Abstract
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.
Highlights
Ontake volcano in central Japan (Fig. 1), previous geophysical and geochemical studies have suggested that pore fluids are associated with earthquake swarm activity located below the southeast flank of the mountain
We infer that the resistive zone located between the C1 and C2 conductors was not detected in previous models due to: (1) the lack of MT data in the resistive area and (2) fitting measured MT responses with a smooth 2D TM-mode only inversion model
We show here an improved resistivity model with new dense MT measurements, modeled using a 3D inversion procedure
Summary
This sensitivity test shows clearly that the filled model responses do not fit the measured data, the high phase values (i.e., > 45°) and low apparent resistivity of off-diagonal components in the 0.01 s − 0.1 s period range. While sensitivity to the C-2 conductor is less than for C-1 (owing to the location of C-2 at depth and at the edge of the data coverage), changes in apparent resistivity and phase are significantly larger than the estimated errors This sensitivity test shows that the C-2 conductor is required by the data and a robust model feature. Additional observations would be required to constrain this feature outside of our current data coverage
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