A 3D electrical resistivity model around the focal zone of the 2017 southern Nagano Prefecture earthquake (MJMA 5.6): implications for relationship between seismicity and crustal heterogeneity

Seismic swarm areas below the southeast flank of Ontake volcano, central Japan, provide an important opportunity to study interactions between seismicity, volcanic processes and crustal fluid. On June 25, 2017, an M5.6 earthquake occurred in the Ontake swarm area where geochemical and geophysical studies suggest that pore fluid pathways from the lower crust and mantle affect fault rupture. To clarify the electrical resistivity distribution (that reflects pore fluids, altered sediments and temperature), audio-frequency and broadband magnetotelluric data were measured at 35 sites around the aftershock area of this earthquake. A 3D resistivity inversion model based on these observed magnetotelluric data shows the following key features: (1) two conductive zones (C-1 and C-2) underlie springs where isotope studies indicate fluids of mantle or lower crustal origin and (2) aftershock hypocentres locate in a resistive area between these two aseismic conductive zones (C-1 and C-2). The relationship between seismicity and conductivity suggests that the C-1 and C-2 conductors can be interpreted as interconnected pore fluid, high temperature and/or sediment under aseismic elastic conditions. In addition, the fault rupture of the M5.6 earthquake was located near the boundary between the central resistive and conductive C-2 zone, indicating stress accumulation associated with heterogeneity of rock, temperature and/or pore fluid distribution. If these features are observed generally in seismic areas, surveys of resistivity structure could contribute to estimating the magnitude of potential earthquakes and evaluation of risk.