The INFREP Network: Present Situation and Recent Results

VLF/LF (20 300 kHz) radio waves propagation is affected by different factors such as meteorological conditions, solar bursts and geomagnetic activity. At the same time, variations of some parameters in the ground, in the atmosphere and in the ionosphere occurring during the preparatory phase of earthquakes can produce disturbances in the propagation of the previous signals along their radio paths: these disturbances are the radio precursors. Since 2009, several VLF/LF radio receivers have been installed throughout Europe in order to realize a European (VLF/LF) radio network for studying the VLF/LF radio precursors of earthquakes, called the INFREP network. In this paper, at first the description of the present situation of the INFREP network is presented, that is: the location of the receivers, the location of the VLF/LF transmitters whose signal is sampled, the daily download of the data collected by the receivers on the INFREP server and the method of data analysis used in order to individuate possible radio precursors. Then the results obtained on the occasion of recent (2016-2017) seismic activities which occurred in the “sensitive” zone of the INFREP network are presented. The first case examined is the October 30, 2016 earthquake with Mw = 6.5, which occurred in Central Italy, near Norcia small town; this earthquake was preceded by a strong shock (Mw = 5.9) which occurred 4 days before. The second case presented is the strong (Mw = 6.7) offshore earthquake which occurred on July 20, 2017, near the coast of Turkey and Kos island (Greece) and the third case is the August 8, 2017 earthquake with Mw = 5.0, which also occurred near the coast of Turkey and Kos island (Greece). In all the previous cases radio anomalies were revealed in some radio signals collected by the receiver loHow to cite this paper: Biagi, P.F., Colella, R., Schiavulli, L., Ermini, A., Boudjada, M., Eichelberger, H., Schwingenschuh, K., Katzis, K., Contadakis, M.E., Skeberis, C., Moldovan, I.A. and Bezzeghoud, M. (2019) The INFREP Network: Present Situation and Recent Results. Open Journal of Earthquake Research, 8, 101-115. https://doi.org/10.4236/ojer.2019.82007 Received: April 2, 2019 Accepted: May 24, 2019 Published: May 27, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
The VLF radio signals lie in the 20 -80 kHz frequency band. These radio signals are used for worldwide navigation support, time signals and for military purposes. The LF radio signals lie in 150 -300 kHz frequency band and are used for long way broadcasting by few (this type of broadcasting is going into disuse) transmitters located all over the world. The VLF signals propagate in the earth-ionosphere wave-guide mode along great circle propagation paths. The LF signals are characterized by a ground-wave and a sky-wave propagation mode.
The first one generates a signal that propagates in the channel ground-troposphere, while the second one generates a signal which propagates using the lower ionosphere as a reflector.
VLF/LF radio signal propagation is affected by different factors such as meteorological condition, solar bursts and geomagnetic activity. At the same time, variations of some parameters in the ground, in the atmosphere and in the ionosphere, occurring during the preparatory phase of earthquakes, can produce disturbances in the previous signals and these disturbances are the radio precursors. Radio precursors are reported in many previous studies [1]- [20]. These anomalies are pointed out in the intensity and/or in the phase of the radio signals and generally they are connected to earthquakes located within the 5 th Fresnel zone defined by transmitter and receiver location. Mainly, the precursors were revealed in the night time data because the VLF/LF radio signals propagation is less disturbed during the night than during the day, due to the dynamic status of the lower atmosphere. According to the most convincing models [ [25] the processes occurring during the preparatory phase of strong earthquakes determine a particular lithosphere, atmosphere, and ionosphere coupling and cause the variation of the medium in which radio signals propagate, affecting especially radio propagation in the VLF/LF bands.
The INFREP network has been developed in 2009 in order to study the VLF/LF radio precursors and some results have been published [26]- [31]. In the recent years the INFREP network was implemented and modified.
Here at first the current situation of the INFREP network is presented; then

Status Quo of INFREP Network
A web site [32] has been organized; the site shows the network, describes the research Teams involved and indicates the main references.
The network currently consists of nine receivers located as follows: two in Italy, Romania and Greece; one in Austria, Portugal and Cyprus. The radio receivers were manufactured by an Italian factory and measure the intensity (electric field strength) of 10 radio signals in the bands VLF (20 -80 kHz) and LF (150 -300 kHz), with 1 minute sampling rate. The signals radiated by existing VLF-LF broadcasting stations located in Europe are used. Generally, each receiver collects 5 VLF and 5 LF signals; in any case, the selection of the signals to collect is based on the quality of local reception. The location of the transmitters and receivers is shown in Figure 1. The labels and frequencies of the transmitters are reported in Table 1. The data collected are transmitted every day to the server located at the Department of Physics of the University of Bari (Italy) that is the central node of the network. The different temporal trends (10 for each receiver), can be seen using the INFREP web site where also the sampled data are stored protected by username and password.
In order to reveal possible radio precursors, the data had to be analyzed for discovering "anomalies", which differs from normal variations of the data trends. In INFREP, analysis of the radio data is performed only on the night-time data for all VLF/LF radio bands, between 21.00 and 24.00 (UTC). Each day is therefore represented by 3 hours, that is, taking into account the 1 min sampling rate used, 180 data (minutes). For the analysis of the data sets and detection of potential anomalies, there are several methods available that can be used [33] [34] [35] [36]. INFREP employs the Wavelet analysis [36]. Using the "Morlet function", the Wavelet transform of a time signal is a complex series that can be usefully represented by its square amplitude, i.e. considering the so-called Wavelet power spectrum. The power spectrum is a two dimensions plot ( Figure 2) that, once properly normalized with respect to the power of the white noise, gives information on the strength and precise time of occurrence of the various Fourier components which are present in the original time series. Generally, color from blue to red indicates increase in the power strength; so, red zones define anomalies. INFREP has implemented a software able to apply the Wavelet analysis on the radio data automatically at the end of each day. The analysis is performed on those 15 days [2700 data (minutes)] or 20 days [3600 data (minutes)] preceding each day; this day is indicated on the spectrum by a vertical white line ( Figure  2); the part of the spectrum after the day is related to 15 days data without any frequency added to avoid border effects. At the moment, the software operates on the night time data of four signals collected by each of the receivers: CIP, CRE, GRE and IT-Aq. Referring to Figure 1 and Table 1 the situation is the following: CIP (using DHO, GBZ, EU1, MCO transmitters); CRE (using DHO, EU1, FRI, MCO transmitters); GRE (using CZE, GBZ, EU1, RRO transmitters); IT-Aq (using DHO, GBZ, EU1, MCO transmitters). The results obtained with the Wavelet analysis are protected in the INFREP web site by a further username and password.

