Some Physical Mechanisms of Precursors to Earthquakes

The existence of precursors to earthquakes at different heights of the earth's ionosphere is investigated. We analyze a mechanism for the generation of low-frequency large-scale zonal flows by higher frequency, small-scale internal-gravity waves in the electrically neutral troposphere. The nonlinear generation mechanism is based on parametric excitation of convective cells by finite amplitude internal-gravity waves. Measured density perturbations arising due to zonal flow generation may confirm the seismic origin of this mechanism. We also investigate nonlinear propagation of low-frequency seismic-origin internal-gravity perturbations in the stable stratified conductive E-layer. The predicted enhancement of atomic oxygen radiation at wavelength 557.7 nm due to the damping of nonlinear internal-gravity vortices is compared with the observed increase of the night-sky green light intensity before an earthquake. The good agreement suggests that ionospheric internal-gravity vortices can be considered as wave precursors of strong earthquakes. These precursors could be a tool for predicting the occurrence of a massive earthquake or volcano.