Noise reductions of VLF signals and excitation/attenuation of waves with small wave periods before earthquakes

Various types of changes in the characteristics of very low frequency (VLF) signals before earthquakes have been presented during the past few decades. Most of these changes have been observed on data with time sampling of the order of a few tenths of a second or of the order of minutes. Improvements in this sampling in recent years have indicated three new types of changes whose onsets have been observed a few minutes or tens of minutes before the earthquake. These changes manifest themselves as reductions in the VLF signal amplitude and phase noises, and excitation and attenuation of waves with small wave periods in both of these signal characteristics [1-5].In this work, we present these changes and list the parameters in the time and frequency domains that are significant for statistical analyses. A central issue is the relationship of the changes with the characteristics of earthquakes, the observed signals, and their spread in the surrounding area. The presented analyses were conducted on data recorded by a VLF receiver in Belgrade, Serbia. References: [1] A. Nina, S. Pulinets, P.F. Biagi, G. Nico, S.T. Mitrović, M. Radovanović, L.Č. Popović, “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)”, Science of The Total Environment, 710, 136406, 2020.[2] A. Nina, P. F. Biagi, S. T. Mitrović, S. Pulinets, G. Nico, M. Radovanović,  L. Č. Popović, “Reduction of the VLF signal phase noise before earthquakes”, Atmosphere 12 (4), 444, 2021.[3] A. Nina, P. F. Biagi, S. A. Pulinets, G. Nico, S. T. Mitrović, V. M. Čadež, M. Radovanović, M. Urošev,  L. Č. Popović, “Variation in the VLF signal noise amplitude during the period of intense seismic activity in Central Italy from 25 October to 3 November 2016”, Frontiers in Environmental Science, 10, 10:1005575, 2022.[4] A. Nina, “Analysis of VLF Signal Noise Changes in the Time Domain and Excitations/Attenuations of Short-Period Waves in the Frequency Domain as Potential Earthquake Precursors”, Remote Sensing, 16(2), 397, (2024)[5] A. Nina “VLF Signal Noise Reduction during Intense Seismic Activity: First Study of Wave Excitations and Attenuations in the VLF Signal Amplitude”, Remote Sensing, 16(8), 1330, 2024.