Controls on the size and mobility of deep-seated landslides in the North Tanganyika - Kivu Rift region, Africa

The size and mobility of landslides control their impact on both landscapes and communities. Despite their importance to understanding landslide mechanisms and associated hazards, few studies have examined the factors controlling these two characteristics, particularly at a large scale. This is especially the case for deep-seated landslides that occur across diverse geomorphological and lithological settings. Further, most research focuses on recent landslides and thus fail to consider historical processes that could be associated with environmental conditions that differ from the contemporary ones. Here, we investigate the influence of geomorphology and lithology on the size and mobility of old and recent deep-seated landslides in the North Tanganyika-Kivu Rift region in Africa, an under-researched mountainous environment located in the tropics. Based on a comprehensive inventory of ~2500 landslides, we show that mobility increases with size, especially for the old landslides. These old landslides are significantly larger than the recent ones, likely due to potential progressive landslide growth over time and  influenced by the region’s paleoseismic activity. The main controls on both the size and mobility of deep-seated landslides are lithology and, to a lesser extent, fluctuations in Lake Kivu’s level during the Holocene. Landscape rejuvenation by migrating knickpoints associated with rifting also plays a key role in determining landslide size: in rejuvenated landscapes, landslides tend to be larger than those in relict landscapes. The presence of these large landslides favours the development of smaller ones along their margins, reflecting the influence of path dependency on landslide occurrence and size. Our findings underscore the importance of considering the chronology of landslide occurrence and the long-term legacy of landscape evolution in shaping landslide characteristics.