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 (similar to 80 MHz) 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 notion s on the lithosphere-atmosphere-ionosphere coupling. Gravity waves in the a tmosphere are speculated to be generated by quasi-periodic movements of tec tonic blocks or yield of gases along faults. Conversion of large-scale grav ity waves above a seismically active region into ionospheric disturbances i nvolves 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 gradien t-drift plasma instability. These meter-range irregularities could be the r eason for the back-scattered VHF radio waves. Some experimental tests for v erification of this hypothesis are suggested.