## 3 GNSS Data and Results  
Daily GNSS data from stations located in the Ionian Islands and western mainland Greece and the Peloponneese were processed for the period from 2009 to 2021. The analysis intended to determine the crustal velocity field of the broad area, detect possible pre-seismic displacements and study co- and post-seismic deformation. The continuous GNSS stations extend from the northern Corfu Island to the Strofades Islets, south of Zakynthos. In western mainland Greece and the Peloponneese, the stations are located in the broad area of Patras Gulf and in the city of Pyrgos in the western Peloponnees (Figure 8). Sites KASI, SPAN, PONT, KIPO, VLSM, ZNTE, STRF, KTCH and RLSO belong to the National Observatory of Athens (NOA network) [42, 43]. The commercial network of METRICA SA (HexagonSmartNet) [85] provided data for stations KERK, PAXO, ARGO, ZAKY, AGRI and PYRG. In Cephalonia Island, the sites SISS, KARA and SKAL are part of the National and Kapodistrian University of Athens network (NKUA net) [54]. Meanwhile stations EXAN and DRAN in Lefkada Island and FISK and ASSO in Cephalonia are part of the GNSS network of the Corinth Rift Laboratory (CRL GNSS net) [86], which is the only international Near-Fault Observatory (NFO) of the European Plate Observing System. Finally, PAT0 station in the city of Patras is a European Reference Frame (EUREF) station [87]. Figure 8: Horizontal velocity vectors (black arrows) for the continuous GNSS sites (blue triangles) of the broad Ionian Islands area. Red arrows indicate the horizontal velocity component estimated for the period after the strong (\(M>6\)) earthquakes in the area (ITRF 2014). The raw GNSS data were processed using the Bernese v5.2 software [88]. Several GNSS stations of the EUREF and IGS were included in the processing together with the local GNSS sites, while auxiliary files were introduced in the procedure. The absolute antenna phase center corrections were used, together with precise orbital solutions from the Center for Orbit Determination in Europe (CODE) and the Vienna Mapping Functions for the tropospheric modeling. The FES2004 model ([[http://holt.oso.chalmers.se/loading](http://holt.oso.chalmers.se/loading)]([http://holt.oso.chalmers.se/loading](http://holt.oso.chalmers.se/loading)); accessed on 11 January 2022) was used for the tide-loading corrections. The precise double-difference method was used for static-mode solutions. Several ambiguity-resolution strategies were applied, based on the length of the formed baselines between the GNSS stations. The daily calculated coordinates for the GNSS stations were evaluated for the repeatability error on a weekly basis, and solutions were excluded in cases of large deviations from the weekly solution. The processing resulted in the estimation of high-precision station coordinates. Time series were formed, annual velocities were calculated (Figure 8; Tables 1-5) and co-seismic displacements were determined (Appendix B). The daily coordinates of the Ionian GNSS stations were estimated on the global ITRF2014 reference frame. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Site** & **Latitude** & **Longitude** & **Period** & \(\mathbf{V_{East}}\) & \(\mathbf{V_{North}}\) & \(\mathbf{V_{Up}}\) \\  & **(°)** & **(°)** & & **(mm/Year)** & **(mm/Year)** & **(mm/Year)** \\ \hline KASI & 39.7464 & 19.9355 & 1 January 2013–6 May 2017 & 20.91 \(\pm\) 0.04 & 13.75 \(\pm\) 0.04 & 0.39 \(\pm\) 0.09 \\  & & & 4.35 years & & & \\ \hline KERK & 39.4937 & 19.8734 & 1 March 2017–31 October 2021 & 22.15 \(\pm\) 0.03 & 14.57 \(\pm\) 0.03 & \(-\)2.07 \(\pm\) 0.11 \\  & & & 4.67 years & & & \\ \hline PAXO & 39.2108 & 20.1639 & 1 November 2019–31 October 2021 & 24.96 \(\pm\) 0.07 & 16.08 \(\pm\) 0.11 & \(-\)3.1 \(\pm\) 0.30 \\  & & & 2.00 years & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Velocity components for the GNSS stations on Corfu and Paxoi Islands (ITRF 2014).