Multi-spectral detection of statistically significant components in pre-seismic electromagnetic emissions related with Athens 1999, M=5.9 earthquake

A multi-spectral analysis of the kHz electromagnetic time series associated with Athens' earthquake (M=5.9, 7 September 1999) is presented here, that results to the reliable discrimination of the fracto-electromagnetic emissions from the natural geo-electromagnetic field background. Five spectral analysis methods are utilized in order to resolve the statistically significant variability modes of the studied dynamical system out of a red noise background (the revised Multi-Taper Method, the Singular Spectrum Analysis, and the Wavelet Analysis among them). The performed analysis reveals the existence of three distinct epochs in the time series for the period before the earthquake, a “quiet”, a “transitional” and an “active” epoch. Towards the end of the active epoch, during a sub-period which is approximately starting two days before the earthquake, the dynamical system passes into a high activity state, where electromagnetic signal emissions become powerful and statistically significant almost in all time-scales. The temporal behavior of the studied system in each one of these epochs is further searched through mathematical reconstruction in the time domain of those spectral features that were found to be statistically significant. The transition of the system from the quiet to the active state proved to be detectable first in the long time-scales and afterwards in the short scales. Finally, a Hurst exponent analysis revealed persistent characteristics embedded in the two strong EM bursts observed during the “active” epoch. •Detection of statistically significant emissions in a pre-seismic period•Statistical significance of the emissions was tested against a red-noise background.•The emitting system passes into an active state 10 to 18days before the earthquake.•Activation of specific variability modes found to be frequency dependent.•Two days before earthquake all time scales became active and significant.