## 2 Materials and Methods  
### Electromagnetic Signals of Seismic Origin  
The scientific community has studied the possible relationship between electromagnetic phenomena and seismic events since the second half of the 20 th century. There is more and more evidence about a connection between the generation of electromagnetic signals and the occurrence of seismic events (in addition to the well-known variations of the Earth's magnetic field associated with the incidence of the solar wind). This field of study requires combining various disciplines, such as physics, geology, and even biology [7; 8]. Anomalies in electromagnetic signals associated with strong seismic events have been identified, and physical mechanisms have been proposed to explain them [9; 10; 11; 12; 13]. The study of seismic-electromagnetic phenomena has acquired greater importance since the 1980s [7]. Anomalies were found in the behavior of electromagnetic signals before some strong earthquakes, shedding new light on the study of seismic precursors. Several authors have proposed different mechanisms for generating anomalous electromagnetic signals associated with seismic activity. For example, (1) a mechanism that could explain the anomalies is the displacement of the limits between cortical blocks of high and low conductivity [14]. Due to this, the electrical currents in the subsoil would be altered, also altering the induced magnetic fields. (2) Another mechanism could be the electrokinetic effect, which consists of the fact that, in the presence of fluids in the subsoil, the solid phase can be electrically charged, while the liquid phase acquires the opposite charge [13]. The latter, when flowing, generates an electric current that can induce magnetic fields. (3) On the other hand, the piezomagnetic effect occurs when the magnetization of ferromagnetic rocks changes when subjected to stress, giving rise to an alteration in the magnetic field in the form of very low-frequency signals [15]. (4) An alternate mechanism could be associated with the formation of microfractures in the rock, in the presence of fluids. As the pressure increases before the seismic event, the rocks dehydrate and lose conductivity. These ruptures can also cause the separation of ionic bonds, generating a variation in the potential difference, which can generate electromagnetic signals [13; 15; 16]. [PERSON] and [PERSON] [17] have proposed four stages before large-magnitude earthquakes based on the Wenchuan event, M8.0, on 12 May 2008. The first stage would consist of the accumulation of mechanical stresses. The pressure would close the subsurface microfractures, altering their \(\rho_{a}\). In the final phase of this stage, electromagnetic signals would be recorded. In the case of the Wenchuan event, this stage lasted for more than two years. The second stage would consist of a blockade of the subsoil. At this point, the microfractures would have closed entirely, and tectonic activity (and therefore seismicity) would have been reduced to a minimum, avoiding the generation of electromagnetic anomalies. The third stage would consist of an \"unblocking\" and an expansion of the subsoil. The temperature would rise, generating electromagnetic signals in the infrared. Microfractures would re-form, and small seismic events would occur with very low-frequency electromagnetic emissions. The fourth stage would take place hours or days before the big event and would give rise to anomalies in the electric field of the ionosphere. Although the authors base their model on the observations associated with this specific event, they could be considered for the study of other events. Solar activity can also be a source of electromagnetic signals, for which it is necessary to distinguish between these and those possibly associated with seismic activity [18]. Likewise, anthropic activity can generate electromagnetic noise, especially in areas where there are electrical power distribution systems [19].