The Seismicity of Africa

Abstract

Summary The seismicity of Africa has been studied for the period January 1963 to December 1970. Epicentres have been relocated using the method of Joint Epicentral Determination (Douglas). Magnitude-frequency graphs suggest that since 1963, earthquakes with body wave magnitude mb≥ 4.8 are being well determined for the northern half of Africa and with mb≥ 4.7 for the southern half of Africa. The epicentres have been plotted on new geological maps and for the East African rift system there is a good correlation between seismicity and recent faulting. The study confirms that the western rift is more seismically active than the eastern rift and the eastern rift is most active south of about 3° S. Special studies have been made of the series of events associated with the 1966 March 20 and May 17 events in Uganda, with the filling of the Kariba Dam, with deep mining near Johannesburg and with the Ceres earthquake of 1969 September 29. A comparative study has been made of P wave arrivals for seismic stations near and away from the rift system and an attempt made to map the region of P slowing down associated with the rifting. Fourteen fault plane solutions are now available for Africa including the Red Sea and Gulf of Aden. These suggest that the tensional stress field is directed NE-SW for the Red Sea and Gulf of Aden and WNW-ESE for the regions of rifting in Africa.