West Africa is considered a region of low seismicity. However, the monitoring of earthquake activity by local seismic arrays began very early (as early as 1914) in West Africa but seismic catalogs are very incomplete. In 1991, Bertil studied the seismicity of West Africa based on networks of seismic stations in Ivory Coast and neighboring countries. The reference work of Ambraseys and Adams as well as the recent earthquakes given by the international data centres on the seismicity of West Africa were also used for the computations of earthquake hazard parameters. Different earthquake event data have been compiled and homogenised to moment magnitude (M w ). The obtained catalog covers a period of over four centuries (1615-2021) and contains large historical events and recent complete observations. The complete catalog part has been subdivided into four complete subcatalogs with each a level of completeness. The minimum magnitude and the maximum observed magnitude are equal to 2.89 and 6.8 respectively for the whole catalog. The seismic code software developed by Kijko was used to calculate the earthquake hazard parameters. The results give a b value of 0.83 ± 0.08 for the whole period and preliminary seismic hazards curves are also plotted for return periods 25, 50 and 100 years. This is a good and practical example showing that this procedure can be used for seismic hazard assessment in West Africa.

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How to cite this paper: Teitchou, M.I.,

The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas have porphyritic microlitic textures with a more pronounced magmatic fluidity than the felsic lavas displaying trachytic textures. The lavas are composed of phenocrysts, microlites and microphenocrysts of olivine, clinopyroxene, plagioclase and iron-titanium oxides for the basanites and of greenish clinopyroxene, alkali feldspar, and titanomagnetite for the felsic lavas. Chemical microprobe analysis indicates that the olivine crystals are magnesian (Fo 73-78 ). Clinopyroxene crystals have a composition of diopside (Wo 47-) in the basaltic lavas and diopside near the hedenbergite pole in the trachytes phonoliths and titanomagnetite (TiO 2 : 21.13% -22.36% and FeO: 68% -68%). Chemical analyses on whole rocks show that all the lavas belong to the same series and the felsic lavas come from the differentiation of basanite lavas by fractional crystallization of the minerals therein. The basanites originate from a low rate of partial melting of an OIB-type mantle. Contamination and mixing processes are suspected. Lavas of similar composition are found in other volcanic centres of the Adamawa plateau and the continental and oceanic sectors of the Cameroon Volcanic Line, in particular the Kapsiki plateau, Mounts Cameroon and Bamenda.

Mount Cameroon is the site of explosive eruptions with the emission of ash, slag and lava flows. Mudflows, known as Lahars, can also be observed. These Lahars are mainly found in the lower part of the south-eastern slope. The outcrops observed on this flank show that these Lahars are very extensive and are avalanches of volcanic material, which flowed down the slopes of the volcano after the eruptions. The different outcrops observed are separated by pyroclastic products on which the lahars rest. The components of these lahars are generally black or grey basalts. The main granulometric characteristic of the deposits of these lahars is their heterometry. The size of the material varies from a few meters to the clay fraction. Large concentrations of mineable material are found in the watercourses. These materials are exploited in order to produce aggregates for civil engineering. This exploitation has consequences on the immediate environment. In addition to climatic factors, several parameters have influenced the mobilities and emplacement of the lahars on the south-eastern slope of Mount Cameroon: the slope, the volume and thickness of the deposits, the nature and physico-hydric characteristics of the deposits. The persistence of strombolian volcanic eruptions on Mount Cameroon and the particularly rainy context predispose it to other future episodes of lahar flow. Zogning, A. (1988). Le Mont Cameroun, un volcan actif: contribution à l'étude de géographie physique appliquée (Doctoral dissertation, unpublished doctoral thesis, University of Yaounde, Cameroon).