Abstract
Coseismic electromagnetic (EM) signals that appear from the P arrival were observed in a volcanic area during the 2016 Kumamoto earthquake. In this study, we conduct numerical simulations to explain the coseismic EM signals observed for a M5.4 aftershock of the earthquake. Initially, we adopt a water-saturated half-space model, and its simulation result for a receiver with a depth of 0.1 m suggests that the magnetic signals do not show up at the arrivals of P, refracted SV–P and Rayleigh waves because the evanescent EM waves just counterbalance the localized magnetic signals that accompany P, refracted SV–P and Rayleigh waves. Then, we conduct numerical simulations on a seven-layer half-space model in which the second layer corresponds to an aquifer analogy and the six other layers refer to air-saturated porous media. When only the electrokinetic effect is considered, the simulated coseismic magnetic signals still appear from the S arrival. The combination of electrokinetic effect and surface-charge assumption is also tested. We find that signals before the S arrival are missing on the transverse seismic, transverse electric, radial magnetic and vertical magnetic components, although the situation on horizontal magnetic components is improved to an extent. Then, we introduce an artificial scattering effect into our numerical simulations given that the scattering effect should exist in the volcanic area. New numerical result shows good agreement with the observation result on the signal appearance time. Hence, the combination of electrokinetic and scattering effects is a plausible explanation of coseismic EM signals. Further investigations indicate that coseismic electric and/or magnetic signals are more sensitive to the scattering effect and the aquifer thickness than seismic signals.
Highlights
Coseismic EM signals were recorded by the observation stations located in Iwo-yama area during the 2016 Kumamoto earthquake
We try to explain the coseismic EM signals observed for the M5.4 aftershock by conducting numerical simulations on several different models
The simulation result for a receiver located at (− 80,890, 8680, 0.1) m shows that the arrivals of P, refracted SV–P, S and Rayleigh waves can be found on either seismic or electric signals, whereas only the arrival of S wave can be found on magnetic signals (Fig. 4a)
Summary
The existence of the seismo-electromagnetic phenomena has been confirmed by field observations in previous decades Layered model consisting of porous media saturated by air or water Ren et al (2016b) conducted numerical simulations by adopting an eight-layer half-space model consisting of porous and solid materials; they found that coseismic EM signals can be recorded by a receiver located in the top solid layer, where the electrokinetic effect is inoperative In this case, the coseismic EM signals are contributed by the evanescent EM waves. If only the electrokinetic effect is considered, the simulation result of the seven-layer half-space model consisting of water-saturated and air-saturated porous materials shows evident difference from the observation result on the magnetic signals before the S arrival. For the used sevenlayer half-space model, the coseismic EM signals recorded by the receiver nearby the ground surface are contributed
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