuous absolute gravity measurements made between February 2011 and March 2012 and repeated relative gravity measurements in the vicinity of Shinmoe-dake volcano, which commenced erupting in late January 2011. We find that 20 of 24 eruptive events are associated with precursory short-term gravity decreases occurring over 5-6 hours followed by quick recoveries lasting 1-2 hours. Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm. To isolate the gravity signal associated with volcanic processes, we compared gravity measurements made at 15 sites in March 2011 and again in March 2012. The gravity changes and crustal deformation observed during the one year period are well explained by 6 × 106 m3 inflation of a magma reservoir at a depth of 9 km and intrusion at shallower depths of a dike with dimensions of 10 km × 0.5 km × 0.5 m.
Key words: Absolute gravity, precursory gravity signal, magma transport, Shinmoe-dake volcano.


Corresponding author E-mail: okubo@eri.u-tokyo.ac.jp


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