## 4 ETL Susceptibility Assessment in Cephalonia Based on GIS-Based Analytic Hierarchy Process
### ETL Susceptibility Model
Another reason is that PGA has been used in many studies in order to determine earthquake-induced displacements [49, 113, 114, 115]. PSHA in the framework of the present study, in terms of PGA, was adopted by [PERSON] et al. [65], who applied the Cornell and McGuire method [116, 117] that requires a seismotectonic model. Regarding the latter, the seismogenic crustal area source model of the European Seismic Hazard Model (ESHM), proposed by the Seismic Hazard Harmonization in Europe (SHARE) project [118, 119], was used. The required seismicity parameters for PSHA, i.e., the b-value, earthquake rate and maximum magnitude for each zone, were taken from [PERSON] et al. [65]. The Modified Gutenberg-Richter (MG-R) model [120, 121] was adopted as the earthquake occurrence model. The Ground Motion Prediction Equation (GMPE) of [PERSON] and [PERSON] [122] was used for the prediction of the PGA distribution. It is worth noting that this GMPE, obtained using Greek data, can be considered as reliable, as it has been applied in many PSHA studies in Greece (e.g., [123, 124, 125]). The R-CRISIS software [126] was utilized to obtain PGA for a return period of 475 years for Cephalonia Island, considering a rock basement and faulting types corresponding to reverse or strike-slip focal mechanisms (Figure 7f). In terms of land use, the majority of researchers use combinations of vegetation, land cover and land use from existing maps prepared after visual interpretation of aerial photographs and more recently from automatic or semi-automatic processing of satellite imagery at various scales [127, 128] and in different regions, maps showing changes in vegetation or land use [129, 130, 131, 132]. The rationale behind incorporating vegetation into landslide susceptibility models is that vegetation depends on slope stability. The normalized difference vegetation index (NDVI) gives a quantitative estimation of the vegetation growth and biomass in the study area and constitutes an index used in landslide susceptibility studies worldwide. NDVI values range from +1.0 to -1.0. Very low NDVI values (for example, 0.1 or less) correspond to no vegetation cover and exposure of the unvegetated area to surface erosion processes and subsequent slope failures [133]. Moderate NDVI values ranging approximately from 0.2 to 0.5 correspond to sparse vegetation, while high NDVI values ranging approximately from 0.6 to 0.9 correspond to dense vegetation [133]. Thus, the lowest NDVI value has the highest impact on the occurrence of landslides. In this study, Sentinel 2 satellite images were used (acquisition date 21.04.11) and the index was calculated by using the red and near infrared spectral bands [134] (Figure 8a), according to the following formula NDVI = (NIR \(-\) R)/(NIR \(+\) R), where NIR and R are the observed reflectance in the near infrared and red portions of the electromagnetic spectrum, respectively. Regarding the hydrometeorological parameters, the average annual precipitation, leads to an increase in landslide susceptibility by affecting the water content of rock/soil. In this case, gridded precipitation data were collected from the Hellenic National Meteorological Service [88], based on which the spatial distribution of the average annual rainfall in Cephalonia was modelled in raster form (Figure 8b). Concerning the environmental parameters and the influence of the drainage network, the distance from drainage network is used according to the following approach: rivers and streams can influence the probability of landslide occurrence, as their erosive processes especially at the base of the slopes and the intense incision across active faults and increase the instability conditions of the slopes and the susceptibility of landslides. As the distance from the branches of the drainage network increases, the susceptibility to landslide events decreases. Rivers and streams in the study area were derived from the topographic maps of the Hellenic Military Geographical Service in 1:50,000 scale [89; 90; 91; 92] (Figure 8c).