A highly conductive zone revealed by an improved Network-MT method beneath the Kii Peninsula, southwestern Japan

摘要

The Philippine Sea plate is subducting beneath the Eurasian Plate at southwestern Japan arc along the Nankai Trough.Historical great earthquakes have frequently occurred along this trough. In addition to these mega-thrust earthquakes,deep low-frequency tremors have also been recently recognized at Moho depth forming a narrow zone along the strike of the arc. These tremors are considered to be probably generated by the movement of fluid discharging from the dehydrating slab.To investigate subsurface electrical conductivity structure in Earth's deep structure, the magnetotelluric (MT) method is generally used. The Network-MT method, in particular, is the most suitable method to determine deep and large scale conductivity structure. In order to reduce limitations from telecommunication facilities on which the original Network-MT method strongly depends, we modified the Network-MT method so as to use electrodes purpose-built for the method and local metallic telecommunication lines.We used this improved Network-MT observation technique over the Kii Peninsula and undertook the two-dimensional conductivity modelling along the profile crossing the central part of the Kii Peninsula. The model is characterized by a large and highly conductive zone in the central part of the peninsula between the Conrad discontinuity and the upper plane of the Philippine Sea slab. The zone is consistent with spatial distribution of earthquakes of the following two types. Hypocenters of deep low-frequency tremors located in this highly conductive zone. A few hypocenters of 'usual' earthquakes were recognized in the zone but they are localized at the outer boundary area of this highly conductive zone. The presence of fluid can explain the occurrence of 'usual' earthquakes, occurrence of deep low-frequency tremors, and enhancing conductivity in the highly conductive zone.