ANALYSIS OF TEC MEASUREMENTS ABOVE ACTIVE SEISMIC REGIONS

Ionospheric perturbations by natural geophysical activity, such as vol canic eruptions and earthquakes, have been studied since the great Ala skan earthquake in 1964. Measurements made from the ground show a vari ation of the critical frequency of the ionosphere layers before and af ter the shock. In this paper, we present an experimental investigation of the electron density variations before earthquakes when the low-al titude satellite TOPEX-POSEIDON is above active seismic regions. Fluct uations in the integrated electron density or TEC (Total Electron Cont ent) between the satellite and the ground are measured to correct the altimetry measurement. The satellite was launched on August 10, 1992 a nd an analysis of the TEC fluctuations above the main earthquakes (M-s > 5) is performed up to the end of 1995. Due to different constraints , the number of earthquakes considered in this study is only 706. Incr eases as well as decreases of TEC are observed above some epicentres. Three events are detailed and a statistical study is performed with th e DORIS data. The corresponding number of TEC perturbations con-elated in space and in time (up to 48 h before these earthquakes) is 238, i. e., 34% of the cases. A comparison is made with TEC data randomly sele cted during the lifetime of the satellite, and ionospheric perturbatio ns are observed in 16% of the cases. This study indicates that it is p ossible to use TEC measurements to survey these phenomena. (C) 1998 El sevier Science B.V.