REGIONAL GEODYNAMIC IMPLICATIONS OF THE MAY JULY 1990 EARTHQUAKE SEQUENCE IN SOUTHERN SUDAN

Several large earthquakes occurred in the south of the Sudan in May and July 1990. The focal mechanism solution of the main shock (May 20, 1990), one of the largest events in Africa ever recorded, shows left-lateral strike-slip faulting. One of the nodal planes is parallel to the Aswa fault zone, a major Proterozoic tectonic feature. A line connecting the epicenters of the main events of May and July also strikes in the same direction. Field observations and satellite imagery also indicate left-lateral horizontal reactivation movement along this large structural feature during Cenozoic times. The fault plane solutions of the Harvard Centroid Moment Tensor catalog for earthquakes between 1977 and 1989 in the East African Rift System show extensional horizontal slip component striking about northwest-southeast. All these facts lead the interpretation of this wide, diffuse fault zone as an active intracontinental transform area linking the two main branches of the rift system. If one considers this displacement and the slip directions obtained from the centroid moment tensors solutions as characterizing the motion between the Nubian and Somalian plates, new constraints on the regional geodynamics can be inferred; the Somalian block appears to be moving southeastwards relative to the African block. The rupture occurred in at least three steps, affecting a zone roughly 50 km long. The large aftershocks of 24th May have significantly different fault plane solutions, indicating the complexity of the rupture process. Nevertheless, the tension axes of the main aftershocks always strike N-S. Thus there is an exchange of the intermediate and pressure axes for the aftershocks on May 24 relative to the main shock.