The seismicity of the east african rift system

Abstract

Epicentres for the whole of Africa for the period 1963–1970 have been relocated (Fig.1) and their relationship to the rifting studied. For the Gulf of Aden and Red Sea most of the epicentres are found to be related to the spreading zones along their axes. In East Africa, the epicentres appear scattered but are found to be closely associated with the recent rift faulting in the ancient Precambrian crust. Earthquake mechanism studies show the Gulf of Aden and Red Sea to be associated with tensional stress fields directed north easterly and the rifting in East Africa to be related to a tensional stress field in an east south easterly direction. By looking at ray paths from various groups of earthquakes to various African recording stations, an attempt has been made to map the region of P slowing down associated with the rifting (Fig.2). The P wave delays are related to the negative Bouguer gravity anomalies and a possible model for the structure of the lithosphere is presented (Fig.3).