Context and theory

Context and theory

The region of interest corresponds to the centre of the western branch of East African rift valley extending from North Tanganyika region to the Virunga Volcanic Province (VVP). That densely populated area is affected by a combination of several types of geohazards.

 

Earthquakes

The region is characterized by a strong seismicity (800 “felt seisms” between 1909 and 1954 in DRC, Rwanda and Burundi; the greatest historical known earthquakes in East Africa, in terms of magnitude, recorded close to the studied region). Recently, moderate magnitude earthquakes have hit the Kivu basin area and caused important damages and casualties (e.g. 2008 Bukavu-Cyangugu Mw 5.9). Due to the demographic increase, more and more people and infrastructures are exposed to the seismic risk in these regions.

 

Volcanoes

The dormant volcanoes in the Eastern part of the VVP represent a potential threat but require more assessment. The region also hosts the two most active volcanoes of Africa: Nyiragongo and Nyamulagira. Their activities have serious impacts on the environment (e.g. 927 ha of tropical primary forest devastated during the 2010 Nyamulagira eruption). Its neighbour, the Nyiragongo volcano, hosts a (semi-) permanent ~200 m wide lava lake, the largest on Earth. In Africa, this volcano is also probably the most dangerous to human beings as it directly threatens the ~700.000 inhabitants of Goma city, which is located ~15 km south. In January 2002, such as in 1977, fissures opened on the southern flank of the volcano and fast lava flows destroyed about 10 % of the city, killed nearly 150 persons, made 100.000 people homeless and triggered a humanitarian crisis. The long-term socio-economic impact is still felt nowadays. Goma is a strategic economic centre for the Great Lake Region and its population is growing rapidly, also due to the important displaced populations that settle in camps during conflicts. In addition, the high concentration of volcanic CO2 gas dissolved in the lake Kivu water could potentially lead to a limnic eruption, i.e. a brutal gas release leading to massive casualties by asphyxiation in the whole region (e.g. lake Nyos in Cameroun in 1986). A sub-lacustrine magmatic eruption or a major landslide are considered as possible trigger mechanisms for a lake overturn. The passive volcanic degassing causes CO2 accumulation in on-land depressions up to lethal concentration, called mazuku. That hazard is observed in many areas along the northern shore of Lake Kivu including Goma and Sake urban centers where it represents a serious danger to the population and cattle.

 

Mass movements

Throughout the targeted region, landslides mass movement of a portion of the slope or the slope itself (soil or rock) represent the most severe hazard at local scale. A chronological and spatial inventory of geologic/geomorphic events that occurred in the DRC, Rwanda and Burundi since 1900 is shown by the on-line database compiled by the RMCA. In the study area, mass movements can be triggered both by climatic events (strong rainfall) as well as seismic or volcanic activity. The potential impact of these landslides is increased by anthropogenic factors. In cities such as Bujumbura and Uvira, for instance, both the rift escarpment and the Lake Tanganyika limit urban expansion, forcing people to settle on unstable slopes.

 

Such disasters, unforeseen and often sudden events, are the result of the spatial conjunction of a hazard and vulnerable stakes (people, assets, lifelines, activities) in a given place and time. It is fundamental to try assessing the probability of occurrence of a disaster (i.e. the risk).

 

  hazards types in-depth study
(magnitude, frequency, duration, mechanism and intensity)
RISK LEVEL       = X
  analysis of the vulnerability,
i.e. the susceptibility of threatened elements to suffer
from damages if a given hazard occurred

 

Vulnerability 

Vulnerability is measured through the exposure to perturbations and the adaptive capacity or resilience of the vulnerable systems.

An added value of GeoRisCA is to develop a methodology to assess the vulnerability to multi geohazards within the studied area at regional and at local scale, for selected urban sites, considering the evolution of vulnerability through time, e.g. due to improved education, increased income, denser social networks and evolution of coping mechanisms.

 

 

Risk perception

The risk assessment is influenced by an additional factor: the risk perception. People's behaviour when facing a danger are not only influenced by the socio-economic context, but also by their hazard knowledge, the role of political institutions and the role of religion. In summary, people with the higher risk perception are likely to be best prepared in the event of a disaster.