Small-period wave excitations in the amplitude of VLF signals before earthquakes: differences for periods with and without intense seismic activity

The application of Fast Fourier Transform (FFT) to the amplitudes of very low frequency (VLF) signals recorded with a sampling time of 0.1 s showed that waves with wave periods below 5 s could be excited during seismic activity. The beginnings of these excitations were observed several minutes or tens of minutes before earthquake events, but studies have shown differences in the cases of earthquakes that occur during a period of intense seismic activity (PISA) and in a period without intense seismic activity (PWISA). Namely, in the first case, the Fourier amplitude is higher in the domain of wave periods from about 2 to 4 s compared to other values, while in the second case, almost discrete values of the periods in which the excitations are visible are recorded. This study presents a comparison of the characteristics of excited waves during the considered PISA and PWISA. The amplitude analysis of the 20.27 kHz ICV signal emitted in Italy and recorded in Serbia is given for one PISA in Central Italy (from 26 October to 2 November 2016) and four earthquakes that occured near Kraljevo, Serbia (Mw 5.4, and ML 4.4), in the Tyrrhenian Sea (mb 5.1), and in the Western Mediterranean Sea (ML 4.3) during PWISA (from 3 to 9 November 2010). Although more reliable conclusions require statistical analysis, the significance of the presented results lies in the indication that the aforementioned differences may indicate the possibility of a multi-day period during which earthquakes can be expected in a localized area.