Electromagnetic Signal Associated with Seismic Waves: Case Study in the North Central Algeria Area

This study aimed to investigate the temporal variations in electromagnetic signals related to crustal tectonic activity in northern Algeria, using an intermittent magnetotellurics (MT) measurement system that was launched in December 2014. On 23 December 2014, a moderate earthquake (Mw = 4.9) occurred at 08:00:21 (UT) in the village of Hammam Melouane, exactly 36.3 km northeast of the magnetotellurics station. This seismic event provided a unique opportunity to observe, clearly and distinctly, a co-seismic electromagnetic signal from the natural electromagnetic field in the magnetotellurics time series. Analysis of the recorded signals revealed that the co-seismic electromagnetic signal was synchronized with the P-wave arrival time. Based on the time–frequency misfit criteria analysis method, we conducted a quantitative comparison between the co-seismic telluric and magnetic horizontal component time series, in addition to the horizontal components of a seismic trace record, which were collected from the seismological station nearest the magnetotelluric site. Good waveform similarity was found between the electromagnetic and seismic signal components, with a high temporal frequency misfit envelope and a high fit value. After the arrival of the seismic waves, the north–south co-seismic electric and magnetic component shifted to a higher frequency compared to the east–west component, which was similar to the seismic velocity components. Thus, to investigate the generating mechanism, a one-dimensional geoelectric model was derived using the magnetotelluric data, and the polarization trajectories of the co-seismic electromagnetic signal were analyzed. The results indicated that the electrokinetic effect was likely the mechanism generating the observed co-seismic electromagnetic signals.