Long-Memory and Fractal Traces in KHz-MHz Electromagnetic Time Series Prior to the ML=6.1, 12/6/2007 Lesvos, Greece Earthquake: Investigation through DFA and Time-Evolving Spectral Fractals

This paper presents evidence of long-lasting and chaotic trends in one-month kHz-MHz electromagnetic disturbances collected prior to a ML=6.1 shallow earthquake (June 12, 2017, 12:28:38 GMT, 38.84° N/26.36° E, 12 km deep, 37.5 km SSE of Mytilene town, Lesvos island, Greece) recorded by a telemetric ground station (39.23° N/26.27° E) located only 44 km away from the earthquake's epicenter. All analyzed earthquake occurrences (4.0 ≤ ML< 6.1) formed tight groups in both time and space which is significant for the investigation. The analysis is implemented via detailed timeevolving sliding-window two-slope DFA and power-law analysis of 4096 samples per window allowing hidden, potentially precursory, pre-earthquake trends to emerge. The classical two-exponent DFA results support the aspect of possible pre-earthquake activity 10-12 days prior to the ML=6.1 earthquake, for the 3-10 kHz antennas (both EW-NS orientations) and the 41-46 MHz ones, by simultaneously presenting a sudden increase of a parameter calculated from the two DFA exponent data. The time evolution of the power-law fractal-analysis data indicates activity 12-13 prior to the event, however, only for the 3 kHz antennas. Hurst exponents calculated in various analysis segments indicate persistency during the main pre-earthquake activity as well as persistency-anti-persistency changes. Potential pre-seismic activity prior to two other earthquakes of ML=5.0 and ML=4.6 is investigated and discussed. The precursory activity of reported time-series is discussed.