PUBLICATIONS OF THE ASTRONOMICAL OBSERVATORY OF BELGRADE104, Page 105-112, https://doi.org/10.69646/aob104p105
PROCEEDINGS of the XX Serbian Astronomical Conference, Belgrade, October 16 - 20, 2023, Edited by Jelena Petrović, Dušan Marčeta and Ana Lalović
Published by: Astronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia

Abstract. Studies of pre-earthquake disturbances in the ionosphere since the middle of the last century point to several types of potential precursors to these natural events. A large number of them are detected as disturbances in the Global navigation satellite system (GNSS) and very low / low frequency (VLF/LF) signals a few days before the earthquake. However, the analysis of the VLF signal emitted in Italy and received in Belgrade in the period around the earthquake in Kraljevo that occurred on 3 November, 2010 indicates a potentially new type of these precursors. It is visible as a reduction of the VLF signal noise which starts several minutes or several tens of minutes before earthquake. In addition, Fourier transform of the recorded data indicates wave excitations at discrete waveperiods under 1.5 s, and wave attenuations at other waveperiods. In this paper, pilot studies of the noise reduction of the amplitude and phase of the mentioned VLF signal in both time and frequency domains are presented. The data recorded by the Absolute Phase and Amplitude Logger (AbsPAL) receiver located at the Institute of Physics Belgrade on November 3, 4 and 9, 2010, and during the period of intense seismic activity when almost 1000 earthquakes occurred in Central Italy from October 25 to November 3, 2016, are analyzed. The obtained results indicate possible differences in the signal noise reduction in cases when the observed earthquake follow and not follow previous earthquakes in approximately the same area. Namely, in the first case, the signal noise reduction may already be present so that no new reductions occur before subsequent earthquakes. On the analyzed sample of 35 earthquakes of minimum magnitude 4, the signal noise is reduced in over 90% of cases. In the remaining three cases, it cannot be clearly concluded whether this reduction exists due to its reduced value over a longer time period, which can be explained by the intense seismic activity in that period.