  It had been observed by Linkov, Petrova and Osipov (1992) that there exist
periodic 4-6 hours pulses of 200 microHz seismogravitational oscillations ( SGO
) before 95 % of powerful earthquakes. We explain this by beating between an
oscillation eigenmode of a whole tectonic plate and a local eigenmode of an
active zone. The beating transfers the oscillation energy from the remote zone
of the tectonic plate to the active zone, triggering the earthquake.
Oscillation frequencies of the plate and ones of the active zone are tuned to a
resonance by an additional compression applied to the active zone due to
collision of neighboring plates or the magma flow in the liquid underlay of the
astenosphere ( the upper mantle). In the case when there are three or more SGO
with incommensurable difference frequencies the SGO beating pattern looks
quasi-random, thus masking the non-random nature of the beating process.
Nevertheless, we are able to discuss a possibility of the short term
earthquakes predictions based on an accurate monitoring of the beating
dynamics.
