This thesis presents results based on wave measurements performed by the DEMETER spacecraft. This French spacecraft was operating between 2004 and 2010 at the altitude of ~660 km. We focus on measurements of the electromagnetic waves and plasma density in the vicinity of imminent earthquakes. In a first part, we present a study of plasma density variations in the vicinity of a very powerful earthquake in Chile ( Mω 8.8) which occurred on February 27, 2010. Data recorded before the main shock along orbits close to the future epicenter showed increasing plasma densities. In a second step, a statistical analysis using 4 years of data to monitor density variations under similar conditions has been performed. This study shows that a large increase of the plasma density is very uncommon at this location and at this time. In a second part, a statistical study (~6.5 years) of variations of VLF wave intensity is shown. We have analyzed all available data measured close (in time and in distance) to large earthquakes (M> ou = 5). Data related to more than 12000 earthquakes have been compared with an unperturbed background distribution. We confirm the previously reported results of a statistically significant decrease of the wave intensity at frequencies of about ~1.7kHz. This effect seems to be related to a decrease of the lower ionospheric boundary which affects the VLF waves propagating in the Earth-ionosphere waveguide.
