Reduction of the VLF Signal Phase Noise Before Earthquakes

Abstract

In this paper we analyse temporal variations of the phase of a very low frequency (VLF) signal, used for the lower ionosphere monitoring, in periods around four earthquakes (EQs) with magnitude greater than 4. We provide two analyses in time and frequency domains. First, we analyse time evolution of the phase noise. And second, we examine variations of the frequency spectrum using Fast Fourier Transform (FFT) in order to detect hydrodynamic wave excitations and attenuations. This study follows a previous investigation which indicated the noise amplitude reduction, and excitations and attenuations of the hydrodynamic waves less than one hour before the considered EQ events as a new potential ionospheric precursors of earthquakes. We analyse the phase of the ICV VLF transmitter signal emitted in Italy recorded in Serbia in time periods around four earthquakes occurred on 3, 4 and 9 November 2010 which are the most intensive earthquakes analysed in the previous study. The obtained results indicate very similar changes in the noise of phase and amplitude, and show an agreement in recorded acoustic wave excitations. However, properties in the obtained wave attenuation characteristics are different for these two signal parameters.

Variation in the VLF signal noise amplitude during the period of intense seismic activity in Central Italy from 25 October to 3 November 2016

“…The presented analysis of the noise amplitude in a localized area during PISA is a pioneer study and the confirmation of the obtained conclusions requires additional case studies or statistical research which should be carried out in future. However, a comparison of the study of the VLF signal characteristics related to separated relatively strong EQs given in Nina et al (2020) and Nina et al (2021a) indicates similarities with conclusions obtained in this study for the corresponding periods. The potential horizontal displacements and enlargements of the area related to the seismic zone indicated in this study are also shown in the studies of the atmospheric chemical potential (ACP) and TEC.…”

“…The procedure for the determination of the noise amplitude, A noise , used in this study is described in Nina et al (2015) and applied in the studies of the noise reductions in the amplitude and phase presented in Nina et al (2020) and Nina et al (2021a), respectively. It is based on processing of the signal amplitude A recorded by the AbsPAL receiver with time resolution of 0.1 s, and on determining of:…”

“…As in the study presented in Nina et al (2020) and Nina et al (2021a), in this analysis we assumed that p = 5% (the study given in Nina et al (2015) indicates that the choice of this value has no essential significance for relevant analyses).…”

“…Previous studies indicate that typical perturbations (variations of the electron density (positive and negative) at specific altitudes of the ionosphere registered by ionosondes, the total electron content (TEC) variations registered by GNSS receivers, changes in the VLF/LF signal amplitude (or phase), its periodic fluctuations and the "terminator time" shift) are registered in all ionospheric regions mostly a few days before the occurrence of stronger earthquakes (Hayakawa, 1996;Molchanov et al, 1998;Biagi et al, 2001bBiagi et al, , 2006Miyaki et al, 2001;Molchanov et al, 2001;Pulinets and Boyarchuk, 2004;Rozhnoi et al, 2004;Yamauchi et al, 2007;Hayakawa et al, 2010;Liu et al, 2018;Perrone et al, 2018;Pulinets et al, 2022). However, recent research presented in Nina et al (2020) and Nina et al (2021a) points out a possibility for a new type of precursor that occurs up to a few tens of minutes before an EQ (including some cases of weak earthquakes whose magnitude can be less than 3) and manifests itself through a reduction in the VLF signal amplitude and phase noise, respectively. These studies present analyses of several EQs whose epicentres were relatively close to the considered signal propagation path.…”

Two-Dimensional Correlation Analysis of Periodicity in Noisy Series: Case of VLF Signal Amplitude Variations in the Time Vicinity of an Earthquake

“…Those precursors are primarily associated with changes in the VLF signal amplitude and/or phase [1][2][3][4][5] and the amplitude minimum time shift during solar terminator periods (the so-called "terminator time") [6][7][8][9][10]. In addition, the most recent research suggests that there are reductions in the noise of the VLF signal amplitude and phase a few tens of minutes before the observed type of disaster [11][12][13]. Although various data processing procedures have been applied in previous studies, there is still no way to reliably predict an earthquake.…”