Signal-Processing Investigations on the Precursors of Strong Earthquakes Using GPS-TEC, ULF, and VLF Data

The seismo-electromagnetic studies have been in progress since 1998 at Agra station. In the present paper, ionospheric GPS-TEC, ground-based ULF/VLF measurements were investigated in light of four strong earthquakes (M ≥ 6.8) that occurred around the Indian subcontinent in different periods. These three datasets are processed by using advanced signal-processing techniques in time and frequency domains. To analyze these datasets, a period of 16 days (including the day of the earthquake) was considered. For each day, only one minute of data was taken into account, with the time of the earthquake being the midpoint of that minute. The precursors are obtained in all the datasets considered before the occurrence of earthquakes. In TEC, ULF, and VLF data, significant changes are observed 2 to 15, 2 to 7, and 5 to 13 days before earthquakes, respectively. Significant results are obtained in time and frequency domains and the variations of solar and magnetic storm activities have also been examined thoroughly to check the validity of these variations. Further, these variations are interpreted in terms of lithosphere-atmosphere-ionosphere coupling mechanisms available in the literature.