Lower Ionospheric Turbulence Variations during the Intense Tectonic Activity of October, 2018 at Zakynthos Area, Greece

In this paper we investigate the ionospheric turbulence from TEC observations before and during the intense seismic activity of October of 2018 at Zakynthos area (λ = 20.51°E, φ = 37°34'N), (Main shock: ML 6.6, 60.6 km SW of Zakynthos). The Total Electron Content (TEC) data of 5 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analyzed using Discrete Fourier Analysis in order to investigate the TEC variations. The results of this investigation indicate that the High-Frequency limit fo, of the ionospheric turbulence band content, increases as the site and the moment of the earthquake occurrence are approaching, pointing to the earthquake locus. We conclude that the LAIC mechanism through acoustic or gravity wave could explain this phenomenology.


Introduction
It is generally accepted that the original cause of the earth surface perturbation is the increasing stress and the rapture of the rocks at the earthquake preparation area where radon release takes place. The coupling of radon with the atmosphere at the earth surface results in the increase of the Ionization, Temperature [1], Electromagnetic field and disturbances in the air electrical conductivity. This perturbation is transmitted to the Ionosphere by some LAIC Mechanism. The proposed possible hypothesis on the mechanism of coupling between lithospheric activity and ionosphere is Transmission through the: 1) Chemical chan-How to cite this paper: Contadakis, M.E., Arabelos, D.N., Vergos, G.S. and Scordilis , E.M. (2019) Lower Ionospheric Turbulence Variations during the Intense Tectonic Activity of October, 2018 at Zakynthos Area, nels [2], 2) Atmospheric oscillation (or acoustic) channels [3] and (3) Electromagnetic channels [2] [3]. Finally, a hypothesis of electrostatic channel has been proposed by Freund [4] on the basis of positive hole charge carriers release in crustal rocks, alongside electrons. When the positive holes arrive at the Earth's surface, they can cause massive ionization of the air molecules and positive surface potential. Subsequently, these perturbations are transmitted to Ionosphere.
From all these hypothesis for the LAIC mechanism, we believe that the hypothesis of the Atmospheric oscillation (or acoustic) channel [3] is the most suitable for the explanation of our observations so far [5] [6] [7], since the observed frequency band of the suggested Gravity waves of our work complies with the observed frequency bands of the Internal Atmospheric Gravity waves (Acoustic standing waves) by Horie et al. [8] and Molchanov et al. [9] [10]. Thus according to the LAIC mechanism through Acoustic channel, Acoustic or Gravity waves affect the turbulence of the lower ionosphere. Subsequently the produced disturbance starts to propagate in the ionosphere's waveguide as gravity wave. The inherent frequencies of the acoustic or gravity wave range between 0.003 Hz (period 5 min) and 0.0002 Hz (period 83 min), which according to Molchanov et al. [9] [10] correspond to the frequencies of the turbulent produced by tectonic activity during the earthquake preparation period. During this propagation, the higher frequencies are progressively damped. Thus, observing the frequency content of the ionospheric turbidity we will observe a decrease of the higher limit of the turbidity frequency band. Our investigations so far, on the occasion of strong earthquakes approve this view. Never the less, since the approval of the theoretical view depends mainly on the compliance of the observational results to the theoretical clues, further observational results are always welcome.
In this paper we investigate the ionospheric turbulence from TEC observations before and during the intense seismic activity of October of 2018 at Zakynthos area (λ = 20.51˚E, φ = 37˚34'N), (Main shock: ML 6.6, 60.6 km SW of Zakynthos

Seismotectonic Information
On October 26, 2018 (22:54 GMT) a strong (M L = 6.6) shallow (h~10 km) earthquake occurred ~50 km off the SW coast of Zakynthos Island, southern Greece (37.34˚N, 20.51˚E). The main shock was followed by an aftershock sequence that lasted for almost three months.
Strong earthquakes have occurred in the broader region in the past with magnitudes up to 7.2 (Table 1, Figure 1). Their generation is related to significant   Table 1). http://www.isc.ac.uk/. https://www.globalcmt.org/CMTsearch.html), assumes that both the above seismic faults could be responsible for its generation. The main shock was followed by a remarkable number of aftershocks, the strongest of which occurred four days later (on October 30, 15:12 GMT) with M L = 5.4, while within the same day and a few hours earlier (02:59 GMT), another strong aftershock with similar magnitude (M L = 5.3) occurred, with its epicenter very close to that of the main shock. Table 2 and the map of Figure 2 give information on the aftershock activity during the first four months after the occurrence of the main shock.    Table 3 displays the 5 EUREF stations while Figure 3 displays the locus of the five GPS stations and the main shocks.

TEC Variation over Mid Latitude in Europe
IONOLAB system provides comparison graphs of its TEC estimations with the estimations of the other TEC providers of IGS in its site. In this work only TEC estimations in perfect accordance among all providers were used. The TEC values are given in the form of a Time Series with a sampling gap of 2.5 minute. The IONOLAB TEC estimation system uses a single station receiver bias estimation algorithm, IONOLAB-BIAS, to obtain daily and monthly averages of receiver bias and is successfully applied to both quiet and disturbed days of the ionosphere for station position at any latitude. In addition, TEC estimations with high resolution are also possible [13].

