Electromagnetic response to undersea earthquakes in marine layered model

In this study, we adopt a horizontally layered model which consisting of air, seawater and undersea porous rock and develop an analytically-based method to calculate the seismic and EM fields generated by undersea earthquakes. We conduct numerical simulations to investigate the characteristics of the EM response in three case (the receivers located at the seafloor, in the seawater near the sea surface and in the air, respectively). The results show that two kinds of EM signals can be identified in the EM records at these receivers. The first is the early EM wave arriving before the seismic waves and the second is the coseismic EM fields with apparent speed of the seismic waves. The EM signals observed at the seafloor are mostly stronger than those observed in the seawater and air near the sea surface. We applied this method to simulating the EM response to the 2022 Mw 7.3 earthquake that took place in the sea near Fukushima, Japan. At the receiver with 80 km epicentral distance at the seafloor, the predicted coseismic electric and magnetic signals reach the amplitudes of 2 μV/m and 2 nT, respectively. The results suggest a possibility to monitor the EM disturbances associated with marine earthquakes and use them to serve the earthquake early warning or earthquake mitigation.