Many electronic density disturbances in the ionosphere were reported by Blanc (1985) in the aftermath of high-intensity earthquakes. These observations can be interpreted (at least up to the highest point of the F layer) as the result of gravity waves or acoustic waves travelling in the upper atmosphere, generated by ground movements close to the epicentre or at great distances in the case of seismic waves travelling around the planet (Artu, 1998). Understanding ionospheric disturbances observed days before or after seismic events is much more difficult (Parrot et al., 1993.)
Setting aside pre-seismic acoustic waves emissions, some scientists have offered different hypotheses, such as for examples the redistribution of electric charges on Earth’s surface and then in the upper atmosphere (Pulinets et al, 1994). More recently, measuring the total electron content (TEC) provided evidence of significant ionospheric disturbances following an earthquake (Calais et Minster, 1995). This study was extended by Zaslavsky et al. (1997). Researchers analysed TEC data depending on seismic activity using the TOPEC-POSEIDON satellite and DORIS beacons. Earthquakes were selected with a magnitude of over 5.0 on the surface wave magnitude scale and so that the epicentre is less than 300 km from the satellite ground track. Data was studied 48 hours before the event.
Some cases show ionospheric disturbances when the satellite is over an earthquake’s epicentre. But natural TEC variations can be affected by several ionospheric characteristics; therefore, a statistical survey was conducted over 706 seismic events. The number of time-correlated (within 48 hours before the event) and space-correlated (when the satellite is above the epicentre) disturbances reached 238. These disturbances include rises as well as falls in the TEC. They were studied and compared with natural TEC variations; results show for example that there is no rise in disturbances at noon local time, as would be the case if these anomalies had been mixed with naturally-occurring variations.
Statistical analysis of the correlation between seismic events the TEC