The Kivu basin, studied and monitored by the GORISK Scientific Network, is located in the western branch of the East African Rift Sytem (EARS), along the border between D.R. Congo and Rwanda.
The EARS (Fig. 1) is an intra-continental break-up system in Africa, which extends from North Ethiopia to Mozambique. It separates into two branches between South Ethiopia and South Tanzania, namely the western and eastern branches, which surround the Tanzanian craton. The rift branches are subdivided into asymmetric basins bounded by normal faults. Unlike the Eastern branch, the Western Rift experiences intense seismic activity and moderate volcanism. Its basins are linked by transfer zones, where volcanic provinces sometimes develop.
The Kivu rift basin corresponds to a complex half-graben bounded, to the West, by steep escarpments and, to the South and North, by two volcanic provinces (Fig. 2): The S-Kivu Volcanic Province (SKVP) and the Virunga Volcanic Province (VVP). Only the VVP currently shows volcanic activity. That activity is restricted to Nyiragongo and Nyamulagira, which are two of the most active volcanoes in Africa. The center of the Kivu basin is filled by the 485m-deep Lake Kivu. Located in the area where the trend of the western rift branch changes from NNE to NNW, the Kivu basin is tectonically highly active.
This intense volcano-tectonic activity triggers a large amount of natural hazards. Volcanic activity at Nyiragongo and Nyamulagira volcanoes threatens directly more than 1 million persons and –combined to anthropic activities- affects the fragile ecosystem of the Virunga National Park, an endangered UNESCO World Heritage. In addition, a large amount of volcanic products potentially affect the environment, the air, and the water quality. Earthquakes are frequent. They involve building and other human infrastructures damages, and cause casualties. Earthquake-triggered landslides are also frequent, mainly west and south of Lake Kivu. Landslides also occur with the heavy rains and intense erosion. In the Virunga Volcanic Province, the main edifices of some dormant volcanoes are cut by deep gullies of dense temporary hydrographical networks. Those networks are activated during heavy rains, triggering torrential events, which develop into mudflows when large amounts of unconsolidated materials are available. Finally, Lake Kivu is characterized by huge and increasing amounts of dissolved gases (methane and carbon dioxide) at depth. Although the dissolved gases concentration is not close to saturation, a large and brutal mixing of layered waters in the lake (e.g. caused by an earthquake, mass movement or an eruption) could trigger a limnic eruption, i.e. a sudden degassing of the lake. Such an event would have catastrophic consequences on the surrounding population (> 2 million persons) and animals living in the vicinity of the lake, as it already occurred, for example, at Lake Nyos (Cameroon) in 1986.