Abstract
Electromagnetic wave data from years 2009 2010 which were recorded by the extremely-low-frequency (ELF) station at Lulin Observatory in Taiwan (120.87°E, 23.46°N; 2862 m) are analyzed to explore the wave properties and their association with seismic and geomagnetic activities. Specifically, the power spectra, the ellipticity and the orientation of the below-10-Hz wave signals are studied. The correlation coefficients of these ELF signals and geomagnetic activity indices Dst and Kp are within -0.2 to 0.2, indicating geomagnetic activities are not likely sources for these emissions. In the frequency versus time (f-t) spectrograms, a few events exhibit a frequency shift between 4 and 6 Hz and the possible association with seismic activities of magnitudes (M) larger than 4 in the vicinity of Taiwan are examined. The subset of earthquakes in Taiwan with M/r2 (r is the epicenter distance in kilometer from the Lulin station) of greater than 6 × 10-4, a total number of 50, show a higher association with ELF wave events showing a frequency down-shift within 7 days prior to the earthquakes and a frequency bounce-back within 7 days after the earthquakes. The epicenters of these earthquakes are all within a distance of 100 km from the Lulin station and with a depth of 50 km, suggesting that the detected ELF frequency-shift wave events maybe are pre-earthquake electromagnetic signals.
Highlights
Over the past decades, there have been many observation reports of electromagnetic disturbances in the frequency range of 10-3 to 108 Hz before large earthquakes (e.g., Kalashnikov 1954; Johnston 1989; Parrot 1990; Hayakawa and Molchanov 2008)
ELF magnetic wave data collected by the ELF station at Lulin Observatory between October 2009 and September 2010 were analyzed to investigate the features of waves with frequency below 10 Hz
Wave frequency shift between 4 and 6 Hz were identified and their correlations with geomagnetic activities were examined and found to be insignificant (Fig. 3). Correlations of these wave events with earthquakes (M > 4) in the Taiwan region were studied for ±7 days within the occurrence of earthquakes
Summary
There have been many observation reports of electromagnetic disturbances in the frequency range of 10-3 to 108 Hz before large earthquakes (e.g., Kalashnikov 1954; Johnston 1989; Parrot 1990; Hayakawa and Molchanov 2008). Schekotov et al (2007) analyzed 4 to 6 Hz radio frequency data to avoid interferences from global geomagnetic activities (frequencies < 2 Hz) and the background Schumann resonance signals (frequencies > 7 Hz); they found this band emissions in the Kamchatka region started 5 days before an earthquake and persisted for 5 days after the quake for the period from 24 February 2003 to 6 April 2003 In their work, they found, for the probable quake-related radio signals, the spectral power ratio of the radio signals recorded along the north-south and the eastwest directions exhibited enhancement, while the standard deviation of the ellipse orientation angle and the ellipticity decreased and the polarization of waves became more linearized. The angle θ was found to concentrate more at a certain value before large quakes (Schekotov et al 2007), so here we tag the sharp changes associated with focused values of θ in this rather continuous band
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