ATTENUATION AND VELOCITY OF P-WAVES IN THE MANTLE BENEATH THE EAST-AFRICAN RIFT, KENYA

We present the results of mantle P velocity and attenuation tomography from the combined KRISP 1985 and KRISP 1990 teleseismic data sets. We incorporate the results from the KRISP 1990 refraction survey to remo ve crustal effects from the teleseismic data and thereby better image the lithospheric and asthenospheric structure in the mantle beneath th e rift. We find a broad 6% low-velocity anomaly that extends beyond th e limits of the rift, as defined by its bounding faults, and superimpo sed on this a narrower 6% low-velocity zone that is confined to the ri ft zone. The latter anomaly has a total contrast of 12% compared to ne arby unrifted cratonic lithosphere. The broad structure persists at al l depths to 165 km, and is aligned with the rift, while the narrower s tructure varies with depth and does not extend along the entire length of the rift covered by our array. The attenuation signal is noisier t han the travel time signal, but shows the greatest attenuation beneath the rift and at the southern and northern ends of our array.