Anomalous behaviors of the VLF signals before earthquakes for VTX-Malda propagation path

We present the results of monitoring of four years (2005, 2007-09) of VLF signals collected in the Malda branch of ICSP, located in Malda, West Bengal and find the correlations, between the ionospheric activities and the earthquakes. Here we use that VLF signals which are transmitted from the VTX station (18.2 KHz). We first study the average signal throughout the year. We plot the so-called standardized calibration curve using the four years data. To establish the correlation between the ionospheric activities and the seismic events, we use the data of the year 2008 and we find that the deviations of the VLF day length (defined as the time difference between sunrise and sunset terminators) are correlated with the seismic event. We find that the highest deviation takes place one day prior to the seismic events. We also calculate the `D-layer preparation time´ (DLPT) and the `D-layer disappearance time´ (DLDT) for the data of 2008 and established the co-relation between the anomalous DLPT and DLDT with the seismic events. We find that the anomalous behaviour of the DLPT and DLDT are also correlated with the seismic events.