Fast Fourier Transform Analysis
The Power Spectrum of TEC variations will provide information on the frequency content of them. Apart of the well known and well expressed tidal variations, for which the reliability of their identification can be easily inferred by statistical tests, small amplitude space-temporal transient variations cannot have any reliable identification by means of a statistical test. Nevertheless looking at the logarithmic power spectrum we can recognize from the slope of the diagram whether the contributed variations to the spectrum are random or periodical. If they are random the slope will be 0, which correspond to the white noise, or −2 which correspond to the Brownian walk noise, otherwise the slope will be different, the so called Fractal Brownian walk [14]. This means that we can trace the presence of periodical variations in the logarithmic power spectrum of TEC variations. As an example, Figure 5 displays the logarithmic power spectrum of TEC variations over the GPS station of Mate at the days of 23 to 25/10/2018. It is seen that the slope of the diagram up to the log(f) = −2.2, is −2. This means that for higher frequencies the TEC variation is random noise. On the contrary the variation of TEC for lower frequencies contains not random variations i.e. turbulent. So we conclude that the upper frequency limit fo of the turbulent band is 738.6 μHz. Or, equivalently, the lower period limit Po of the contained turbulent is 22.563 minutes.   shown that at the day of the earthquake a strong dependence of the upper frequency f o (lower period P o ) limit of the Ionospheric turbulent content with the epicentral distance is observed. In particular, the closer of the GPS station to the active area the higher frequency f o /lower period P o limit is. This dependence Open Journal of Earthquake Research is not shown at the "quiet" day. As it is seen from Figure 6 and Figure  to 100 min [9] [10] or 20 to 80 min [8].     Hobara et al. [15] in a study on the ionospheric turbulence in low latitudes concluded that the attribution of the turbulence to earthquake process and not to other sources, i.e. solar activity, storms etc. is not conclusive. Nevertheless in our case, the steady monotonic, time and space, convergence of the frequency band upper limit f o increment, to the occurrence of the East Aegean strong earthquakes is a strong indication that the observed turbulence is generated by the respective earthquakes preparation processes. The qualitative explanation of this phenomenology can be offered on the basis of the LAIC: Tectonic activity during the earthquake preparation period produces anomalies at the ground level which propagate upwards in the troposphere as Acoustic or Standing gravity waves [16] [17]. These Acoustic or Gravity Waves affect the turbulence of the lower ionosphere, where sporadic Es-layers may appear too [18], and the turbulence of the F layer. Subsequently, the produced disturbance starts to propagate in the ionosphere's waveguide as gravity wave and the inherent frequencies of the acoustic or gravity waves can be traced on TEC variations [i.e. the frequencies between 0.003 Hz (period 5 min) and 0.0002 Hz (period 100 min)], which, according to Molchanov et al. [9] [10] and Open Journal of Earthquake Research Horie et al. [8] correspond to the frequencies of the turbulent induced by the LAIC coupling process to the ionosphere. As we move far from the disturbed point, in time or in space, the higher frequencies (shorter wavelength) variations are progressively attenuated. It has to be noted that the original cause of the earth surface perturbation is the increasing stress and the rapture of the rocks at the earthquake preparation area where radon release takes place. The coupling of radon with atmosphere at the earth's surface results to the increase the ionization, temperature [1], electromagnetic field and to disturbances in the air electrical conductivity. This perturbation is transmitted to the Ionosphere by some LAIC Mechanism. The proposed possible hypothesis on the mechanism of coupling between lithospheric activity and ionosphere is transmission through the: 1) chemical channels [2], 2) atmospheric oscillation (or acoustic) channels [3] and 3) electromagnetic channels [2] [3]. Finally, a hypothesis of electrostatic channel has been proposed by Freund [4] on the basis of positive hole charge carriers release in crustal rocks, alongside electrons. When the positive holes arrive at the Earth's surface, they can cause massive ionization of the air molecules and positive surface potential. Subsequently these perturbations are transmitted to Ionosphere. From this hypothesis for the LAIC mechanism, we believe that the hypothesis of the atmospheric oscillation (or acoustic) channel [3] is most suitable for the explanation of our observations, since the observed frequency band of the suggested gravity waves of this work comply with the observed frequency bands of the Internal Atmospheric Gravity waves (Acoustic standing waves) by Horie et al. [8] and Molchanov et al. [9] [10], as it is already mentioned.

Conclusion
The results of our investigation, on the case of the recent East Aegean tectonic activity, indicate that the High-Frequency limit f o , of the ionospheric turbulence content, increases as we are getting closer to the site and the time of the earthquake occurrence, pointing to the earthquake location. We conclude that the LAIC mechanism through acoustic or gravity wave could explain this phenomenology. That is, tectonic activity during the earthquake preparation period produces anomalies at the ground level which propagate upwards in the troposphere as Acoustic or Standing gravity waves. These Acoustic or Gravity waves affect the turbulence of the lower ionosphere, where sporadic Es-layers may appear, too, as well as the turbulence of the F layer. Subsequently, the produced disturbance starts to propagate in the ionosphere's wave guide. Thus observing the frequency content of the ionospheric turbulence we will observe a decrease of the higher limit of the turbulence frequency band, as a result of the differential frequency attenuation of the propagating waves.