## 3 GNSS Data and Results
### Neighboring Stations-Overall Image  
Together with the continuous GNSS stations in the Ionian Islands, stations located in western mainland Greece (AGRI, KTCH) and western and northwestern Peloponnese (PAT0, RLSO, PYRG) were also processed (Table 5). The goal was to study the velocity field, ground deformation and differential motions in the broad region of the Ionian Sea, focusing on the area close to the highly activated central Ionian Islands. The deduced velocity field of the area shows the differentiation of the motion between western Greece and the Peloponnese, resulting in the opening of the Patras Gulf with an estimated rate of about 8.0 \(\pm\) 0.5 mm/year. As previously described, the seismic activity on Cephalonia has not caused any displacement to the broad area, as co-seismic deformation was observed only at local sites. The Lefkada 2015 event affected only the northern AGRI station, located in western Greece. It was only the strong 2018 Zakynthos event that caused noticeable static displacement in both western Greece and the Peloponnese (Appendix B). The most distinct co- and post-seismic deformation was recorded at PYRG station. A small displacement was also observed at KTCH station (Figure S3). The strong events in the Ionian Islands did not appear to affect the velocity field of these stations, which exhibited a linear type of motion throughout the processed period. The only noticeable alterations were observed in the vertical component of PAT0 station (NW Peloponnese) (Figure S3). In this station, the vertical component shows a non-linear motion. Initially, up to the end of 2015, an uplift pattern was observed, which since the beginning of 2016 has reversed to a subsiding motion. A similar pattern also was observed at the neighboring RLSO site. The overall image of the velocity field in the broad Ionian Islands area shows a gradual rotation of the horizontal velocity vectors (Figure 8). GNSS stations located to the north exhibited a NW horizontal motion, while towards the south, the horizontal vector component shifted to a SW motional direction. The latter has been combined with a decrease in the amplitude of the horizontal velocity from -26 mm/year in the northern Ionian Islands, to -13 mm/year in Zakynthos Island and -7 mm/year in Strofades Islet, respectively. The transition from a northward to southward motion pattern occurs in Cephalonia Island. A similar rotation pattern is also observed at the stations located in western Greece and the NW Peloponese, with the transition occurring between AGRI and KTCH stations, roughly at the same latitude as Cephalonia. Analyzing the baseline-length changes for GNSS stations located on different Ionian Islands, as well as between stations in Ionian and western Greece, information about the tectonic status of the area can be extracted (Appendix C). In the northern Ionian Sea, the distance between Corfu and Paxoi has remained almost unchanged (V\({}_{\text{KERK-PAXO}}\) = -0.60 \(\pm\) 0.11 mm/year). Intenense lengthening was calculated between the northern islands (Corfu, Paxoi) and Lefkada (i.e., V\({}_{\text{PAXO-SGAN}}\) = 7.55 \(\pm\) 0.11 mm/year). The latter indicates an extensional regime between the northern and the central Ionian Islands. Extension with a lower rate is also observed by the baseline changes between the stations located in the central and southern Ionian Islands. Fluctuations in the baseline velocity rates were detected before and after the strong seismic events that occurred in the area during the years 2014-2018. According to the baseline velocity between stations on the islands of Lefkada and Cephalonia, lengthening was taking place during the period before the 2014 seismic events at Cephalonia (i.e., V\({}_{\text{PONT-VLSM}}\) = 2.91 \(\pm\) 0.04 mm/year). For the period following the 2015 Lefkada earthquake, the northern and the southern regions were detected in the northern and the southern regions. The northern and the southern regions were detected in the northern and the southern regions.