Upper mantle structure using P wave data from an east African array station

Abstract

A seismic array of ten short period seismometers has been set up at Kaptagat, 10 km west of the Elgeyo escarpment in Kenya, by the Durham University Geology Department. This study concerns the analysis of teleseismic arrivals. The method of onset time analysis is used to calculate values of dT/dΔ and azimuth of approach for 34 events from epi central distances of 18 - 99º. Large systematic variations in slowness and azimuth are observed as a function of the great circle azimuth to the epicentre. These are similar to those found at other arrays but are of unusually high amplitude. Prom consideration of onset time residuals and the velocities of regional earthquake arrivals at Kaptagat it appears that these effects are not caused by variations in near-surface structure or by errors in the array geometry. An explanation is developed in terms of sharply dipping interfaces beneath the array, and the preferred model is of a mantle low velocity zone with sloping boundaries underneath the Gregory Rift. This anomalous zone attenuates rapidly westward to sink below mantle material typical of the stable areas of Africa. P-wave delay times are measured relative to Bulawayo for 78 events in the distance range 24 - 99º. Substantial values confirm the presence of a considerable low velocity body beneath the array and the absence of large variations with azimuth implies ultra-low velocities with relatively moderate dips on the zone boundaries. A reinterpretation of Rayleigh wave phase and group velocities for the AAE-NAI path is made and found to be compatible with the top of the low velocity mantle material sinking eastward from the Gregory Rift underneath normal shield-type topmost mantle. Although the model derived is probably not continuous throughout the East African rift system, the upper mantle structure beneath the Gregory Rift is similar to that suggested to exist below oceanic ridges.