TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 65 (No. 6), pp. 539-550, 2013

Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano

Koki Aizawa1, Takao Koyama1, Makoto Uyeshima1, Hideaki Hase1,2, Takeshi Hashimoto3, Wataru Kanda2, Ryokei Yoshimura4, Mitsuru Utsugi5, Yasuo Ogawa2, and Ken'ichi Yamazaki4

1Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan
2Volcanic Fluid Research Center, Tokyo Institute of Technology, Ookayama 2-12-2, Meguro-ku, Tokyo 152-8551, Japan
3Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, N10W8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan
4Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
5Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, Minami-Aso, Kumamoto 869-1404, Japan

(Received November 1, 2012; Revised April 18, 2013; Accepted May 17, 2013; Online published July 8, 2013)

Abstract: Three sub-Plinian eruptions took place on 26-27 January 2011 at Shinmoe-dake volcano in the Kirishima volcanic group, Japan. During this event, GPS and tiltmeters detected syn-eruptive ground subsidence approximately 7 km to the WNW of the volcano. Starting in March 2011, we conducted broad-band magnetotelluric (MT) measurements at a site located 5 km NNW of the volcano, beneath which the Shinmoe-dake magma plumbing system may exist. In addition, temperature monitoring of fumaroles and hot-springs near the MT site was initiated in July 2011. Our MT data record changes in apparent resistivity of approximately ±5%, along with a ±1° phase change in the off-diagonal component of the impedance tensor (Zxy and Zyx). Using 1-D inversion, we infer that these slight changes in resistivity took place at relatively shallow depths of only a few hundred meters, at the transition between a near-surface resistive layer and an underlying conductive layer. Resistivity changes observed since March 2012 are correlated with the observed temperature increases around the MT monitoring site. These observations suggest the existence beneath the MT site of pathways which enable volatile escape.
Key words: Magnetotellurics, temperature monitoring, resistivity monitoring, structural change.

Corresponding author E-mail: aizawa@sevo.kyushu-u.ac.jp

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