Ultra low frequency (ULF) amplitude anomalies observed at Agra (India) and their association with regional earthquakes

Employing a three-component search coil magnetometer (f = 0.01–30 Hz) the ultra low frequency (ULF) magnetic field observations have been started in a noise free rural site at Agra (India) since 20 April, 2007. We have analysed seven months of nighttime data for the period 01 May, 2007–30 November, 2007. The data show seven cases of occurrence of ULF bursts with periods ranging from a few minutes to an hour in a wide frequency range up to 15 Hz. The occurrence of such ULF bursts have been analysed statistically and polarization parameter criteria tested. The polarization parameter analysis shows that five of these bursts occurred from sources under the ground (Z/X > 1, where Z and X are vertical and horizontal components of the magnetic field emissions) whereas the rest of the two bursts were caused by sources in the ionosphere and magnetosphere (Z/X < 1). It is further seen that the occurrences of the former bursts are correlated very well with some regional earthquakes (M ⩾ 4.5) which occurred at distances less than 628 km from the observing site at Agra. While the causes of the ULF anomalies are not well known, it is expected that the ULF bursts occurred due to microfracturing of earth crust during stress accumulation.