Some typical landslide phenomena in Western Greece, in terms of the geological composition and structure of the affected materials, induced by heavy rainfalls are thoroughly studied and analyzed. The studied cases involved composite landslides on Flysch and Neogene weak rock materials that constitute the most critical landslide prone geological formations in Greece. The studied cases named Platanos, Platanitis and Karya include representative landslide sites as regards to the geological composition and structure of the displaced formations. Thus:
flysch formation includes different lithological units as shales, marls, siltstones, sandstones and conglomerates and is closely related to the Alpine orogenesis, suffering intense past tectonic movements and
the Plio-Pleistocene sediments include clayey marls, marlstones and siltstones.
Landslides represent a major threat to human life, property, infrastructure and natural environment in most regions of the world. They are recognized by the scientific and politic authorities as having a major socio-economic impact and they represent a significant hazard for the population and the properties in particular locations. Landslide hazard expressed as the probability of occurrence within a reference time period and is a function of the spatial and temporal probability (Varnes, 1984; Guzzetti et al. 1999; Lee and Jones, 2004). The spatial probability of landslide initiation (susceptibility) is mainly related to static causal factors (slope inclination, material properties, etc.), while temporal probability is mainly related to dynamic causal factors such as rain input, increased groundwater levels and drainage, earthquakes, etc. (Van Westen et al. 2005).
One of the most frequent causes of landslides in the Western part of Greece is a change in groundwater levels, either by natural drainage conditions or by an increase in groundwater due to periods of excessive rainfall. The presence of groundwater affect slope stability by increasing the effective weight of the saturated materials, creating appreciable pore pressure and tending to weaken soft rocks and unconsolidated materials. Other factors such as: variant lithological composition, intense folding and jointing as well as high relief energy favor the manifestation of such phenomena. However, the predominant factors relative to the triggering effects are mainly heavy rainfalls and active fault tectonics. The interrelation between landslide events and precipitation for Western Greece was initially established by Koukis et al. (1996).