Numerical modeling of the co-seismic electromagnetic signals observed during the 2014 Mw 4.9 Hammam Melouane earthquake, Northern Algeria

Abstract

The seismic activity in northern Algeria provides the opportunity to observe earthquake-related electromagnetic signals through magnetotelluric (MT) measurements. Through an MT station, we measured the two telluric and three orthogonal magnetic field components. On December 23rd, 2014, a moderate earthquake (Mw = 4.9) occurred in the village of Hammam Melouane (Algeria), located 36 km northeast of the MT site. This event allowed the observation of a co-seismic electromagnetic signal in the MT time series. The analysis of the electromagnetic time series using Fourier transform revealed the spectral characteristics of these co-seismic electromagnetic signals with a high spectral amplitude that started at the frequency of 0.3 Hz. Then, we simulate the electric and magnetic responses of the Hammam Melouane earthquake based on the electrokinetic mechanism. We conducted the simulations using a point source model with a realistic layered earth model and compared the synthetic signals to the observed data in the frequency band 0.3–6 Hz. The result shows that the synthetic electric signals match the measured data in both amplitude and waveform for the parts of the recordings after the arrival of the S wave with a phase shift. Similarly, the synthetic magnetic field agrees with the observed data in the waveforms with a phase shift, but their amplitudes are smaller than the observed data. In the latter part of the records (>18 s), a poor disagreement can be seen between the synthetic and observed electromagnetic field signals in their shape and amplitudes. We investigated the rotation-induced magnetic field effect as a possible mechanism for the generation of additional co-seismic magnetic signals. The synthetic magnetic fields resulting from the magnetometer rotation modeling are in agreement with the observed magnetic data in terms of amplitudes after the arrival of seismic waves. The main conclusion of this study reveals that the electrokinetic effect is responsible for the generation of co-seismic electric fields while the rotation of magnetic sensors is likely the main cause of co-seismic magnetic fields.