Variation in natural short-period ionospheric noise, and acoustic and gravity waves revealed by the amplitude analysis of a VLF radio signal on the occasion of the Kraljevo earthquake (Mw = 5.4)

Reduction of the VLF Signal Phase Noise Before Earthquakes

“…As it can be seen in Table 1, their magnitudes were greater than 5 for two events (the first one near Kraljevo, and in the Tyrrhenian Sea) while other two had magnitude larger than 4. These magnitudes are larger than those related to other EQs considered in [33]. During the considered four time intervals, additional weaker EQs also occurred near the considered signal propagation path.…”

“…However, contrary to the previous cases this pioneer study which indicates a possible new ionospheric precursor of earthquake is provided only for the amplitude i.e., variations of the phase, is not considered. For this reason, in this study we extend the research presented in [33] and investigate if the recorded changes in amplitude noise are also visible in analysis of the VLF signal phase, and if excitations and attenuations of the acoustic and gravity waves can be visualized from the recorded phase. We show analysis of the phase of the ICV signal emitted in Italy and recorded in Serbia in periods around four EQs with magnitude greater than 4 which are connected with the noise amplitude reductions in study shown in [33].…”

“…In this study we analyse phase of the ICV signal emitted by a transmitter located in Isola di Tavolara, Italy (40.92 N, 9.73 E) and received in Belgrade, Serbia (44.8 N, 20.4 E) in time periods around four EQs, considered in [33] and connected to the short-term noise amplitude reduction. Two of these events occurred near Kraljevo, Serbia, one in the Tyrrhenian Sea (TS) and one in the Western Mediterranean Sea (WMS) (their epicentres are shown in map given in Figure 1).…”

“…These lower ionosphere disturbances are detected as the solar terminator shift [13,36,37] and deviations of the time evolutions of signal characteristics from values recorded during days with unperturbed conditions [5,38] (in time domain), and as variations in the wavelet power spectrum [5,39] (in frequency domain), and all of these changes are shown for both signal amplitude and phase. The recent analysis presented in [33] shows reduction of the amplitude noise of the VLF signal less than one hour before the earthquake occurred near Kraljevo, Serbia, on 3 November 2010 (the seismotectonic model of this event is presented in [40]), as well as excitation and attenuation of the acoustic waves. In addition, the similar changes in the amplitude noise are also recorded for 12 other earthquakes with different magnitudes during three whole days.…”

“…Research of AGW in the lower ionosphere is also presented in several studies. They relate to disturbances induced by the solar terminator [28], geomagnetic storms [29,30], tropical cyclones [19,31,32] earthquakes [33], solar eclipse [34], and they are based on analyses of the VLF/LF signals.…”