The structure of the upper mantle beneath East Africa

Abstract

This study concerns the analysis of arrivals at the Kaptagat array in Kenya set up by the Durham University Department of Geological Sciences. The array consisted of ten short period seismometers and was sited 10 km vest of the Elgeyo escarpment, which forms the western boundary of the Gregory Rift. Onset time analysis was used to determine the slowness and azimuth of approach for teleseisraic arrivals. Large deviations from the expected slowness and azimuth are found and it as shown that the major cause of these anomalies is the low velocity upper mantle which is assumed to exist beneath the Kenyan domal uplift. A preliminary analysis of 60 teleseismic arrivals shows that the anomalies cannot be explained in terms of a single plane interface or any plane structure. It is shown that if the data is to be explained by a single boundary then the structure must thin both to the north and west of the station. The structure was thus assumed to be caused by a structure which is ellipsoidal in plan and hyperbolic in section and optimum structures were calculated for various assumed velocities. Relative teleseismic P-wave delay time data between Kaptagat and Bulawayo, which confirms the presence of low material beneath the station, was, reinterpreted on the basis of the model proposed and was found to be consistent with a depth to the bottom of the structure between 150 and 300 km. The arrival of a phase corresponding to a reflection from the top of the proposed structure was searched for in the records from local earthquakes using velocity filtering techniques but no consistent arrival could be identified with confidence. A preliminary study showed that the postulated seismic structure was not inconsistent with the available gravity data.