Central Italy
On October 26, 2016 an earthquake with M w = 5.9 occurred in Central Italy, near Norcia small town; followed by the main shock (M w = 6.5), which occurred 4 days later (October 30, 2016). The focal depth reported was equal to 8 km and 9 km, respectively. The epicenter of the main shock was 12 km shifted towards a south west direction with respect to the first one. The earthquakes occurred in the "sensitive" area of the INFREP network. Unfortunately, at the time of the earthquake, part of the INFREP network was undergoing a major reorganization. Among the data collected by the 4 receivers where the Wavelet analysis is automatically performed each day only those from CIP receiver are available ( Figure 3). Starting several days before the first earthquake, two anomalies appeared one after the other in the two VLF signals, the night time intensity of which is analyzed online. The two signals are radiated by the DHO transmitter (23.4 kHz) and by the ICV transmitter (20.27 kHz). The 5 th Fresnel zones of the radio paths brush the border of the Dobrovolsky area [37] of the previous main shock ( Figure 3). The power spectra related to DHO transmitter in the period 17-30 October are reported in Figure 4; the anomaly starts at October 19. The power spectra related to ICV transmitter in the same period are reported in Figure 5; here the anomaly starts at October 23.
The power spectra related to the other two signals (EU1 and MCO), automatically analyzed at this site, were controlled and they did not show any clear disturbance in the considered period. So, the possibility of a malfunction of the receiver is very low.

Western Turkey
On July 20, 2017 a strong (M w = 6.7) earthquake occurred offshore, near the coast of Turkey and Kos island (Greece); on August 8 an earthquake with M w = 5.0 occurred practically in the same zone. The focal depth was 10 km for both the events. The epicentres are inside the "sensitive" area of the INFREP network. In this case the online Wavelet power spectra of four radio-signals for each one of the CIP, CRE, GRE and IT-Aq receivers are available.   At first the situation related to the July earthquake is described. In Figure 6 and IT-Aq site were considered. Take into account that this signal is not collected by the GRE receiver. The situation is described in Figure 8, showing that none disturbance is present on both the power spectra of the DHO signal.   Next, the August 8, 2017 (M w = 5.0) earthquake is described. Note that Figure  6 can be referred also to this earthquake (the epicenter is about the same of the previous earthquake). In Figure 9 the power spectra on August 7, 2017 related to DHO, GBZ, EU1 and MCO signals collected by the CIP are reported. From Figure 9, an anomaly appears on the signal radiated by the DHO transmitter, starting 6 -7 days before the earthquake mentioned above; on the contrary, the power spectra related to the GBZ, EU1 and MCO don't reveal any clear disturbance. As before, the presence of anomaly only in one signal minimizes the possibility of a malfunction of the receiver. In order to exclude a possible malfunction of the DHO transmitter (the signal of which shows the anomaly) the power spectra related to this signal obtained in the same day in CRE and IT-Aq site were examined. The situation is described in Figure 10, showing that there is no disturbance present on both the power spectra of the DHO signal.

Discussion
Disturbances in VLF-LF radio waves can be connected to adverse meteorological conditions around the receiver location or to anomalous geomagnetic activity as reported in previous analyses [15] [16] [26] [38] [39]. These causes have been checked as regards the periods when the anomalies presented above occurred Open Journal of Earthquake Research Figure 9. The power spectra obtained on August 7, 2017 of the four radio signals represented in Figure 6. and no clear correspondence was noticed in all the cases. The Wavelet analysis performed at each site on four signals and, when possible, on the same signal at different sites allows to reduce the possibility that the anomalies presented are related to problems connected to the transmitter or to the receiver. So, a connection of the anomalies with the seismicity can be considered.
On the basis of the many papers (see references in the introduction), the VLF-LF radio signals anomalies have the following general peculiarities: 1) they appear at least 10 -15 days before the subsequent earthquake, 2) the magnitude M w of the earthquake is equal-greater than 5.5, 3) the epicenter is located inside the 3 rd -5 th Fresnel zone of the radio path in the central zone or near the receiver.

Conclusion
Currently