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

Abstract

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 central Main Ethiopian Rift (CMER) and southern Main Ethiopian Rift (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 Somali plates moving apart with the presumed contribution of the NNE(NE)–SSW(SW) extension controlled by the Arabian Plate. The formation and polyphase reactivation of faults in the changing regional stress field significantly increase the rocks' tectonic anisotropy, slope, and the risk of slope instabilities forming. According to geostatistical analysis, areas prone to landslides in the central and southern MER occur on steep slopes, almost exclusively formed on active normal fault escarpments. Landslide areas are also influenced by higher annual precipitation, precipitation seasonality, vegetation density, and seasonality. Deforestation is also an important predisposition because rockfalls and landslide areas typically occur on areas with bushland, grassland, and cultivated land cover. A detailed study on active rift escarpment in the Arba Minch area revealed similar affinities as in a regional study of MER. Landslides here are closely associated with steep, mostly faulted, slopes and a higher density of vegetation. Active faulting forming steep slopes is the main predisposition for landslide formation here, and the main triggers are seismicity and seasonal precipitation. The Mejo area situated on the uplifting Ethiopian Plateau 60 km east of the Great Rift Valley shows that landslide occurrence is strongly influenced by steep erosional slopes and a deeply weathered Proterozoic metamorphic basement. Regional uplift, accompanied by rapid headward erosion forming steep slopes together with unfavourable lithological conditions, is the main predisposition for landslide formation; the main triggers here are intense precipitation and higher precipitation seasonality.

Landslides and Gullies Interact as Sources of Lake Sediments in a Rifting Context: Insights from a Highly Degraded Mountain Environment

“…The ages of large landslides are undetermined and clearly much older, potentially over 1 kyr [25], and have natural origins associated with environmental factors that have not been recorded during the last decade [78]. Although some of these large landslides show altered morphologies from erosion, we argue with confidence on the completeness of this inventory [13].…”

“…The role of faults in explaining the presence of large landslides has been highlighted in previous studies within the region [13,79]. Here, we further consider the seismotectonic control on the occurrence of old bedrock landslides through the analysis of the rejuvenated landscapes created by the upstream migration of fault-related knickpoints.…”

“…Overall, our landslide inventory provides information that complements two recent mapping efforts that only partially cover the study area [13,79]. The authors of [13] looked at a few rather large-scale and old landslides, whereas [79] investigated recent small-scale landslides.…”

“…Overall, our landslide inventory provides information that complements two recent mapping efforts that only partially cover the study area [13,79]. The authors of [13] looked at a few rather large-scale and old landslides, whereas [79] investigated recent small-scale landslides. Note that the latter study provides an inventory that includes, under the name "landslide", a lot of mass-movements that are distributed along gully banks and therefore directly associated with the formation of these erosion processes.…”

“…The increased sediment deposition rate in Lake Abaya since the 1970s resulted in water level rise and decline in water quality [11]. Preliminary field assessments indicated that concentrated sediment sources, dissecting the land and impacting agricultural production in the catchments of the two lakes, are predominantly gullies and landslides [10,[12][13][14]. However, little information exists for their distribution and influence on Abaya and Chamo.…”