Electrical Conductive Fluidized Zones and Their Influence on the Earthquake Nucleation, Growth, and Arrest Processes of the 2016 Kumamoto Earthquake Sequence, Kyushu Island, Japan

Abstract

Crustal earthquake ruptures tend to nucleate near fluidized zones. However, it is relatively unknown whether fluidized zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal area of the 2016 Kumamoto earthquake sequence by using 200 sites broad-band magnetotelluric data, and discuss its quantitative relationship to earthquake nucleation, growth, and arrest processes. The result shows that the earthquake hypocenters are all located within 10 km from low-resistivity fluidized zones < 30Ωm. The ruptures that nucleated along the outer edge of the low-resistivity fluidized zones tended to become large earthquakes, whereas those that initiated either distal to or within the fluidized zones did not. The ruptures were arrested by high-temperature (>400°C) fluidized zones, whereas shallower low-temperature (200°C–400°C) fluidized zones either promoted or arrested the ruptures. These results suggest that the distribution of mid-crustal fluids contributes to the nucleation, growth, and arrest of crustal earthquakes.

Electrical Resistivity Structure Around the Atotsugawa Fault, Central Japan, Revealed by a New 2‐D Inversion Method Combining Wideband‐MT and Network‐MT Data Sets

“…The fault‐valve hypothesis of Sibson (1992) suggests that overpressured fluid reduces the frictional fault strength, inducing the earthquake and promoting upward fluid flow from deep fluid‐rich reservoirs to the shallower crust, but the fluid pressure drop after the fault rupture subsequently reverses and rises once again. Recently, it is suggested that the overpressured fluids not only contribute to the initiation of earthquakes but also contribute to the final magnitude of earthquakes (Aizawa et al., 2021). Based on the fault‐valve hypothesis, it is likely that electrical conductors under the UF, AF, and TOFZ contain suprahydrostatic fluids, which can induce intraplate earthquakes at the faults in the future.…”

“…(2020) found mid‐to‐lower‐crustal conductors associated with seismogenic structures of some strong earthquakes. Moreover, in Japan, several electromagnetic induction surveys have identified electrical conductors close to seismically active regions and have discussed the influences of fluids on earthquakes (e.g., Aizawa et al., 2021; Ichihara et al., 2011; Ogawa & Honkura, 2004; Ogawa et al., 2001; Uyeshima et al., 2005; Yoshimura et al., 2008).…”