| The Western Rift Valley is the west branch of the East African Rift System. This rift valley is one of the most seismically and volcanically active region in the African Continent. The Western rift valley is characterized by the shallow seismic activities with normal faulting type. Volcanoes Nyiragongo and Nyamuragira standing at the north of Lake Kivu are characterized by frequent basaltic eruptions in the recent history. The most catastrophic disastrous eruption occurred on Jan. 17, 2002 and the thick lava flows covered over about 13 percent of Goma city, 80 percent of its commercial district and killed 100 people. The Bukavu and Goma (D.R.C.), and Gisenyi (Rwanda) are the main cities located around the lake Kivu margin. The whole population in these cities is more than one million. From the seismic and volcanic studies, field observations and testimonies of inhabitants in the basin, it is revealed that, since 1997 the lake Kivu basin had became more active than before. Several moderate earthquakes with local magnitude more than 4.0 accompanied with foreshocks and aftershocks have been recorded in basin every year. This seismic activity is supposed to be related partly to the reactivation of faults in the basin area and partly to volcano-magmatic activity. The last earthquake of Mw 6.2 occurred recently at the western escarpment of the Rift system near the lake Kivu on October 24, 2002. The damages caused by this event were recorded on houses in Bukavu, Lwiro, Kalehe, Nyabibwe, Kiroshe, Gisenyi and even in farther city like Kigali in Rwanda. The most heavily damaged zone was the territory of Kalehe where 90% of masonry D were destroyed and masonry C affected. In total 8 persons were killed and 1700 houses damaged. Very slight damages were observed in Goma and Bukavu. Landslides phenomenon, which has been already observed in Bukavu area since 1997, was accelerated by the occurrence of this earthquake. In addition to the seismic activity prevailing, some factors such as: the regressive erosion, the presence of aquifer nape, the deforestation, and parcelling on the steep slopes are contributing to the landslides phenomenon and inundation in Bukavu, where big catastrophic event may be expected in the future. Except some seismological and geological results available, no serious studies have been done on soil engineering; foundation soil, special treatment of slope instability and subsidence, modification of strong-motion parameters. Every year several people are killed directly by earthquake or by landslides and inundations. The earthquakes associated to the Nyiragongo eruption on January 17, 2002 killed 4 people, injured 18 and damaged more than 400 houses, most of masonry D in Gisenyi. Therefore, geophysical monitoring must be reinforced quickly around Lake Kivu region in order not only to understand what are occurring beneath the basin and volcanic regions but also to reduce the geophysical risk in future. |
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