Abstract
Abstract. In this paper the Total Electron Content (TEC) data of eight Global Positioning System (GPS) stations of the EUREF network (AUT1, Thessaloniki, TUC2, Crete in Greece, MATE, Matera, LAMP, Lampedusa in Italy, GAIA, in Portugal, RABT, Rabat, EVPA, Evpatoria in Ukrain and TRAB, Trabson in Turkey) were analysed using wavelet analysis in order to detect any frequency dependence of the correlation between TEC over different stations. In the same time frequency dependence of Dst (Global geomagnetic field disturbances) and TEC variations over each GPS station are searched in order to detect any correlation between them. The main conclusion of this analysis is that the components of TEC variation with periods <3 h are more suitable in searching for earthquake precursors. On the base of this conclusion the analyzed TEC series are searched for possible precursory phenomena on the occasion of the seismic activity of the last quarter of 2005 in the area of Greece. An exalting (i.e. an increase in the amplitude) of variations with periods up to the tidal ones (period of 6 h,8 h,12 h) may be observed a month before and during the seismic activity over the stations TUC2 and AUT1 and may be attributed to this tectonic activity. Statistical properties of the 1.5 h component of the Total Vertical Electron Content (TVEC) over the nearest GPS stations (TUC2 and AUT1) of the areas of the seismic activity indicate that this component present characteristic exalting in the time period of 15 days before the shock.
Highlights
Since we are interesting for Total Electron Content (TEC) variation over Greece for this preparatory stage, in which we investigate the general characteristics of TEC variations, the TEC data of eight widely spread over Mediterranean Global Positioning System (GPS) stations of the EUREF network (GAIA, in Portugal, RABT, Rabat, EVPA, Evpatoria and TRAB, Trabson) were analyzed
In order to get a more accurate determination of the site which is responsible for the TEC disturbance, since all the GPS-stations in the broader area present Total Vertical Electron Content (TVEC) exalting, we compare the statistical properties of the 1.5 h component together with that of the signal for all the GPS stations in order to detect the attenuation effect which indicate the closest to the epicenter GPS station for the strongest event which is the Kythera earthquake of 8 January 2006
The overall variations of Dst do not correlate with TEC variations in different sites
Summary
It is well known that TEC variations reveal ionospheric variations of different nature: i) Global, like earth revolution, earth rotation, earth-tides, variations of the geomagnetic field, Solar activity etc. (Svets et al, 2004; Biagi et al, 2003; Crowley and McCrea, 1988; Afraimovich, 2000); ii) Local such as atmospheric or underground explosions, volcanoes, extreme weather phenomena (Calais et al, 1998; Jacobson and Carlos, 1988; Afraimovich et al, 2000); iii) Earthquakes (Afraimovich et al, 2001; Molchanov et al, 2004; Molchanov et al, 2005; Meister et al, 2002; Liperovsky et al, 2002; Shvets et al, 2004). It is well known that TEC variations reveal ionospheric variations of different nature: i) Global, like earth revolution, earth rotation, earth-tides, variations of the geomagnetic field, Solar activity etc. In order that TEC variations over a particular site are used as earthquake precursory diagnostic a concrete sense of the interrelation of TEC variations over different sites as well as their respond to the geomagnetic field variations would be of great interest. In the same time frequency dependence of Dst and TEC variations over each GPS station are searched in order to detect any correlation between them. Comparative analysis of TEC variation over the stations of the network has been performed in order to detect any unusual characteristic of TEC variation over the area of Greece, which can be related to the seismic activity
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