(v) Hydrogeological conditions

In this research, the classification is based on River basin managemen<sup>t</sup> plans from the Greek Ministry of Environment, Energy and Climate Change/Special Secretariat for Water (2012) [41], where the highest rating (4) was given to debris, alluvial–marine deposits whose permeability is crucial for slope failure.

(vi) Rainfall

It is well known that high precipitation can increase both the groundwater level and the pore pressure in a soil mass/weathered mantle or aquifer, and accordingly it constitutes the main triggering causal factor of landslides [39]. The data that we used were provided by Attica meteorological stations of the National Observatory of Athens (NOA). NOA has published reports presenting the locally encountered conditions [42]. Those data were analyzed using kriging interpolation in order to acquire a rainfall layer of information for the upcoming GIS geoprocessing. In addition, the rating was based on the statistical analysis made by Lalioti and Spanou (2001) for Greece during the period 1991–1998 [43]. In this research, the class 400–800 mm is the one with the greater amount of rain (mean annual) in the Greek territory, so the highest rating for this study corresponds to 4.

## (vii) Land Use

Land use is a crucial parameter in controlling soil erosion as it is related to the vegetation covering which in its turn provides a protective layer on the earth and regulates the transfer of water from the atmosphere to the surface, soil and underlying rocks [44]. The vegetation data used in this study was extracted from the EU Corine Land Cover 2018 database and its rating is based on Rozos et al. [33]. According to them, the higher rating was given to the cultivated areas, due to the maximum percentage of landslide density that is observed.

#### (viii) Distance from streams

The closer a slope is to a stream, the less stable it is. This happens, due to the fact that streams may adversely affect stability by eroding and saturating the bottom zones of the slopes [45]. The hydrographic network for DIAS geodatabase was generated using the digital elevation model of 5 m pixel size resolution as well as ArcGIS algorithms referring to hydrology processing (Fill, Accumulate, Flow direction based on Strahler classification).

For the examination of this parameter, buffer zones were created around the streams at distances of 50, 100, 150 and 200 m. The classes of the buffer zones are shown in Table 2 and its ranking was based on Rozos et al. [33], suggesting that the most prone class to landslide is that of 0–50 m. This implies that as the distance from the hydrographic axes decreases, the highest percentage of landslide density increases.

#### (ix) Distance from tectonic elements (e.g., faults)

There is an increase in the occurrence of slope failures at areas close to fault zones, because as the distance from a tectonic element decreases, the fracture of the rock and the degree of weathering increases [46], while the structure of the surficial material is affected

causing selective erosion and forcing the movement of water along fault planes to decrease slope stability [47,48]. In Attica Region, many active faults were mapped particularly in west and northeastern part of its peninsula as well as in some islands (such as those of Salamina, Kithira). The digital fault database was provided by the Hellenic Survey of Geology and Mineral Exploration and from the National Observatory of Athens [49]. The classes of the buffer zones are shown in Table 2, with the most prone class to landslide to be that of 0–50 m (rating: 4) [33].

(x) Elevation

The combination of elevation, precipitation and erosion-weathering process contribute to landslide manifestation. The elevation data used in the model were derived from highresolution DEM (5 m pixel analysis) provided by the Greek Cadastre S.A. The classes of the buffer zones are shown in Table 2 and its ranking was based on the landslide statistical analysis made by Koukis and Ziourkas (1991) for Greece during the period 1949–1991 [37]. In this research, the category 200–600 m is related to the highest number of slope failures that happened in Greece, so this class is associated with a rating of 4.

The above data were rated so as to be used in the development of the interaction matrix (Table 2).
