Seminar: Subionospheric very-low frequency (VLF) and low frequency (LF) radio waves as tools for monitoring seismic electromagnetic precursors
We report on electromagnetic (EM) phenomena in association with earthquakes (EQs). In the last decades, several studies have been devoted to the investigation of the short-term earthquake prediction with a time scale of about one week. First, we examine the physical mechanisms proposed to explain seismic EM precursors. Secondly, we introduce and describe the wave propagation of VLF and LF transmitter signals in the Earth’s ionosphere. The propagation of such signals is characterized by a low path attenuation over long distances, up to 20 000 km. In addition, the radiated transmitter energy is confined to the wave-guide between the ground and the ionospheric D-layer. We show an example of VLF/LF wave propagations during the occurrence of two earthquakes in the Croatian regions at geographic coordinate: 16°E, 45°N. The first and second EQs happened, respectively, on March 22 and December 29, 2020, with magnitudes Mw equal to 5.4 and 6.4. We analyze the amplitude variations of VLF and LF transmitters localised in Czech, Italy, Romania and Turkey few weeks before the earthquakes occurrences. We discuss the observed anomalies in the radio wave signals which may be considered as precursors due to the ionospheric disturbances of the transmitter ray paths above the earthquakes preparation zones.