Electromagnetic response to undersea earthquakes in a layered ocean model

Electromagnetic response to undersea earthquakes in a layered ocean model Qianli Cheng, Yongxin Gao   We adopt a horizontally layered model consisting of air, seawater and undersea porous rock and develop an analytically based method to calculate the seismic and electromagnetic (EM) fields generated by undersea earthquakes. We conduct numerical simulations to investigate the characteristics of the EM response at 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, namely, the early EM wave (seismic-to-EM conversion at the seafloor interface) arriving before the seismic waves and the coseismic EM fields with apparent speeds 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. The method is applied to simulating the EM response to the 2022 Mw 7.3 earthquake that took place in the sea near Fukushima, Japan. At a receiver with 76 km epicentral distance at the seafloor, the predicted coseismic electric and magnetic signals reach 2 μV/m and 2 nT, respectively, which are within the detectability of the current EM equipment. This suggests a possibility to monitor the EM disturbances associated with undersea earthquakes and use them to serve the earthquake early warning, helping to mitigate the societal impact of large earthquakes.