Electromagnetic heterogeneity of the seismogenic region of 1962 M6.5 Northern Miyagi earthquake, northeastern Japan

Abstract

Magnetotelluric surveys were carried out to investigate the electrical resistivity structure of the seismogenic region of the 1962 Northern Miyagi Earthquake (M6.5) in Miyagi Prefecture, northeastern Japan. A 2-D inversion reveals the existence of a deep conductive zone and overlying resistive zone in the upper crust. A positive aeromagnetic anomaly is also recognized above the resistive zone, which is interpreted as a buried Kitakami granitoid pluton. By comparison with the seismicity, we found that the microearthquakes occur just above the deep conductor and in the resistive zone, and that several S-wave reflectors are just above the deep conductor. Since the S-wave reflectors suggest the existence of fluid beneath them, the deep conductor can be interpreted as a fluid-filled zone. Moreover, we suggest that the seepage of the fluid from the conductive fluid-filled zone to the resistive granitoid pluton can become a trigger of the earthquakes.