Simultaneous ULF/VLF amplitude anomalies observed during moderate earthquakes in Indian region*

Abstract

A three-component search coil magnetometer to monitor ultra low frequency (ULF) magnetic field emissions (frequency = 0.01 -30 Hz) and an Absolute phase and amplitude data logger (AbsPAL) to monitor the phase and amplitude

variation of very low frequency (VLF) 19.8 kHz fixed frequency NWC (Australia) transmitter signals have been installed at Bichpuri, Agra (Geograph Lat. 27.2°N, Long. 78^oE) in India and regular simultaneous observations have been taken since 1 Aug. 2002. The analysis of the nighttime VLF amplitude data has been carried out for the initial period or nine months between 1 Aug. 2002 and 30 Apr. 2003 and it is seen that the amplitude decreased abnormally by 1-10 dB from normal nighttime daily averages on six days during the whole period under consideration. The results of ULF data analysis show that the background magnetic field amplitudes of the three components, which are usually low. in the range of 0.03-0.30 nT are enhanced to the range of 0.26-0.96 nT, either on the same days or within ± 2 days of the decrease in amplitude of the VLF signal. The possible causes for the observed ULF/VLF amplitude anomalies are examined and it is found that they are caused by moderate seismic activities that occurred along the VLF propagation path in the region. The statistical analysis of the data using mean and standard deviation around the mean is carried out, which supports the results satisfactorily.