Spatio-temporal evolution of electromagnetic field pre- and post-earthquakes

Utilizing the electromagnetic field observations of DEMETER satellite， we statistically analyze the spatio-temporal evolution of electromagnetic field during the strong earthquakes with magnitude over than 7.0 occurred from 2005 to 2009. For each earthquake， using the power spectral density data of electromagnetic field in the ELF/VLF frequency band (370—897 Hz) from 90 days before the earthquake to 30 days after the earthquake， in the area within ±10° in latitude and in longitude from the epicenter， the background model is built up with the same period. A formula is applied to extract the perturbation amplitude of the electromagnetic field relative to the background field observation model during the earthquake time， and the perturbation amplitude is used to statistically study the spatio-temporal variations of electro-magnetic field for the 45 earthquakes， which had been chosen in this paper. Statistical analyses on the perturbation amplitude of magnetic field shows that 35 earthquakes exhibit anomaly that the maximum of perturbation amplitude exceeds 2.2 times standard deviation， while the result of the electric field shows that 39 strong earthquakes exhibit anomalies with the maximum perturbation higher than twice standard deviation， and the maximum electromagnetic perturbations mainly appear in the places within ±4° to ±10° in latitude and in longitude from the epicenter. The temporal variation of the perturbation amplitude shows there are three types of variation patterns. The abnormal electromagnetic perturbation that occurs before earthquakes displays that the distribution of epicenters is related to latitude. However， the perturbation amplitude of electromagnetic field over the random non-earthquake region is weak， without displaying prominent characteristic of anomalous variation.