## 1 Introduction  
In this paper, we will deal with the delicate research on the possible precursors of earthquakes, i.e., those anomalous phenomena that might appear before earthquakes because they are caused by their preparation phase and thus anticipate their imminent occurrence. The earthquake precursors could be simply classified as seismic precursors and non-seismic precursors: the former are eventual modifications of seismicity or seismological parameters before the occurrence of an earthquake, while the latter, the object of our present research, are alterations of some geophysical parameters, such as atmospheric chemical and physical characteristics, radon emission, and, furthermore, possible electromagnetic disturbances up to the ionosphere [1; 2; 3]. Historically, some evidence of electromagnetic precursors has been reported in the data from ground magnetic observatories--for example, before the M6.9 Spitak (Armenia) 1988 and M7.1 Loma-Prieta 1989 earthquakes by [PERSON] et al. [4] and [PERSON] et al. [5]. A model to explain the generation of Ultra Low Frequency (ULF) electromagnetic waves responsible for such pre-earthquake disturbance was proposed by [PERSON] and [PERSON] [6] and based on electromagnetic pulses produced by the separation of electric charges at the fault level due to micro-fracturing activity during the preparation phase of the earthquake. Variations of this explanation have been proposed; e.g., [7], introducing a better model for microfracturing rate variation on the time and improving the geometry of the fault. However, one of the most important theories was put forward by [PERSON] [8; 9], which proposed the generation of positive holes (called p-holes) due to the peroxy defects on the rocks also induced by stress on the rock. [PERSON] proposed such theory as a \"universal\" explanation of all other precursors: for example, he proposed that the detected Thermal-InfraRed anomalies could be explained by photon emission in such wavelengths due to the recombination at the Earth's surface of the peroxy bonds, as well as their location atop topographical profiles by the electrical migration of p-holes in the lithosphere due to the \"tip electric effect\" [9]. Even if p-holes are a different source with respect to the one initially proposed, the propagation mechanism of the eventual pre-earthquake ULF waves could, in principle, remain unchanged. Other theories have also been developed and warrant consideration--for example, supposing that the release of p-holes at the Earth's surface can produce an accumulation of electric charges that could gradually create a perturbation in the atmosphere and then propagate up to the ionosphere in the form of plasma bubble, interacting and following the magnetic field, as numerically simulated by [PERSON] et al. [10; 11]. Such simulation opened a scientific discussion, as [PERSON] and [PERSON] [12] found that some assumptions in the model were too simplified, and [PERSON] and [PERSON] replied with an update of the original work, showing that the model seems, in any case, valid [13]. Furthermore, [PERSON] et al. [14; 15] found some possible inaccuracies in the model of [PERSON] et al., proposing another model that improved the calculus of the conductivity in the ionosphere, but the value on the ground of the electric field strength signal necessary to create a small perturbation of the ionosphere was found to be very large, even if it is still possible. As stated by the same authors of all these papers, more pieces of evidence are necessary to understand if such phenomena exist and which could be the most reliable model to describe them. This paper aims to provide some further observational points for such discussion. According to [PERSON] and [PERSON] [16], the ionization of the air could be also produced by the release of radon instead of (or in addition to) the p-holes.