Study of earthquake precursors using Very Low Frequency (VLF) signals received at Cooch Behar in eastern India

With an aim to monitor the sub-ionosperic changes a Very Low Frequency (VLF) signal receiving station has been installed at Cooch Behar in Eastern India $[26.334^{\circ}\mathrm{N} ; 89.326^{\circ}\mathrm{E}]$. VLF signals transmitted from different navy transmitter e.g. VTX $[8.433^{\circ}\mathrm{N} ; 77.733^{\circ}\mathrm{E}]$, NWC $[21.816^{\circ}\mathrm{S}$; $114.165^{\circ}\mathrm{E}]$, NDT $[32.082^{\circ}\mathrm{N}; 130.827^{\circ}\mathrm{E}]$ around the world are received by an E-field whip antenna and a computer records the amplitude of the amplified signals at the rate of 4 data points per second. Diurnal variation of the data is analyzed and it is observed that the shifts of the Terminator Time during both the sun rise and sunset time are very prominent. Interestingly these perturbations are followed by several moderate size earthquakes. Primarily the VLF signals appear to be very promising for short time earthquake precursor study.