It is well established that earthquakes could be a major cause of the ionospheric perturbations and the consequent disturbances in the VLF signals. It is believed that the seismic events can perturb the height and ions density profile of the ionosphere and the ground conductivity can be affected also. We observe that the daytime (especially during the sunrise and sunset time) and night time VLF signal both in amplitude and phase shows anomalous behavior during and prior to the seismic events in the context of Indian scenario. We use three methods viz. (1) Sunrise-sunset terminator time shift method, (2) D-layer Preparation/Destruction (DLPT/DLDT) method and (3) Night time fluctuation method and observe that most of the anomaly in the VLF signal occurs on one to three days prior to the earthquakes.

The correlations between signal anomaly with earthquake magnitude for (1) Terminator time (left) method, (2) DLPT/DLDT (center) method and (3) Night time fluctuation (right) method. The anomaly ix maximum one two, one and three days before the earthquake respectively (see Sasmal et al., 2009; Chakrabarti et al., 2010; Ray et al., 2011 for details).

We are developing automatic alert system throughout the India to collect and analyze the VLF data simultaneous collected from different parts of the country and try to observe the seismo-ionospheric correlations with a much detail analysis. Our networking system to collect the data from all running stations using the VPN technology is perfectly ready.

Currently the analysis of the phase of the VLF data for long propagation path is going on. Also we are in process of comparing the anomaly in the signal with ionospheric parameter like Total Electron Content (TEC) recorded from GPS.

The schematic diagram of the Automatic Seismic Alert System