A crustal seismic refraction line along the axis of the S.Kenya Rift

Abstract

The first phase of the Kenya Rift International Seismic Project was carried out in 1985 (KRISP 85) and included both earthquake and explosion programmes. In the explosion programme, 13 shots ranging in size between 100 kg and 1140 kg were fired in lakes and boreholes and recorded by 50 3-component stations deployed at 3.5 km intervals along the southern half of the rift axis and at 1 km intervals across it near Susua. Phases identified on the normalized and true amplitude record sections for the axial line include Pg, Pm, Pn, and intracrustal reflections. Time term and ray-tracing methods were used to interpret the data which suggest a new crustal model for this part of the rift. The velocity at the top of the basement is 6.05 km/s and its depth varies between 2 and 6 km. The intracrustal reflections are best explained by velocity increases to 6.5 km/s at about 15 km depth and a high velocity layer, about 4 km thick, at a depth of about 25 km. An anomalous upper mantle velocity of 7.5 km/s was observed at a depth of 34–35 km. The crustal structure appears to be rather different from that suggested by the 1968 low-resolution experiment in the northern part of the rift and emphasises the need for further seismic studies of the Kenya rift.