Despite the constant presence of microseismicity, certain parts of major faults in the WGoC appear to be locked, accumulating stress that is released in major events. The last destructive earthquake in the area was the 15 June 1995 M\({}_{\rm s}\) = 6.2 event, which severely damaged the town of Aigion [29]. The source of this earthquake was a north-dipping, low-angle (33\({}^{\circ}\)) normal fault, with more recent data excluding its association with the East Helike fault [22]. The rupture surface of the 1995 event appears to be locked, with the observed microseismicity stopping abruptly at the eastern end of the study area (Figure 1). Other major earthquakes have also occurred in the past at the eastern end of the WGoC, with epicenters towards the northern coasts. Significant earthquakes have struck the WGoC in the historical era (i.e., before 1900). One of the more notable was the 373 BCE earthquake, with an estimated minimum magnitude of M = 6.6 [36], which caused a tsunami that destroyed the ancient city of Helike [37]. An earthquake of similar macroseismic characteristics struck the same area on 26 December 1861 (M\({}_{\rm w}\) = 6.5 according to Boiselet [38]), triggering a tsunami and causing a strip of plain to submerge under the sea [37]. Both earthquakes are attributed to the north-dipping Helike normal fault, most likely its eastern branch [22]. Recently, the contemporary town of Helike was in the epicentral area of a swarm that started in May 2013, persisting for several months and involving a few earthquakes with magnitudes ranging from 3.3 to 3.7, but which was attributed to activity near the root of Pirgaki fault [17; 39]. Other historical earthquakes that struck the town of Aigion occurred in 1748, 1817, and 1888, with estimated magnitudes of 6.3-6.6 [33]. The area of Patras was affected by historical earthquakes in 1785, 1804, and 1806, with magnitudes between 6.1 and 6.4, while the town of Nafpaktos was struck by earthquakes in 1462, 1703, 1714, 1756, and 1831, with magnitudes in the range from 5.9 to 6.5 [33]. In the instrumental era (1900 to today), before the 15 June 1995 earthquake, the previous major event with a magnitude \(\sim\)6 occurred in the WGoC on 30 May 1909, while intermediate-depth events with magnitudes of 6.4 and 5.9 occurred in 1965 and 1993, respectively; the epicenters of these events were located at the eastern end of the study area. Although no M\({}_{\rm w}\)\(\geq\) 6.0 earthquake has struck the WGoC since 1995, moderate magnitude events (M\({}_{\rm w}\)\(\approx\) 5) have occurred in recent years. On 14 July 1993, an M\({}_{\rm w}\) = 5.5 event with a strike-slip focal mechanism [30; 40] struck near Patras, causing considerable damage. Although the locally mapped structures trend SW-NE, it has been suggested, based on the aftershock distribution, that the earthquake ruptured a NW-SE sinistral-slip fault [41]. A notable case is the earthquake doublet of January 2010 near Effgalio, with an M\({}_{\rm w}\) = 5.3 event being followed by an M\({}_{\rm w}\) = 5.2 four days later, both related to blind E-W-trending normal faults [32; 42; 43]. This activity likely played a role in the triggering of another M\({}_{\rm w}\) = 5.1 earthquake on 7 August 2011 to the west, near Nafpaktos, characterized by a similar normal faulting style. Later, in 2013-2014, the WGoC presented strong signs of microseismic activity, with an earthquake swarm initiating in September 2013 at the offshore region between Nafpaktos and Psa thopyrgos that slowly migrated eastwards, triggering earthquake clusters and culminating in an M\({}_{\rm w}\) = 5.0 event, offshore of Aigion, on 7 November 2014 [21; 44]. A second seismic swarm excitation, characterized by bilateral spatiotemporal migration in the offshore area between Nafpaktos and Psa thopyrgos, involved an M\({}_{\rm w}\) = 4.9 earthquake on 21 September 2014, activating a patch related to the westernmost edge of the Psa thopyrgos fault, near its junction with the Rion-Patras fault zone [3].