Abstract
The Earth's natural pulse electromagnetic field (or ENPEMF) is a new concept of geophysics area in recent years. The physical mechanisms of its field source about which there are a variety of hypothesis, is generally considered as a reflection of dynamic change of the geological structure in the “tectonic-electricity” transformation process on the earth surface, that is, macro piezoelectric effect and electrokinetic effect is gathered by micro. Meanwhile, rock fracture and the spread of the strain wave can also excite electromagnetic effect, while observation equipment on ground can capture the wide spectrum of electromagnetic radiation wave. Its frequency and amplitude of the ENPEMF signals will be show some anomalies before an earthquake. In order to find the distribution relationship between the frequency and time, we adopt some time-frequency analysis methods to deal with ENPEMF signals. In this paper, an improved algorithm named Wigner-Ville Distribute based on Dual Ensemble Empirical Mode Decomposition (DEEMD-WVD) and the Hilbert-Huang Transform (HHT) are used to process ENPEMF data obtained before the Lushan earthquake (M S 7.0, 2013-04-20), it reveals clear preseismic time-frequency characteristics of ENPEMF, namely, a strong-weak-strong pattern.
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