Possible mechanism of the over-horizon reception of FM radio waves during earthquake preparation period

Kushida noticed anomalous over-horizon reception of FM radio waves (-80MHz) before earthquakes when epicenters were in a certain sensitive region. The suggested method of earthquake prediction recognizes precursors in supposedly back-scattered waves from the ionosphere several days or few weeks before the seismic events. We try to clarify the possible underlying physical mechanism for this method within the framework of existing notions on the lithosphere-atmosphere-ionosphere coupling. Gravity waves in the atmosphere are speculated to be generated by quasi-periodic movements of tectonic blocks or yield of gases along faults. Conversion of large-scale gravity waves above a seismically active region into ionospheric disturbances involves formation of sporadic E-layer. The sporadic E-layer in turn may be regarded as a free energy source of generation of small-scale (a few meters) ionospheric plasma irregularities, presumably due to the modified gradient-drift plasma instability. These meter-range irregularities could be the reason for the back-scattered VHF radio waves. Some experimental tests for verification of this hypothesis are suggested.