Main Ethiopian Rift landslides formed in contrasting geological settings and climatic conditions

Abstract. The Main Ethiopian Rift (MER), where active continental rifting creates specific conditions for landslide formation, provides a prospective area to study the influence of tectonics, lithology, geomorphology, and climate on landslide formation. New structural and morphotectonic data from CMER and SMER support a model of progressive change in the regional extension from NW – SE to the recent E(ENE) – W(WSW) direction driven by the African and Somalian plates moving apart with the presumed contribution of the NNE(NE) – SSW(SW) extension controlled by the Arabic Plate. The formation and polyphase reactivation of faults in the changing regional stress-field significantly increase the rocks' tectonic anisotropy and the risk of slope instabilities forming. According to geostatistical analysis landslides in the central and southern MER occur on steep slopes, almost exclusively formed on active normal fault escarpments. Landslides are also influenced by higher annual precipitation, precipitation seasonality, vegetation density and seasonality. A detailed study on active rift escarpment in the Arba Minch area revealed similar affinities as in regional study of MER. Landslides here are closely associated with steep, mostly faulted, slopes and a higher density of vegetation. Active tectonics and seismicity are the main triggers. The Mejo area situated on the uplifting Ethiopian Plateau 60 km east of the Rift Valley shows that landslide occurrence is strongly influenced by steep erosional slopes and deeply weathered Proterozoic metamorphic basement. Rapid headward erosion, unfavourable lithological conditions and more intense precipitation and higher precipitation seasonality are the main triggers here.