Unusual shifts in terminator times of the VLF signals before the Pakistan earthquake (M=7.4), occurred on 18th Jan., 2011

On 18th Jan., 2011, at 20:23 UT, an earthquake of magnitude 7.4 occurred in Southwestern Pakistan (latitude 28° 54´ N, longitude 64° E). We present the results of the analysis of Very Low Frequency (VLF) radio signals, received at three receiving stations located in India. We analyze the VLF signal around this earthquake day for four different VLF propagation paths namely DHO-IERC(Sitapur), VTX-Pune, VTX-ICSP(Kolkata) and NWC-IERC. We observed significant shifts of the `sunrise terminator time´ (SRT) for DHO-IERC and VTX-Pune paths. We also observed the SRT-shift for VTX-Kolkata path, but here the shift is not so strong. We found no significant shifts of SRT for NWC-IERC propagation path. Higher deviation in the VTX-Pune path as compared to VTX-ICSP path could be due to the formers proximity to the epicentre. Similarly, DHO-IERC path is over the epicentre while that of NWC-IERC path is away from the epicenter. This could be the reason why the effect in DHO-IERC path is stronger than that in NWC-IERC path.