## 4 Discussion
### Some Features of the Largest Concentrations  
In the results shown in Figure 3, it seems to be observed that the content of the high frequency of the magnetic field signal of the possible earthquake precursor becomes more intense as the earthquake is approaching. There is not a simple explanation for this, and the present theories would suggest the contrary direction. From a seismological point of view, the crack dimension is supposed to increase toward the earthquake (e.g., [43; 44]), the duration of the electromagnetic pulses eventually generated by the microfracture is supposed to be proportional to the crack dimension, and the emitted frequency is supposed to be proportional to the inverse of the pulse duration [6; 7]. Even supposing that the fault is simplified as a two planar faces capacitor forming an oscillating circuit with the resistance and inductance of the ground (it can be the fluid patches in the rocks that act as a \"transmission line\" for the higher conductivity of fluids), the frequency \"\(\omega\)\" is supposed to decrease with the crack size \"\(l\)\", which increases (a similar calculus that found that \(\omega\propto\frac{l}{l}\) is provided by [PERSON] et al. [45] and [PERSON] et al. [21]). These calculations and theories would not support the result presented in this paper, as they point to the opposite conclusion. However, first of all, we think that the problem is really open. Furthermore, we are not sure that the electromagnetic disturbances observed in the ionosphere are directly produced on the fault, as other phenomena can act at the ground-lower atmosphere interface, as suggested by [PERSON] et al. [10; 11], who simulated the formation of ionospheric disturbances with accumulation on the Earth's surface of positive-holes released from the crack in the fault or a very different and complex chain of processes that influence the ionosphere as a consequence of the global atmospheric electric circuit alteration induced by air ionization from radon release, as theorized by [PERSON] and [PERSON] [16]. In the last two mechanisms, it would be very difficult for the frequency of the supposed generated signal to be related to any crack evolution model, so it would be complicated to predict if the frequency of the magnetic field signal can increase or decrease toward the earthquake occurrence. Further studies will be necessary both in theory and in the observations, as shown here. We finally investigated if the earthquakes involved in the absolute maximum concentrations of Figure 3A-C share some common features, such as focal mechanism or the location of the hypocenter beneath the sea or the land. A similar investigation on the _Swarm Ne_ data and on earthquake occurrences over sea or land has been reported by [PERSON] et al. [46]. Even if the authors did not find a particular \"preference\" in such classification, they found a better correlation for higher magnitude events, as was also detected in the present work. First of all, we found that the earthquakes that are associated with the three maximum concentrations in the different analyzed signal bands (146 EQs, 137 EQs, and 146 EQs, respectively) are actually different events (except for two that are shared in all three concentrations), so the increase in frequency toward the earthquake occurrence could be apparent, and it is possible that the frequency of the generated signal depends on other parameters such as those investigated in the following. Table 3 reports the number of earthquakes and their percentage in the specific class. The focal mechanism has been checked, searching for the same earthquake in the global Centroid Moment Tensor (CMT) catalog [47; 48] with a maximum tolerance of 2 min on the origin time, 2.5 on the magnitude estimation, and 2 degrees in the epicenter localization.