VHF-electromagnetic evidence of the underlying pre-seismic critical stage

Based on the study of pre-seismic very high frequency (VHF) and very low frequency (VLF) electromagnetic signals, we attempt to establish a set of necessary conditions referring to the underlying critical stage of the earthquake preparation process. This study combines concepts from spectral analysis associated with critical point hypothesis, results from laboratory experiments of rupture and seismological arguments. These conditions are fully satisfied in the case of the VLF-VHF pre-seismic signals associated with the Kozani-Grevena earthquake in Greece.