Seismicity of the East African Rift System determined from the Kaptagat array

Abstract

The seismicity of East Africa has been investigated using the Durham University seismometer army at Kaptagat. Events recorded by this station originate from five main seismic regions. Three of the five regions, the Kavirondo Rift, the Siria Fault and the area from Entebbe to the Ruwenzori Mountains, form east-west bands of activity. The activity In the remaining regions, the Western Rift, In particular the Ruwenzori Mountains, and the Gregory Rift shows a north-south distribution. Earthquakes from the Western Rift are associated with the boundary faults. In the Kavirondo Rift the events are associated with the eastern end of the graben and have been used to Infer an easterly extension of the faults. Within the central Gregory Rift the shocks are associated with the axis of the rift. Where the Gregory Rift passes Into the North Tanzania Divergence and the Turkwel Depression the events are distributed across the width of the Rift. From the slope, b, of the cummulative frequency-magnitude curve It Is suggested that the Western Rift can be divided Into a northern and southern section. The former section Is the younger and Is tectonically similar to the Kavirondo graben. The Eyasi and Gregory Rifts have shown the same value for 'b' and seem to be tectonically similar. However, they differ In the upper cut off limit on the magnitude of the earthquakes. The pattern of seismicity In the Gregory Rift and the travel time, compared to normal shield structure, across the rift have been Interpreted to give a model for the crystal structure within the rift. The model consists of a continuous velocity Increase with depth Interrupted by a drop In velocity. The anomalous material which the model represents also has a low Q value of approximately 100.