## 4 Discussion
### Seismic Anisotropy in the WGoC
attributed to the strike of local structures [48; 75], particularly to mapped offshore faults with significant strike-slip component (e.g., [17]). In the present study, however, station PYRG exhibits a much different NW-SE \(\overline{\varphi}\) direction in Cat1 (142.2\({}^{\circ}\)), albeit with a very significant error (\(\pm\)41.6\({}^{\circ}\)). On the other hand, in Cat3, where more measurements are available, station PYRG shows a more E-W-oriented \(\overline{\varphi}\), although with a still large uncertainty (102.4\({}^{\circ}\)\(\pm\) 29.6\({}^{\circ}\)). This is due to the significant scatter in \(\varphi\) measurements (Figure S16), while a secondary \(\varphi\) maximum at a SW-NE direction, that would be in agreement with previous studies, is also observed. The existence of two groups of \(\varphi\) measurements at stations PYRG and HELI indicates either a strong spatial dependence or that shear-waves propagate through two distinct media with different anisotropic properties [21; 77; 78]. The measurements at the neighboring station SERG are very consistent, with \(\overline{\varphi}\) = 58.4\({}^{\circ}\)\(\pm\) 7.5\({}^{\circ}\). Further west, stations EFP and MALA present an E-W to WSW-ENE \(\overline{\varphi}\) direction, consistent with previous findings [48; 75]. Regarding the Magoula seismic array, station MG00, with \(\overline{\varphi}\) = 82.2\({}^{\circ}\)\(\pm\) 7.5\({}^{\circ}\), is in agreement with the results of [48]. Other stations of the array present variability in their \(\overline{\varphi}\) values, ranging between 60.7\({}^{\circ}\) and 106.3\({}^{\circ}\) (Figure S16). Station MG06, however, with more consistent measurements and smaller error, yields \(\overline{\varphi}\) = 85.6\({}^{\circ}\)\(\pm\) 8.9\({}^{\circ}\), in general agreement with MG00. The WSW-ENE \(\overline{\varphi}\) orientation at station MALA (68.1\({}^{\circ}\)\(\pm\) 11.0\({}^{\circ}\)) appears as a smooth transition in relation to the respective \(\overline{\varphi}\) at the neighboring stations EFP and MG00, and is also consistent with the strike of the Nafpaktos fault. Observations of \(\overline{\varphi}\) at stations MALA and SERG favor the existence of structurally controlled anisotropy [41; 79; 80]. Further west, stations AGRP and VVK present an E-W to WNW-ESE \(\overline{\varphi}\) direction, more consistent with \(\sigma_{\text{Hmax}}\)[48; 75]. Notably, in the WGoC the mapped faults' strike changes from E-W in the north to WNW-ESE in the south, the latter being in agreement with \(\overline{\varphi}\) measured at stations ROD3, ZIRI and ALIK. Stations TEME and LAKA show a more pronounced NW-SE direction, consistent with the results of [81], regarding the latter station. In addition, we investigated patterns of changes in the normalized time-delays, \(t_{\text{n}}\), between the fast and slow split shear-waves, possibly associated with the observed evolution of seismicity in the WGoC. We point out three cases of linear decrease in \(t_{\text{n}}\) values, which could be related to stress-relaxation due to microcrack coalescence. The first is associated with the period before the occurrence of the 2013 Helike swarm, between January and May 2013. The second was observed between November 2013 and January 2014, i.e., near the beginning of the fluid-driven, long-range seismic sequence. In particular, a cluster within group #4 occurred in late November 2013, while mid- to late December marks the beginning of the back-front, or the end of the \"injection\" period (\(\sim\)120 days). The pore-pressure pulse then travels at a speed of \(\sim\)0.13 km/day until January-February 2014 at the eastern end of group #4 and part of groups #6 and #8 (\(Y\) = 23-30 km in Figure 6). This epicentral area north of the Pashopyrgos fault has been previously proposed as the source of a strain transient event of equivalent \(M_{\text{w}}\) = 5.3 that occurred on 3 December 2002 and was detected by a dilatometer at Trizonia Island [82].