A Seismic Investigation of the Kenya Rift Valley

Abstract

In August 1985 the crustal structure underlying the southern part of the Kenya Rift Valley was investigated by long-range explosion seismology. the experiment (KRISP 85) consisted of two seismic lines in the central sector of the rift, one along the axis and the other across it. Interpretation of the data, including time-term analysis and ray tracing has shown that the thickness of rift infill varies from about 6km below Lake Naivasha to about 2 and 1.5km below Lake Magadi and Lake Bogoria respectively. the underlying material has a P-wave velocity of 6.05 ± 0.03 km s-1 which suggests that the rift is underlain by Precambrian metamorphic basement. A localized high-velocity zone identified to the east of Nakuru may be due to basic intrusive material. the P-wave velocity increases discontinuously to 6.45 ± 0.2 km s-1 at a depth of 12.5 ± 1.0 km below sea level. This depth is similar to that inferred for the brittle-ductile transition zone from a study of local seismicity in the Lake Bogoria region. A high P-wave velocity layer (7.1 ± 0.2 km s-1) occurs at 22 ± 2 km depth below sea level which might be associated with a sill-like basic intrusion in the lower crust. an upper mantle velocity of 7.5 ± 0.2 km s-1 (unreversed) is reached at a depth of 34.0 ± 2.0 km below sea level. This implies that only moderate crustal thinning has occurred beneath the central sector of the rift. No evidence was obtained for the existence of a continuous‘axial intrusion’ reaching to shallow levels below the rift and associated with crustal separation as suggested by previous studies.