Many disturbances of the electronic density in the ionosphere after large amplitude earthquakes have been reported by Blanc (1985). These observations can be interpreted, at least up to the maximum of the F region, as the result of gravity wave or acoustic wave propagation in the upper atmosphere generated by ground shakes not only near the epicentre but also at large distance when seismic waves propagate around the Earth (Artru, 1998). It is much more difficult to understand the ionospheric disturbances observed a few days or a few hours before the earthquakes (Parrot et al.,1993).

Beside pre-seismic acoustic wave emissions, other hypotheses have been suggested appealing to electric charge redistribution on the Earth's surface, then in the upper atmosphere (Pulinets et al., 1994). More recently the measurement of the Total Electronic Content (TEC) allowed to bring out noticeable ionosphere disturbances as a result of an earthquake (Calais and Minster, 1995). This study was extended by par Zaslavsky et al. (1997) who analyzed, according to seismic activity, TEC data obtained between the TOPEX-POSEIDON satellite and the ground using DORIS beacons. Earthquakes have been chosen with a magnitude M_s > 5.0, and a distance between the epicentre and the satellite ground track < 300 km. Data from 48 hours before the quake have been studied.

Special case studies show an ionospheric disturbance when the satellite is over a seism epicentre. But natural TEC variations in the ionosphere can be the result of many parameters, a statistical study has thus taken place using 706 seisms. The number of disturbances which are correlated in time (between 0 and 48 hours before the seism), and in space (modification when the satellite is over the epicentre) is 238. Those TEC disturbances appear as increase as well as decrease. They have been studied and compared with natural TEC variations. The results show for example that there are no anomalies increase around noon in local hour as it could have happened if these anomalies have been mixed with natural variations.