Abstract
This study investigates the application of empirical mode decomposition to signals from very low frequency transmitters in Europe that were received in Thessaloniki, Greece, to provide a method for depicting seismic-ionospheric precursor phenomena that occur prior to an earthquake. The basis for ionosphere interactions with seismic phenomena has been well documented in past studies, and the depiction of disturbances applied from the earthionosphere waveguide on the received signals was the purpose of this study. Empirical mode decomposition is a method for processing of nonlinear and nonstationary signals, to decompose them into their functional components, known as intrinsic mode functions. This method can provide high pass filtering to signals, thus depicting a clearer image of any abnormal disturbances in the signals that are not part of the normal noise content. Observations of such precursor phenomena are presented and correlated to earthquakes, to demonstrate the effectiveness of this method.
Highlights
A large amount of research over the last 20 years has indicated the existence of pre, co- and post-earthquake ionospheric perturbations at all levels of the ionosphere (i.e. E, D, F layers)
It is generally accepted that there is a lithosphereatmosphere-ionosphere connection mechanism that works through one of two mechanisms: (i) By the generation of atmospheric gravity waves at acoustic frequencies during the earthquake preparation period. These begin in the area of the epicenter, and subsequently move upwards, to enrich the turbulent content of the ionosphere over the epicenter and to initiate gravity waves that propagate in the waveguide of the ionosphere [Molchanov et al 2004]; or (ii) By ion exhalation and the subsequent variations in the electric field at the site of the earthquake preparation area, which produces variations in the ionosphere over the epicenter area [Pulinets and Ouzounov 2010]
The follow-up of very low frequency/low frequency (VLF/LF) electromagnetic signal propagation is considered to be a reliable diagnostic means for pre, co- and post-earthquake ionospheric perturbations for earthquakes that might occur along the propagation path of the signal
Summary
A large amount of research over the last 20 years has indicated the existence of pre-, co- and post-earthquake ionospheric perturbations at all levels of the ionosphere (i.e. E, D, F layers). To reveal uneven disturbances and to facilitate the detection of possible earthquake-connected perturbations, the use of the Hilbert-Huang transform [Huang and Attoh-Okine 1998, Huang et al 2005] is proposed here, or rather the part of it that is described by the empirical mode decomposition (EMD) method. 2. Empirical mode decomposition Traditional filtering methods are realized in the frequency domain only.
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