## 3 Results
### Spatio-Temporal Analysis of Seismicity
More importantly, a significant pattern of bilateral seismicity migration began to emerge in mid-July 2014 in group #4, originating at Y \(\approx 15\) km (Figure 5, bottom), spreading towards \(Y=25\) km and \(Y=10\) km in a period of two months. Seismic activity at the western part of group #4 was abruptly increased in the form of an aftershock sequence after the occurrence of an \(M_{\rm w}=4.6\) event on 21 September 2014 [48]. The last major seismicity outbreak at the WGoC during the study period began on 7 November 2014, with an \(M_{\rm w}=4.8\) event in group #6 (red), offshore Aigion [48]. The seismic activity spread rapidly in the form of an aftershock sequence, bilaterally along a \(\sim\)7-km-long patch in an E-W direction (Figure A3), but mostly south of the mainshock's epicenter (Figure S22c). Regarding the physical mechanisms behind the spatiotemporal seismicity migration patterns, stress-transfer and pore-pressure diffusion are considered. Slip occurs when the applied shear stress on a fault plane, aided by the pore-pressure in fluid-filled microcracks, surpasses the sum of the cohesive strength of the material and the internal friction. Changes in the level of stress in a specific location may result from Coulomb stress transfer due to the occurrence of an earthquake in a nearby region, which is the main mechanism behind the triggering of aftershocks. Aseismic slip, e.g., due to creeping, also causes Coulomb stress changes that may load a neighboring area, accelerating earthquake occurrence, or relaxing another region. Finally, the diffusion of pressurized fluids can generate earthquakes in the volume behind a triggering front that spreads through the fracture network, increasing pore-pressure and lowering the effective normal stress on a fault plane, thus facilitating slip. When fluids are injected in a homogeneous and isotropic, poro-elastic, fluid-saturated medium, the expanding spherical triggering front of induced seismicity follows Equation (1) [50]:  
\[r(t)_{t>t_{o}}\ =\ \sqrt{4\pi D(t-t_{o})}, \tag{1}\]  
where \(r(t)\) is the triggering front's radius (distance from the injection point), \(t\) is the time since the beginning of the pore-pressure perturbation at time \(t_{o}\) and \(D\) is the hydraulic diffusivity. The same principle may apply as a first approximation to a fluid-saturated crust and an initial pore-pressure perturbation. The determined spatiotemporal seismicity migration rates (Figure 5, bottom) provide a first indication regarding the possible physical triggering mechanism of seismicity in the WGoC. Initiating in September 2013, offshore Nafpaktos, seismicity started to migrate  
Figure 5: (**Top**) Cumulative number of events per spatial group (colours/numbers) for events with \(M_{\rm w}\geq M_{\rm c}=1.8\) of the manual, relocated catalogue of the 2013-2014 seismicity at the WGoC (Cat1). Seismicity rates are marked in events per day (evt/d). The occurrence of events with \(M_{\rm w}\geq 3.0\) is marked by circles on each curve. (**Bottom**) Spatiotemporal projection of epicenters (without magnitude threshold) along the N100”E-oriented profile line A–B of Figure 4 (top). Spatial groups associated with the occurring seismicity are marked by numbers and represented by their respective colours. Inferred seismicity migration patterns are marked by dashed lines, with migration speed in km/day. The initiation of significant sequences is marked by gray vertical lines and labels at the top panel (C# refers to the spatial group number #). However, its evolution cannot be fit with a parabolic envelope, but is better approximated by a roughly linear triggering front, up to a point when it even tends to accelerate as it spreads (Figure A4). Its seismicity migration speed was 0.13 km/d eastwards and 0.03-0.07 km/d westwards between 15 July and \(\sim\)20 August 2014 and then the eastwards migration speed increased to 0.17 km/d. The 21 September 2014 event caused seismicity to spread eastwards at over 0.35 km/d, while the western branch began to expand rapidly at over 1 km/d.