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Earth Planets Space, Vol. 60 (No. 2), pp. 117-122, 2008

LETTER

Magnetotelluric observations around the focal region of the 2007 Noto Hanto Earthquake (Mj 6.9), Central Japan

Ryokei Yoshimura1, Naoto Oshiman1, Makoto Uyeshima2, Yasuo Ogawa3, Masaaki Mishina4, Hiroaki Toh5, Shin'ya Sakanaka6, Hiroshi Ichihara7, Ichiro Shiozaki8, Tsutomu Ogawa2, Tsutomu Miura1, Shigeru Koyama2, Yasuyoshi Fujita1, Kazuhiro Nishimura1, Yu Takagi1, Mikihiro Imai6, Ryo Honda7, Sei Yabe1, Shintaro Nagaoka3, Mitsuhiro Tada1, and Toru Mogi7

1Disaster Prevention Research Institute, Kyoto University, Japan
2Earthquake Research Institute, University of Tokyo, Japan
3Volcanic Fluid Research Center, Tokyo Institute of Technology, Japan
4Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Japan
5Department of Earth Sciences, University of Toyama, Japan
6Department of Earth Science and Technology, Akita University, Japan
7Institute of Seismology and Volcanology, Hokkaido University, Japan
8Department of Civil Engineering, Tottori University, Japan

(Received June 30, 2007; Revised September 3, 2007; Accepted September 10, 2007; Online published February 19, 2008)

Abstract: On 25 March 2007, a damaging earthquake (Mj 6.9) occurred near the west coast of the Noto Peninsula, Central Japan. A wideband magnetotelluric (MT) survey was carried out in the onshore area of the source region immediately after the mainshock, with the aim of imaging the heterogeneity of the crustal resistivity structure. The final observation network had consisted of 26 sites. As a preparatory step for imaging three-dimensional features of the resistivity around the focal region, we constructed two-dimensional resistivity models along five profiles using only the TM mode responses, in order to reduce three-dimensional effects. Four profiles are perpendicular to the fault strike, and a fifth profile is parallel to the strike through the mainshock epicenter. Significant characteristics of the resistivity models are: (1) beneath the mainshock hypocenter, there is a conductive body which spreads to the eastern edge of the active aftershock region; (2) a resistive zone is located in the gap of the aftershock distribution between the mainshock hypocenter and the largest eastern aftershock; (3) one of the largest aftershock occurred at the boundary of the resistive zone described above. These results suggest that the deep conductors represent fluid-filled zones and that the lateral heterogeneity could have controlled the slip distribution on the fault plane.
Key words: Resistivity structure, wideband magnetotellurics, 2007 Noto Hanto Earthquake.


Corresponding author E-mail: ryokei@eqh.dpri.kyoto-u.ac.jp


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