Earth Planets Space, Vol. 61 (No. 8), pp. 957-971, 2009
Satoru Yamaguchi1, Makoto Uyeshima2, Hideki Murakami3, Sirou Sutoh4, Daichi Tanigawa4, Tsutomu Ogawa2, Naoto Oshiman5, Ryokei Yoshimura5, Koki Aizawa5, Ichiro Shiozaki6, and Takafumi Kasaya7
1Department of Earth and Planetary Sciences, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan
2Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan
3Research and Education Faculty, Kochi University, Kochi 780-8520, Japan
4Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan
5Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto 611-0011, Japan
6Department of Management of Social Systems and Civil Engineering, Graduate School of Engineering,Tottori University, Tottori 680-8552, Japan
7Japan Agency for Marine-Earth Science and Technology, 2-15, Natsushima-cho, Yokosuka-shi 237-0061, Japan
(Received November 28, 2008; Revised May 14, 2009; Accepted May 14, 2009; Online published October 19, 2009)
The Network-Magnetotelluric (NMT) method is well-suited for investigating deep and large-scale conductivity structure; however, application of the method is strongly dependent on the availability of telecommunication facilities (specifically, metallic transmission cables). To overcome the problem posed by the progressive replacement of metallic transmission cables with fiber cables, we developed a modified NMT (modified NMT) method consisting of purpose-built electrodes, making use of local metallic telecommunication lines, without a transmission cable. We first applied this modified NMT method over the Kii Peninsula, southwestern Japan, undertaking two-dimensional conductivity modeling along a transect across the central part of the peninsula. The model is characterized by a large (∼20 km wide and depths of 10-60 km) and highly conductive (<10 Ω m) zone in the central part of the peninsula between the Conrad discontinuity and the upper surface of the Philippine Sea slab. This zone contains the hypocenters of many deep low-frequency tremors but regular earthquakes are rare. The zone also corresponds to a high-Vp/Vs area. The presence of fluid in the zone plays a key role in the absence of regular earthquakes, occurrence of deep low-frequency tremors, and elevated Vp/Vs values, as well as enhancing conductivity.
Key words: Network-MT, deep low-frequency tremor, Kii Peninsula, Philippine Sea plate, electrical conductivity, telephone-line network, conductive zone.