The calculation of the energy of EM events on a MHz EM emissions example, recorded prior to the Athens EQ (September 7, 1999, 11:56:49 UT, [38.12°N, 23.60°E], M = 5.9). (a) The recorded amplitude of the analyzed excerpt. The horizontal grey line shows a possible value, above which the recordings are considered fracture-related. (b) The corresponding candidate “fracto-electromagnetic emissions” calculated as the difference . (c) The squared amplitude (power) of the fracto-electromagnetic emissions of Fig. 1(b) . (d) An example of the revealed EM events (between 6400 and 6600 s). Their energy is calculated as the summation of the consecutive squared amplitudes (green areas). The vertical black lines define the limits of the different EM events; the larger of them can be easily identified by inspection.
The EM signal excerpts under analysis. Upper row: The signal excerpts (of ∼5-8 h duration) of the MHz EM emissions recorded before the Kozani-Grevena (left), the Athens (center), and the Methoni (right) EQs. Lower row. The signal excerpt (of ∼6.5 d duration) of the kHz EM emissions recorded before the Athens EQ.
Dependence of the natural time analysis quantities , and relative to amplitude threshold change for the MHz signals detected prior to the significant EQs of (a) Kozani-Grevena, (b) Athens, and (c) Methoni, along with (d) the kHz signal recorded prior to the Athens EQ. The light green horizontal line indicates the entropy limit of , while the solid grey line the value and the grey broken lines a region of around it.
Transformation to the natural time domain. Upper row: the signal excerpts (of ∼5–8 h duration) of the MHz EM emissions recorded before the Kozani-Grevena (left), the Athens (center), and the Methoni (right) EQs that show criticality. Lower row: the transformations to natural time of the corresponding MHz anomalies of the upper row, i.e., the “energy,” , time-series of the corresponding EM events in natural time.
(a) The normalized power spectra for the three MHz anomalies (colored dashed curves) and the kHz anomaly (grey dashed-dotted curve) of Fig. 2 along with the theoretical estimation for (black solid curve), i.e., Eq. (2.75) of Ref. 34 . The inset refers to the range for the easiest discrimination of the different curves. (b) The spectra extended up to .
Evolution of the natural time analysis quantities , and relative to new event occurrence prior to the significant EQ of Athens, considering an area of 160 km around Athens, for different magnitude thresholds, (a) , and (b) . The horizontal lines have the same position and meaning as in Fig. 6 .
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