Three-dimensional resistivity modelling of a seismogenic area in an oblique subduction zone in the western Kurile arc: Constraints from anomalous magnetotelluric phases

• We modelled 3-D resistivity structures in an inland earthquake zone.
• Strong 3-D crustal heterogeneity may cause local concentration of strain.
• Anomalous magnetotelluric phases can be used to constrain resistivity structures.

Wide-band magnetotelluric surveys were performed at 44 sites in the inland earthquake zone on the western margin of the Kurile arc, where strain accumulation has been observed since a high-magnitude interplate earthquake in 2003 (Tokachi-oki earthquake, M 8.0). We estimated the three-dimensional (3-D) resistivity distribution based on magnetotelluric impedance tensors, which contained anomalously large impedance phases. Although anomalous impedance phases have previously been considered a major impediment for magnetotelluric studies, we found that the anomalous phase can be used to constrain the resistivity distribution. The 3-D resistivity model showed that the earthquake zone consists of resistive areas (> 300 Ω-m), whereas the fore-arc and back-arc areas consist of regional conductive layers (< 10 Ω-m) at the surface. In addition, local near-surface conductors surround the resistive earthquake zone. The localized high-resistivity area and surrounding conductors correspond to high-rigidity rocks and thick sediment areas, respectively. This structural contrast suggests that elasticity heterogeneity is strongly distributed in the earthquake area. This heterogeneity could cause local concentration of strain and result in inland earthquakes.