Laboratory studies suggest that electromagnetic emissions in a wide frequency spectrum ranging from kilohertz (kHz) to very high megahertz (MHz) frequencies are produced by the opening of microcracks, with the MHz radiation appearing earlier than the kHz radiation. Earthquakes are large-scale fracture phenomena in the Earth's heterogeneous crust. Thus, the radiated kHz–MHz electromagnetic emissions are detectable not only in the laboratory but also at a geological scale. Clear MHz-to-kHz electromagnetic anomalies have been systematically detected over periods ranging from a few days to a few hours prior to recent destructive earthquakes in Greece. We should bear in mind that whether electromagnetic precursors to earthquakes exist is an important question not only for earthquake prediction but mainly for understanding the physical processes of earthquake generation. An open question in this field of research is the classification of a detected electromagnetic anomaly as a pre-seismic signal associated with earthquake occurrence. Indeed, electromagnetic fluctuations in the frequency range of MHz are known to be related to a few sources, including atmospheric noise (due to lightning), man-made composite noise, solar–terrestrial noise (resulting from the Sun–solar wind–magnetosphere–ionosphere–Earth's surface chain) or cosmic noise, and finally, the lithospheric effect, namely pre-seismic activity. We focus on this point in this paper. We suggest that if a combination of detected kHz and MHz electromagnetic anomalies satisfies the set of criteria presented herein, these anomalies could be considered as candidate precursory phenomena of an impending earthquake.

The altitude distribution of seismic-related direct current (DC) electric field in the Earth-ionosphere layer is computed. This field is generated by the conductivity current flowing in closed atmosphere-ionosphere electric circuit at the preparatory stage of an earthquake. The source of the current is electromotive force caused by upward convective transport and gravitational sedimentation of charged aerosols injected into the atmosphere by soil gases in the course of intensification of seismic processes. It is shown from the solution of a selfconsistent set of non-linear equations that pre-earthquake DC electric field enhancement in the atmosphere can reach breakdown value at the altitudes 1-10 km and initiate numerous chaotic electrical discharges in this region. The generation theory of very high-frequency electromagnetic radiation by such discharges is developed. The calculated spectrum and other characteristics of the radiation are in a good agreement with experimental data on radio emission at the frequencies of *41 and *53 MHz that have been observed on several stations of Crete Island in connection with impending earthquakes.