Ultralow frequency (ULF) magnetic field anomalies observed at Agra and their relation to moderate seismic activities in Indian region

Abstract A three-component search coil magnetometer ( f = 0.01 – 30 Hz ) has been employed to monitor the earthquake-induced magnetic field anomalies at Agra (Geograph. lat. 27.2 °N, long. 78 °E), India. The results show that the magnetic field amplitudes of the three components are normally low in the range of 0.03–0.7 nT, but they are occasionally enhanced to large values in the range of 0.3–5 nT (the X -component along the north–south direction being the lowest). The six month data between September, 2002 and February, 2003 have been analyzed and two cases of abnormally large amplitude enhancements are selected for detailed study. These two cases correspond to 23 November, 2002, and 5–6 February, 2003. The enhancement in amplitudes for these cases are examined in the light of magnetospheric micropulsations and earthquake generated Ultralow frequency (ULF) emissions. For this purpose, the amplitude variation is examined with variations of polarization parameter ( Z / X ) and ∑ K p . It is found that the enhancements are most probably caused by the ULF magnetic field emissions generated during moderate earthquakes (3.5 M