The Characteristics of Magnetic Signals Observed at Lulin ELF Station and Their Association with Earthquakes in Taiwan

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.