Temporal changes in electrical resistivity at Sakurajima volcano from continuous magnetotelluric observations Koki Aizawa a,d, ⁎, Wataru Kanda a,b , Yasuo Ogawa b , Masato Iguchi a , Akihiko Yokoo a,e , Hiroshi Yakiwara c , Takayuki Sugano d a Sakurajima Volcano Research Center, Kyoto University, Yokoyama 1722-19, Sakurajima, Kagoshima 891-1419, Japan b Volcanic Fluid Research Center, Tokyo Institute of Technology, Ookayama 2-12-2, Meguro-ku, Tokyo 152-8551, Japan c Nansei-Toko Observatory for Earthquakes and Volcanoes, Faculty of Science, Kagoshima University, 10861 Yoshino-cho, Kagoshima 892-0871, Japan d Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan e Department of Geophysics, Graduate School of Science, Tohoku University, Aramaki-Aza Aoba 6-3, Aoba-ku, Sendai, Miyagi 980-8578, Japan abstract article info Article history: Received 21 June 2010 Accepted 1 November 2010 Available online 10 November 2010 Keywords: magnetotellurics resistivity volatile degassing hydrothermal system Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and 3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ± 20% and phase change of ±2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The monitoring of subsurface magma is an essential approach in terms of predicting volcanic eruptions and contributing to hazard mitigation. Daily imaging of the location, volume, and physical properties (e.g., pressure and gas fraction) of subsurface magma enables predictions not only of eruption timing, but also its location, duration, and degree of explosivity. Geodetic measurements (strain, tilting, and GPS) are currently the most practical methods with which to investigate changes in subsurface magma, because such data are sensitive to subtle pressure changes and have high temporal resolution. For example, at Sakurajima volcano, Japan, Vulcanian-type eruptions are routinely predicted in advance by up to 1 day based on data from a strainmeter and tiltmeter installed in a tunnel at the volcano (e.g., Ishihara, 1990; Iguchi et al., 2008a,b). However, it is generally difficult to predict an eruption over the coming weeks or months. In addition, some eruptions occur without significant ground deformation. It is a promising procedure to monitor changes in subsurface structure as an indicator of changes in subsurface magma. Previous studies have used seismic methods to investigate changes in structure beneath active volcanoes and geothermal areas (e.g., Foulger et al., 1997; Nishimura et al., 2000; Miller and Savage, 2001; Foulger et al., 2003; Gerst and Savage, 2004; Yamawaki et al., 2004; Nishimura et al., 2006). A recent study of temporal change in seismic structure (4D tomography) beneath Etna volcano, Italy, clearly imaged change in the structure of Vp/Vs ratio (Patanè et al., 2006). The authors attributed the change in Vp/Vs to subsurface magma movement and corresponding degassing of volatiles. Another approach involves using seismic noise records to monitor seismic structure, as reported by Brenguier et al. (2007, 2008). Based on the premise that long-term averaging of seismic noise produces a random source field, the authors imaged changes in seismic velocity at Piton de la Fournaise volcano, Reunion Island. These recently developed seismic methods have given rise to the possibility of monitoring magma movement and predicting eruptions, although such approaches require a dense seismometer network. The monitoring of electrical resistivity structure also shows promise in terms of imaging subsurface magma, because magma Journal of Volcanology and Geothermal Research 199 (2011) 165–175 ⁎ Corresponding author. Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan. Tel.: +81 3 5841 5746; fax: +81 3 3812 6979. E-mail addresses: a-zawa@eri.u-tokyo.ac.jp (K. Aizawa), kanda@ksvo.titech.ac.jp (W. Kanda), oga@ksvo.titech.ac.jp (Y. Ogawa), iguchi@svo.dpri.kyoto-u.ac.jp (M. Iguchi), yokoo@zisin.gp.tohoku.ac.jp (A. Yokoo), yakiwara@sci.kagoshima-u.ac.jp (H. Yakiwara), sugano@eri.u-tokyo.ac.jp (T. Sugano). 0377-0273/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2010.11.003 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores