## 4 Discussion  
### Spatial Distribution of Seismicity and Possible Active Faults  
The high-resolution hypocentral distribution of the present study reveals that the seismic activity is mostly concentrated in a -2-km-thick, low-angle-dipping layer below the gulf, mainly between depths of 7 and 10 km. This is composed of distinct clusters associated with planar structures protruding towards shallower depths (see also Figure A5 and the interactive figure of File S1 in the Supplementary Material). Focal mechanisms (Figure 4 and Table 1) indicate that these clusters are related with normal faults of moderate dip angle. A connection of structures at depth to mapped faults at the surface is not always straightforward, either due to the activation of blind faults, rooting at the fragmented seismogenic layer, or to focal depth uncertainties and biases. The latter case is important, as some systematic differences at hypocentral depths can be observed when compared to other studies. For example, [9] presented relocated seismicity of the period 2000-2015 in the WGoC using data from the CRL network, with focal depths mainly in the range of 6-8 km. Although in the northern part the focal depths of Cat1 are similar to those of [9], a deviation is observed in the southern and western parts of the study area, with absolute locations of our catalogue being 2-4 km deeper. This is likely a bias introduced by the inclusion of HUSN stations at epicentral distances further than 30 km, to better constrain epicenters near the margins of the local network, particularly towards the western part. The use of stations at distances up to 30 km tends to pull hypocenters towards shallower depths, especially at the southern part below Aigion, but also increases scattering in the distribution of epicenters. As a result, the foci in the present study are more clustered, but likely at the cost of a 1-2 km offset in absolute depths. The 2013 Helike swarm is an exception to this, as it has been separately processed using similar data, stations and velocity model as in [6], but considering it as a whole spatial group rather than on a sub-cluster level. Concerning the larger events of the dataset, the 8 June 2014 event exhibits nearly E-W normal faulting (Table 1). [9] related this event with the south-dipping Trizonia fault. However, as observed in cross-section b\({}_{1}\)-b\({}_{2}\) (Figure 4), this cluster of aftershocks seems to be consistent with a north-dipping structure that is likely associated with the Aigion-Selianitika fault system (GRIS503 in [14]: Figure S20) in group #8. As highlighted by [9], this earthquake seems to have also triggered seismicity to antithetic structures, sub-parallel to the Trizonia fault. Furthermore, its activity likely spreads eastwards to the Fasouleika segment, in group #6, particularly by the end of July 2014, in an opposite direction to what was observed in 2003-2004 [5]. Another significant event during the study period is the 21 September 2014 earthquake, which has been estimated at \(M_{\rm w}\) = 4.6 (\(M_{\rm w}\) = 4.9 from MT at NKUA and GI-NOA; Table 1). This event triggered aftershocks at the westernmost part of the 2014 Nafpaktos-Psathopyrgos swarm (group #4). Activity in this part of the WGoC has been related with the junction between the Rion-Patras fault zone, dipping NW and having a dextral strike-slip component, and the Psahopyrgos north-dipping normal fault [9]. The focal mechanism for this event suggests E-W to WSW-ENE normal faulting (strike = 254\({}^{\circ}\), dip = 40\({}^{\circ}\)). Although the cross-section a\({}_{1}\)-a\({}_{2}\) in Figure 4 is oblique to the focal mechanism, the hypocentral distribution shows a better fit with a NW-dipping plane. The 21 September 2014 event seems to coincide with the down-dip extension of the Ano Kastritsi fault. The triggered aftershocks, however, could have been spread to another structure, such as the Panachaikon transfer-fault (Pt.f., Figures 1 and 4; [30]), which is parallel to the Rion-Patras fault but further east.