Tectonic Structures Across The East African Rift Based On The Source Parameters Of The 20 May 1990 M7.2 Sudan Earthquake

Abstract

Earthquakes in Kenya are common along the Kenya Rift Valley due to the slow divergent movement of the rift and hydrothermal processes in the geothermal fields. This implies slow but continuous radiation of seismic energy which relieves stress in the subsurface rocks. On the contrary, the NW-SE trending rift/fault zones such as the Aswa-Nyangia fault zone and the Muglad-Anza-Lamu rift zone are the likely sites of major earthquakes in Kenya and the East African region. These rift/fault zones have been the sites of a number of strong earthquakes in the past such as the Mw = 7.2 southern Sudan earthquake of May 20, 1990 and aftershocks of Mw = 6.5 and 7.1 on May 24, 1990; the 1937 Ms = 6.1 earthquake north of Lake Turkana close to Kenya-Ethiopian border, and the 1913 Ms = 6.0 Turkana earthquake among others. Source parameters of the May 20, 1990 southern Sudan earthquake shows that, this earthquake consists of only one event on a fault having strike, dip and rake of 315o/84o/-3o. The fault plane is characterized by left-lateral strike slip fault mechanism. The focal depth for this earthquake is 12.1 km, seismic moment Mo = 7.65 x 1019 Nm and moment magnitude, Mw = 7.19 (7.2). The fault rupture started 15 seconds earlier and lasted for 17seconds along a fault plane having dimensions of  60 km x 40 km. The average fault dislocation is 1.1 m and the stress drop, , is 1.63 MPa. The distribution of historical earthquakes (Mw ≥ 5) from southern Sudan through central Kenya generally shows a NW-SE alignment of epicenters. On a local scale in Kenya, the NW-SE alignment of epicenters is characterized by earthquakes of local magnitude Ml  4.0, except the 1928 Subukia earthquake (Ms = 6.9) in central Kenya. This NW-SE alignment of epicenters is consistent with the trend of Aswa-Nyangia fault zone, from southern Sudan through central Kenya and further southwards into the Indian Ocean. We therefore conclude that, the NW-SE trending rift/fault zones are sites of strong earthquakes likely to pose the greatest earthquake hazard in Kenya and the East African region in general.