Conductivity structure in and around the Deep Low-Frequency Tremors generation region beneath the western part of the Kii Peninsula in Southwest Japan.

Non-volcanic deep low-frequency tremors (DLT) was detected in the southwest Japan subduction zone by Obara (2002). They are distributed in the fore arc side along the strike of the descending Philippine Sea plate. The generation of tremors may be related to the movement of fluid in the subduction zone judging from the long duration and mobility of the tremor activity,. The network magnetotelluric (NMT) method (Uyeshima et al., 2001) is one of the MT method and is well-suited for investigating deep and large-scale conductivity strcture. Yamaguchi et al. (2009) has developed a modified NMT (modified NMT) method to overcome the problem posed by the progressive replacement of metallic transmission cables with fiber cables and first applied this modified NMT method over the Kii Peninsula. They showed a 2D conductivity model along a transect across the CENTRAL part of the Kii Peninsula. Their model is characterize by a large (∼20 km wide and depths of 10-60 km) and highly conductive (<10 ohm- m) region between the Conrad discontinuity and the upper surface of the Philippine Sea slab. This region contains the hypocenters of many deep low-frequency tremors but regular earthquakes are rare. In this paper, we show a 2D conductivity model along the transect across the WESTERN part of the Kii Peninsula in order to image a 3D structure in and around the DLT region. Our model is characterized by following four regions. (1) A highly conductive region in the wedge mantle. This region contains the hypocenters of many DLT but regular earthquakes are rare. (2) A highly conductive region along the upper boundary of the Philippine Sea slab at a depths of 20-40 km. As this region well corresponds to the high Poisson's ratio region, so we interpret highly conductivity is caused by presence of fluid. (3) A highly resistive region between the surface to ~10km in depth beneath the southern part of the model profile. The region corresponds to the Kumano acidic rocks. (4) A highly conductive region in the shallower part (< 10 km in depth) under the northern part of the model profile. It is interesting that this region well corresponds an earthquake swarm area beneath Wakayama prefecture.