Mobilization rates of landslides in a changing tropical environment: 60-year record over a large region of the East African Rift

The impact of humans on Earth surface processes strongly increased since the offset of the 20th century, often surpassing the importance of natural drivers. In most mountainous regions, landslide mobilization rates may have increased due to human disturbances of the landscape such as deforestation, mining, and road construction. However, assessing the impact of these human disturbances is not straightforward, especially in the Global South where data-scarcity is commonplace. In this study, we use historical aerial imagery to assess the mobilization rates of rapidly-formed landslides and their sensitivity to human impacts over six decades and 21,000 km2 in the North Tanganyika-Kivu Rift region (NTK Rift) in Africa. This tropical region is characterized by rapid societal changes which have drastically altered the natural environment. By estimating the volume of thousands of rapidly-formed landslides, we find that the average mobilization rate in the landscapes rejuvenated by the retreat of rifting-related knickpoints is three times higher as compared to the relict landscapes (ca. 31 [29.2–34.2] m3 km2 year−1 vs 13 [11.9–13.7] m3 km2 year−1). These mobilization rates are dominated by deep-seated landslides. Our mobilization rates are relatively low as compared to observations in other mountainous regions. Yet, the tropical NTK rift also has a more moderate topography and degree of tectonic activity. In addition, our research covers an unprecedented large area and a long period, as such being not biased by local extreme events. In rejuvenated landscapes, roughly 5 % of the sediment mobilization by rapidly-formed landslides is linked to human activity, while in relict landscapes this figure rises to 18 %, notably due to mining and road construction. The role of human activity is limited as compared to the recent occurrence of some large landslides, which seem linked to natural causes and dominate the overall mobilization rates. Moreover, the limited role of human activity must be balanced with the fact that the NTK Rift, although highly populated, remains relatively untouched by major road infrastructure constructions. While previous studies have found that deforestation has a large impact on the landslide risk in the region, its impact on the mobilization rates is much less important. Overall, our results significantly contribute to a better understanding of landslide mobilization and its controlling factors in the context of human-induced environmental change, especially by providing much-needed long-term observations for a currently under-researched type of environment.