Precursors signatures of few major earthquakes in Italy using very low frequency signal of 45.9 kHz

On May 20 and May 29, 2012, earthquakes of magnitude M_W 6.1 and 5.8, respectively, struck Emilia-Romagna of Italy. We present the results obtained from the US Navy VLF Transmitter's NSY signal of 45.9 kHz transmitted from Niscemi, in the province of Sicily, Italy, and received at the Kiel Longwave Monitor, Germany, for 2012. We analyzed the terminator times, D-layer preparation and disappearance times and nighttime fluctuation parameters. We also analyzed trends, dispersion and nighttime fluctuation which gave us possible precursors to these earthquakes. Possible perturbations due to other factors were filtered. Moreover, the findings of these two earthquakes were compared to Central Italy EQ on August 24, 2016, and Tuscany EQ on June 21, 2013. Therefore, this manuscript is the overall analysis of four earthquakes, out of which two were already published before. As our TRGCP path is North–South, the sunrise and sunset times in transmitter and receiver places match making a pathway for VLF/LF smoother and therefore hoping to obtain more natural data. We found many clear anomalies (as precursors) in terminator times 5 days to 16 days before the earthquakes. Moreover, using nighttime fluctuation method, we found clear anomalies 5–13 days prior to main earthquakes. This exactly correlates with the findings of previous authors that ionospheric perturbations are seen from few days to few weeks before the seismic activity. In addition to this, we observed an unexpected decrease in dispersion on certain anomalies where it was supposed to increase, thereby not supporting our finding to some extent. To resolve this problem, we devised a new parameter called dispersion nighttime (dispersion*). On analyzing, this parameter decreases significantly on days of nighttime anomalies, thereby supporting our precursors to much extent.