## 4 Discussion
### Physical Mechanisms for Earthquake Migration
Numerical simulations of fluid transport in heterogeneous fault zones also show anomalous diffusion with exponents greater than 1, as fluid flow is channeled in fractal fault zones with varying permeabilities [69]. In contrast, stress transfer effects due to subcritical crack growth or to cascading effects yield quite low diffusion exponents and sub-diffusion [57; 58], most commonly observed in earthquake diffusion [49; 60]. The super-diffusive behavior of seismicity reported herein may, thus, provide an additional indication of fluid overpressures. Consequently, a candidate physical mechanism for the observed seismicity migration pattern in the offshore region of the WGoC during the \(\sim\)15 months period may well correspond to an initial intrusion of overpressurized fluids from the area of Nafpaktos. In a fluid-saturated and critically stressed crust, fluid overpressures can reduce the effective normal stress along the active fault structures, triggering the observed seismicity offshore Nafpaktos during September-December 2013 (Figure S22b). Fluid diffusion along an E-W-oriented, permeable pathway, defined by the intersection of the active fault structures with the shallow-dipping seismogenic layer beneath the WGoC, may have caused the eastward migration of seismicity. This eventually activated the easternmost part of group #4 and the westernmost part of group #8 during January-February 2014, which included certain \(M_{\mathrm{w}}>3.8\) events (Figure S22b, yellow colour). Increased stresses in this area trigger additional events towards the east during March-May 2014, until 8 June 2014, when the \(M_{\mathrm{w}}=4.3\) earthquake occurred in group #8. The latter initiated an aftershock sequence, including several \(M_{\mathrm{w}}\geq 3.0\) events in group #8 and eastwards in group #6 during July-October 2014. Seismic activity then culminated with an \(M_{\mathrm{w}}=4.8\) event on 7 November 2014 in group #6, offshore Aigion, initiating an aftershock sequence that spread rapidly in an E-W direction and mostly south of the mainshock's epicenter until the end of 2014 (Figure S22c). The overall seismicity migration pattern is, thus, likely produced by a combination of fluid diffusion and stress transfer effects caused by the larger magnitude events. Enhanced permeabilities due to fluid flow in the area of group #4, followed by healing/sealing processes in the active faults (e.g., [70]), may have caused the deficit of the seismic activity, delimited by the back-front observed during March-June 2014 (Figure 6). In mid-July 2014, the seismic activity at group #4 increases again in the same area, as during September-December 2013, possibly triggered by another intrusion of overpressurized fluids. This resembles a conventional fault-valve behavior [71], in which the low-permeability Phylade nappe below the shallow-dipping seismogenic layer acts as a cap for a deeper fluid reservoir evidenced by \(\mathrm{V_{P}}/\mathrm{V_{S}}\) anomalies [5; 65; 72; 73]. A combination of fluid diffusion, aseismic creep, supported by fast earthquake migration speeds, and stress transfer effects caused by the \(M_{\mathrm{w}}>4\) events during September 2014, may have caused the bilateral migration of seismicity that was observed offshore Nafpaktos during July-October 2014.