Abstract:
We have analyzed ULF magnetic field emissions data obtained from identical 3- component search coil magnetometers installed at Indian stations of Agra (Geograph. lat. 27.2◦N, long. 78◦E), Shillong (25.92o N, 91.88o E), and Kachchh (23.78o N, 71o E) for the months of April 2012 and September 2013 in which three major earth quakes occurred, one of magnitude M = 8.5 in Indonesian region and two of magnitudes M = 7.4 and 6.8 in Pakistan. The results of the analysis show the occurrence of ULF bursts 09-16 days prior to the occurrence of these earthquakes at Agra and Shillong stations. Such bursts are not observed at Kachchh station. Analysis of ULF data obtained from a flux gate magnetometer at a remote station Kakioka in Japan (Geograph. lat. 36.2o N, long. 140.2o E) does not show such burst also. The bursts contain electromagnetic pulses of periods 10–15s (f = 0.06–0.1 Hz). The origins of these bursts in lithospheric sources (earthquakes) are confirmed by polarization parameter analysis of data. These ULF data are not influenced by lightning and magnetic storms. A long period analysis of the ULF data for the same months between the years 2010 and 2017 shows that occurrence of such bursts are characterized by either large magnitude shallow depth earthquakes (M > 6) or multiple shallow depth earthquakes of low magnitudes (M < 6). The precursory periods of the recorded ULF bursts match very well with those reported by Singh et al. (2018) who analyzed very low frequency (VLF) and ionospheric total electron content (TEC) data recorded at Agra station.
