Open Access

Modification of the Network-MT method and its first application in imaging the deep conductivity structure beneath the Kii Peninsula, southwestern Japan

  • Satoru Yamaguchi15Email author,
  • Makoto Uyeshima25,
  • Hideki Murakami35,
  • Sirou Sutoh45,
  • Daichi Tanigawa45,
  • Tsutomu Ogawa25,
  • Naoto Oshiman55,
  • Ryokei Yoshimura55,
  • Koki Aizawa55, 25,
  • Ichiro Shiozaki65 and
  • Takafumi Kasaya75
Earth, Planets and Space200961:BF03352946

https://doi.org/10.1186/BF03352946

Received: 28 November 2008

Accepted: 14 May 2009

Published: 9 October 2009

Abstract

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-V p /V s 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 V p /V s 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