Ultra-low-frequency (ULF) and total electron content (TEC) anomalies observed at Agra and their association with regional earthquakes

Since earthquakes are also known to perturb the ionosphere, we have analysed total electron content (TEC) data obtained from a GPS receiver stationed at Agra since June 2006. Analysis of the data for the same period as the magnetic field observations shows that there existed TEC anomalies on all the days when ULF bursts occurred. The TEC anomalies show enhancement and depletion in the data in all cases. While the mechanisms of the ULF and TEC anomalies are not well known, it is expected that the ULF bursts occurred due to microfracturing of the earth's crust during stress accumulation and that the TEC anomalies occurred as a result of the electric field induced in the ionosphere during such processes.