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

Abstract

Recent research shows reductions in the VLF signal noise amplitude that begin before particular earthquakes whose epicentres are more than 100 km away from the signal propagation path. In this paper, we extend this research to studying the noise amplitude during periods of intense seismic activity in a localized area. We analyse variations in the VLF signal noise amplitude over a period of 10 days (25 October–3 November 2016) when 981 earthquakes with the minimum magnitude of 2 occurred in Central Italy. Out of these events, 31 had the magnitude equal or greater than 4, while the strongest one had the magnitude of 6.5. We observe the VLF signal emitted by the ICV transmitter located in Sardinia (Italy) and recorded in Belgrade (Serbia). Bearing in mind that the trajectory of this signal crosses the area in which the observed earthquakes occurred, we extend the existing research to study of variations in the noise amplitude of the signal propagating at short distances from the epicentres of the considered earthquakes. In addition, we analyse the impact of a large number earthquakes on characteristics of the noise amplitude and its reductions before particular events. In order to examine the localization of the recorded changes, we additionally analysed the noise amplitude of two reference signals emitted in Germany and Norway. The obtained results show the existence of the noise amplitude reduction preceding individual strong or relatively strong earthquakes, and earthquakes followed by others that occurred in a shorter time interval. However, the additional noise amplitude reductions are either not pronounced or they do not exist before the considered events in periods of the reduced noise amplitude remain from previous earthquakes. Reductions in noise amplitudes for all observed signals indicate a larger perturbed area through which they spread or its closer location to the receiver. The analysis of daily values of parameters describing the noise amplitude reveals their variations start up to 2 weeks before the seismically active period occurs.

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

“…These periods are chosen to exclude the effects of daily and seasonal changes that are visible in the VLF signal amplitudes and that may affect the observed comparison. In addition, it was taken into account to eliminate the potential influences of other natural phenomena with origin in the atmosphere and from space as well as non-natural causes of variations in the emission and reception of the considered signal (these influences are described in detail in [11] and [13]). For this reason, the reference intervals are chosen in periods when no significant disturbances in meteorological, geomagnetic and space weather conditions were recorded, and when approximately the same values of the amplitude and its noise were recorded as in the quiet period before the Kraljevo 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.…”

Analysis of the Solar Terminator Shift Determined from the VLF Signal Amplitude in the Period Around the Intense Seismic Activity in Central Italy from 25 October to 3 November 2016

“…The propagation path of the observed 20.27 kHz VLF signal is given on the map shown in Nina et al, 2022 (Figure 1). The EQ epicentres are marked with white and magenta crosses, and blue, red and black circles for the EQ magnitudes (M) between 2 and 3, 3 and 4, 4 and 5, and 5 and 6, and for magnitudes greater than 6, respectively (the magnitude types are given in http://www.emsc-csem.org/ Earthquake/ and, for the considered EQs with M ≥ 4, in Nina et al, 2022 (Table 1)).…”

“…The propagation path of the observed 20.27 kHz VLF signal is given on the map shown in Nina et al, 2022 (Figure 1). The EQ epicentres are marked with white and magenta crosses, and blue, red and black circles for the EQ magnitudes (M) between 2 and 3, 3 and 4, 4 and 5, and 5 and 6, and for magnitudes greater than 6, respectively (the magnitude types are given in http://www.emsc-csem.org/ Earthquake/ and, for the considered EQs with M ≥ 4, in Nina et al, 2022 (Table 1)). As can be seen from this map, the locations of the epicentres are primarily grouped in the area where their distances from the ICV signal path (D) are less than 100 km (since the greatest distance from the signal path of this group of epicenters is 99.86 km, in the analysis we chose that the border distance of the considered area is 100 km).…”

“…Processing of recorded amplitudes and phases of VLF signals shows several types of changes that can be considered as possible earthquake (EQ) precursors. They refer to changes in the values of these parameters and the solar terminator time (STT) shift, usually visible a few days before the individual EQs, and the reduction of the signal amplitude/phase noise a few tens of minutes before EQ (Hayakawa, 1996;Molchanov et al, 1998;Yamauchi et al, 2007;Biagi et al, 2001a;Rozhnoi et al, 2010;Biagi et al, 2006;Hayakawa et al, 2010;Miyaki et al, 2001;Molchanov et al, 2001;Rozhnoi et al, 2004;Nina et al, 2020Nina et al, , 2021Nina et al, , 2022.…”