## 3 Results  
### Spatial Distribution of Seismicity  
A total of 8944 events of Cat1 were successfully relocated. As presented in the cross-sections of Figure 4, seismicity is mainly concentrated on a low-angle seismogenic layer at depths between 6.5 km and 10 km below the gulf (see also Figure S1d), reaching \(\sim\)12 km down-dip the activated structures. Several clusters, mainly related to the occurrence of larger events, indicate the activation of higher-angle faults, rooting at the weak layer. The latter is further supported by the reported focal mechanisms (Table 1). The magnitudes of major events mentioned in the present study refer to their respective \(M_{\rm w}\) values calculated herein by spectral fitting (see Section S2 in the Supplementary Material). The westernmost part of the Nafpaktos-Psathopyrgos group #4 (green), activated after the 21 September 2014 \(M_{\rm w}=4.6\) event, could possibly be associated with the Rion-Patras SW-NE-trending dextral oblique-normal fault zone, or with related sub-parallel structures dipping towards NW. The N10\({}^{\circ}\)E cross-sections of Figure S13 imply gradual deepening of seismicity towards the western profiles, which is also highlighted in Figure 4a1-a2, as well as in the N315\({}^{\circ}\)E-oriented cross-sections of Figure S14 and the N100\({}^{\circ}\)E-oriented cross-section of Figure S15. However, the focal mechanisms associated with the larger offshore events of group #4 (e.g., the 21 September 2014 \(M_{\rm w}=4.6\) earthquake) suggest an E-W to WSW-ENE-striking fault. The latter, along with the hypocentral distribution, could indicate an association with the Ano Kastritis fault, further south. The two available focal mechanisms for events below Platanitis support SW-NE faulting, with one being normal and the other dextral, both compatible with the kinematics of the Rion-Patras fault zone. The N10\({}^{\circ}\)E cross-section b1-b2 of Figure 4 contains seismicity of groups #7 (yellow) and #8 (brown), the latter associated with the June 2014 outburst in the gulf. Although the seismic activity is mostly compacted in the depth range 8-10 km, distinct clusters can be associated with individual structures, some being located at shallower depths (6-8 km) towards the north. The 8 June 2014 \(M_{\rm w}=4.3\) event was likely hosted on a north-dipping structure, with a \(\sim\)45\({}^{\circ}\) dip according to the focal mechanism reported by NKUA (Table 1). This makes it partly compatible with the Fasouleika fault segment, if a small degree of listricity is considered. Likewise, the different strike (NW-SE) of the 24/01/2014 \(M_{\rm w}=3.9\)  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline Date & Hour & Lat. (\({}^{\circ}\)N) & Long. (\({}^{\circ}\)E) & Depth (km) & \(M_{\rm w}\)\({}^{*}\) & \(M_{\rm w}\  
event could support an association with the Selianitika or Lambiri faults. Seismicity at the northern part of the group #8 is distributed on a gently dipping layer, with a dip of \(\sim\)23\({}^{\circ}\). This is sub-parallel to the seismicity of group #7, further north, which seems to spread deeper and northwards along the same trend (Figure 4b1-b2, dashed yellow line with a northwards dip of \(\sim\)27\({}^{\circ}\)). Shallower seismic activity in group #7 could be related with antithetic, south-dipping structures, such as the Sergoula fault which is associated with the eastern part of the Trikorfo seismogenic source (GRCS441 in [14]; Figure S20). Figure 4: **(Top)** Map of the relocated seismicity for the period 2013–2014 in the WGoC. Colours and numbers correspond to the 9 spatial groups into which the dataset was divided, as well as the “miscellaneous” group #10 (gray). Beachballs depict the focal mechanisms of major events (Table 1). **(Bottom)** Vertical cross-sections a1–a2 to c1–c2, corresponding to the dashed rectangles shown on the map (top). Beachballs depict the far hemisphere projections of focal mechanisms for the major events.