Spacecraft observations of electromagnetic perturbations connected with seismic activity

Results of a statistical study of intensity of VLF electromagnetic waves observed in the vicinity of earthquakes are presented. A unique set of data obtained by the micro-satellite DEMETER (altitude of about 700 km, nearly Sun-synchronous orbit) and a robust two-step data processing has been used. In the first step, all the measured data are used to construct a map of electromagnetic emissions containing a statistical description of wave intensity at a given point of the satellite orbit under given conditions. In the second step, the intensity measured close to earthquakes is analyzed using the statistical distribution of background intensity obtained in the first step. The changes of wave intensity caused by seismic activity are investigated and their statistical significance is evaluated. Altogether, more than 2.5 years of satellite data have been analyzed and about 9000 earthquakes with magnitudes larger than or equal to 4.8 that occurred all over the world during the analyzed period have been included in the study. It is shown that, during the night, there is a statistically significant decrease by 4 - 6 dB of the measured wave intensity shortly (0-4 hours) before an intense surface (depth less than or equal to 40 km) earthquake.