Earthquakes result from the rapid release of stored elastic strain in the lithosphere, usually in the form of sudden movement of portions of the earth's crust along faults. Earthquakes are classified as shallow focus at depth less than 70 km, intermediate focus between 70 and 300 km and deep focus >300 km. The objective of the study is to find the depth distribution of earthquakes focal depths in African lithosphere. It also finds the relationship between the focal depths and the earthquake magnitudes. The analysis utilized all available recorded earthquake focal depths and magnitudes in Africa from 1900-2000. The focal depths are arranged into depth ranges of 0-5 km, 5-10 km, etc. Two subsurface depth zones, 5-10 km and 30-35 km, have been identified as zones of concentration of earthquake focal depths in Africa. There is a decrease in seismicity from a depth of 15-29 km. It was discovered that the lower crust and the upper mantle have the same number of earthquake foci and this has rheological significance. The studies further revealed the independence of earthquake magnitudes on the focal depths. The research also suggests that there may be two types of earthquake return periods in Ethiopia, at every 55 and 32 years.

Tectonic forces within the earth produce local accumulations of strain that may be released abruptly in the form of seismic energy. Earthquakes result from the rapid release of stored elastic strain in the lithosphere, usually in the form of sudden movement of portions of the earth's crust along faults. The very existence of earthquake is an indication that the earth is not static but is constantly moving, settling and adjusting. Earthquakes occur over a great range of depths within the earth and are classified as shallow focus at depths less than 70 km, intermediate focus of 70-300 km and deep focus above 300 km beneath the surface.

As in the other continents of the world, there is high seismicity in some parts of Africa, especially within the East-African rift. Undoubtedly, Africans have incurred many losses, both in human and material resources, due to these natural hazards. There are many studies of the source mechanisms and depths of the African earthquakes. Foster and Jackson (1998) employed body waveform inversion techniques to study the source parameters for 38 African earthquakes. Chen and Molnar (1983) studied the intracontinental and intraplate earthquakes, and Bram and Schmeling (1976) studied the structure of crust and upper mantle beneath the western rift of East Africa. Earlier, Gregory (1921) and Rykounov et al. (1972) have made researches on the African earthquakes.

The major objective of this study is to, among others, find the depth distribution of earthquakes' focal depths in Africa and relate the depth distribution to the materials of the African lithosphere. The focal depths of earthquakes greater than 40 km and the earthquakes whose magnitudes are greater than 6.0 M on the Richter scale are also documented and analyzed. This study analyzes the focal depths of earthquakes that have occurred in the African continent from 1900-2000, a period of 100 years.

Earth Sciences Journal, Earthquake Focal Depths, Earthquake Magnitudes, Basement Complex, Regional Metamorphism, Granitic Rocks, Granite Greenstone Basement, Cameroon Line, Continental Lithosphere, Brittle-Ductile Transition Zones, Continental Lithosphere, Geologic Materials, Ductile Deformation.