4.2.2. Thresholds for Lithological Classes

For the purpose of studying the role of lithology in the triggering of landslides in Slovenia, we used the Slovenian engineering geological map in scale 1:1,000,000 by Ribiˇciˇc et al. [49] We have reclassified the 29 rock units into four classes (Figure 6): the igneous and metamorphic complex (IG class), limestone and dolomite (LD class), unbound sediments or sedimentary rocks (US class) and bound sedimentary rocks (BS class). Each class represents a unit of similar rock types that occur in Slovenia. The IG class includes diabase, andesites, granites and all types of volcanic sedimentary rocks; the LD class includes all types and forms of these occurring rocks; US class includes all the unconsolidated clastic sediments such as clay, marl, silt, sand, gravel and similar sediments, and the BS class represents all the occurring cemented fine-grained and coarse-grained clastic rocks.

Figure 6 shows the landslides considered for each class, where the class IG has the lowest number (48 of 368), class LD with 72, class US with 106, and class BS with 142 conditions. Overall, 67% of the considered landslides occurred in the areas of sedimentary rocks (bound and unbound), which take roughly half of the total territory of Slovenia (10,026 km2, 49.5%), while landslides in the area of limestones and dolomites (8803 km2, 43.4%) account for only 20% of the considered landslides. This proves the impact of lithology on landslide triggering conditions, as reported, e.g., by Jordanova et al. [29], Peruccacci et al. [39], Palladino et al. [40], Vennari et al. [50] and Gariano et al. [51] Sedimentary rocks are relatively unstable masses that are very susceptible to weathering and consequently accumulate thick eluvium, which is the main source of material for shallow landslides.

**Figure 6.** Subdivision of Slovenia into four main lithological classes based on the engineering geological map by Ribiˇciˇc et al. [49], with indication of the landslides used in the analysis. The donut chart shows the number of landslides in each class.

Figure 7a shows the *ED* conditions in each lithology class in log–log coordinates with the corresponding 5% NEP thresholds, T5,IG, T5,LD, T5,US and T5,BS (Table 1). The same thresholds are shown in linear coordinates in Figure 7b, with the range of duration (*D*) varying from 1 to 120 h. Due to the small number of conditions for classes IG and LD, the uncertainties are too high, and the thresholds cannot be considered significant for these classes [39,47]. Nevertheless, they indicate the differences in the triggering conditions, with the IG class having the highest threshold and bound sedimentary rocks having the lowest. For obtaining more reliable thresholds, more empirical points are needed. However, a clear distinction in the minimum triggering conditions between landslides that occurred in sedimentary rocks and those that occurred in dolomite, limestone, igneous and metamorphic complexes can be currently observed.

**Figure 7.** (**a**) Log–log plot with the *ED* (cumulated event rainfall—duration) conditions that triggered landslides in Slovenia classified in four geological classes and corresponding 5% thresholds (T5,IG, T5,LD, T5,US, T5,BS). (**b**) Same thresholds and related uncertainties (shaded areas) in the range 1 h ≤ *D* ≤ 120 h in linear coordinates. The thresholds with not-acceptable uncertainties are indicated with dotted lines. Legend: IG, Igneous and metamorphic complex; LD, Limestone and dolomite; US, Unbound sediments or sedimentary rocks; BS, Bound sedimentary rocks.
