Determination of source parameters for the May 20, 1990 Southern Sudan earthquake by inversion of teleseismic body-waves

Mulwa JK, Kimata F. "Determination of source parameters for the May 20, 1990 Southern Sudan earthquake by inversion of teleseismic body-waves.". In: Operating Management of Earthquake, Tsunami and Volcano Eruption Observation Systems. Nagoya: Research Center for Seismology, Volcanology and Disaster Mitigation, Nagoya University; 2011.


The May 20, 1990 earthquake which occurred in southern Sudan is so far the strongest earthquake to occur in the eastern part of African continent within the past 21 years. It caused damage in southern Sudan as well as severe shaking in parts of Uganda and Kenya, and was accompanied by aftershocks on May 24, 1990 of moment magnitudes Mw = 6.5 and 7.1.

Inversion of teleseismic body-waves has been undertaken for the purpose of this study in an attempt to reassess the seismo-tectonics of northern and central Kenya as well as southern Sudan. The results show that the best solution for the inversion of teleseismic body waves for the May 20, 1990 earthquake consists of only one event with a source mechanism of 315o/84o/-3o (strike/dip/rake) and 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 a duration of 17 seconds along a fault plane having dimensions of length  60 km and width  40 km. The average dislocation along the fault is 1.1 m and the stress drop,  is 1.63 Mpa.

The distribution of historical earthquakes from southern Sudan through central Kenya 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. This NW-SE alignment of epicenters confirms the existence of an active fault zone, the Aswa-Nyangia fault zone, from southern Sudan through central Kenya and further into the Indian Ocean. However, owing to lack of waveform data for these historical earthquakes, it is not possible to determine the source mechanism of the fault. Further work on inversion of short period waveform data is required so as to precisely determine the fault mechanism of this NW-SE trending fault zone in the central and southeastern parts of Kenya.

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