## 5 Coulomb Stress Changes and Possible Triggering  
The spatial aftershock distribution is known to be affected by the static stress changes caused by the main shock [23]. The change in Coulomb failure function (\(\Delta\)CFF) is used to quantify closeness to failure [24]. A positive value of \(\Delta\)CFF on a fault indicates increased likelihood that this fault will rupture in an earthquake. Values as small as 0.1 bar have been repeatedly shown to influence aftershock activity ([23; 25] among others). \(\Delta\)CFF depends on the changes of shear and normal stress, \(\Delta\tau\) and \(\Delta\sigma\), respectively, along with changes in the fluid pore pressure \(\Delta\)p of the rupture area, expressed by the relation  
\[\Delta\mathrm{CFF}=\Delta\tau\ +\ \mu\ (\Delta\sigma+\Delta\mathrm{p}), \tag{1}\]  
where \(\mu\) is the coefficient of friction. \(\Delta\)p is expressed by the relation [25]  
\[\Delta\mathrm{p}=-\mathrm{B}\ (\Delta\sigma_{kk})/3, \tag{2}\]  
if undrained conditions are considered during the coseismic phase. B is the Skempton's coefficient (\(0\leq\mathrm{B}<1\)), with experimental values between 0.5 and 0.9 shown to be appropriate for rocks [26], and \(\Delta\sigma_{kk}\) is the trace of the induced stress tensor. Assuming a ductile fault zone enclosed by a homogeneous and isotropic medium, Equation (1) becomes  
\[\Delta\mathrm{CFF}=\Delta\tau\ +\ \mu^{\prime}\ \Delta\sigma, \tag{3}\]  
where \(\mu^{\prime}\) is the apparent coefficient of friction [24], which contains the effects of pore fluid as well as the material properties of the fault zone. It is estimated by the equation  
\[\mu^{\prime}=\mu(1-\mathrm{B}) \tag{4}\]  
For this study, the values used to compute \(\Delta\)CFF are \(\mu\)=0.75 and B=0.5 as in [27] and as adopted in other studies in Greece [28; 29]. These values result in an apparent coefficient of friction of \(\mu^{\prime}\)\(\approx\)0.4, suggested by [1] for the area of the central Ionian Islands after testing different values and finding the most appropriate one for this estimation. We calculated the stress changes due to the coseismic slip of the main shock with the causative fault being approximated as a planar area with a length of 7 km and a width of 3 km, equal to the width of the seismogenic layer as explained in the previous section. A seismic moment of \(\mathrm{M_{o}}=1.68\cdot 10^{17}\) N\(\cdot\)m was adopted from the GCMT solution and with the above source dimensions and a shear modulus of 33 GPa, a mean slip of 0.1745 m was estimated. The Poisson ratio was fixed equal to 0.25.  
The Coulomb stress changes were calculated for the faulting type of the main shock (strike = 227\({}^{\circ}\), dip = 45\({}^{\circ}\), rake = 160\({}^{\circ}\)) at a depth of 16.5 km, which is approximately the middle of the seismogenic layer (Figure 5c), and are shown in Figure 6a along with the relocated aftershocks. It is evident that the vast majority of the off-fault activity is located into stress enhanced areas. Given that the faulting type of the earthquakes encompassing the other clusters differs from the one of the main shock, and that the activity was more shallow, the stress field is inverted according to the faulting type of the M4.0 shock (strike = 18\({}^{\circ}\), dip = 62\({}^{\circ}\), rake = 139\({}^{\circ}\)) at a depth of 11.5 km (Figure 6b). The earthquakes that occurred south of the main cluster are located inside areas of positive stress changes, an evidence for possible triggering, although with smaller values than in Figure 6a, possibly due to the difference in the calculation depth.