## 7 Conclusions  
This paper presents one of the approaches to analysing co-seismic displacement by combining the results of GNSS determined coordinates (from stations located in earthquake areas) using the PPP method and subsequent anharmonic spectral analysis of a time series of coordinates. Positioning was undertaken using the PPP method, which, unlike a network solution, only uses data from a designated station and is based on knowledge of the precise positions of the GNSS satellites. One-day solutions are used to achieve the required accuracy. The resulting time series are analysed using an anharmonic analysis. The use of anharmonic analysis for this purpose is innovative. Comparing the time series before and after the earthquake, it is then possible to detect the co-seismic displacement, which can confirm the results of seismic models. The reliability of the size of a co-seismic displacement is determined by using the mean square error (RMSE) of the offset. We tested two approaches to compute RMSE. In the first approach, RMSE is determined from variances produced by PPP processing. The second approach assumes that nothing happens with the GNSS station before or after the earthquake. The dispersion of the approximation data from the time series analysis from before and after the earthquake event is thus given by real errors, which can be used to compute RMSE. The second computational approach of RMSE usually gives a larger value of RMSE. It is usually 2-3 times larger, but in some cases (the NS component in Table 1), it is even 24 times greater. The RMSE from the 2 nd approach corresponds better to the assumed accuracy of determining the difference from PPP positions. The detected co-seismic displacement at the EYPA station with an accuracy calculated using the 2 nd approach RMSE is in decent agreement with the static displacement calculated from the seismic model (see Table 1). In the case of stations in the Zakynthos area, the 2 nd approach for RMSE shows that the Up component of the co-seismic displacement cannot be a reliably determined. The 2.95 sigma errors (1.12, 2.21, 1.65 cm for the STRF,ZAKY, and PYRG stations, respectively) are larger than the absolute values of the Up components of the co-seismic displacements (0.66, 0.70, 0.35 cm for the STRF, ZAKY and PYRG stations, respectively--see also Table 2). Therefore, the 2 nd approach of calculating RMSE gives better information about the accuracy of the detected co-seismic displacement and more credibly assess the significance of the determined co-seismic displacement than the RMSE of the 1 st approach, which is unrealistically small. In summary:  
* A one-day solution using the PPP method give sufficient accuracy for further analysis. * Anharmonic spectral analysis of a coordinate time series, in some cases, gives a more reliable estimate of co-seismic displacement than the difference of arithmetic averages of coordinates before and after a seismic event. * In the 2 nd approach, RMSE determines the dispersion of the approximation data from the time series analysis before and after the earthquake event. * The RMSE from the 2 nd approach gives a more realistic estimate of accuracy of co-seismic displacement than the RMSE from the 1 st approach. The aim of this study was to determine the co-seismic displacement from the GNSS data and to best estimate the accuracy of its determination. The use of the PPP method offers an advantage over the relative network solution in obtaining an \"absolute\" positional change for any given station. The disadvantage may be that the minimum length of the processed measurement, while achieving the required accuracy of the result, is one day. This precludes the possibility of determining co-seismic movements during an earthquake; however, we have shown that it is possible to determine co-seismic displacement. Any afterslips that occur within 1 day are included in the designated co-seismic displacement.