An Analysis of Earthquake Focal Depths in Africa (1900-2000)

Abstract

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.

Keywords: Africa, Lithosphere, Earthquakes, Focal depths, Brittle, Ductile