Tomographic study of local earthquake data from the Lake Bogoria region of the Kenya Rift Valley

SUMMARY The 3‐D P ‐wave and S ‐wave velocity structure in the upper crust beneath the Lake Bogoria region of the Kenya Rift Valley is obtained by the simultaneous inversion of local earthquake arrival‐time data for velocity and hypocentral parameters. The data were obtained from seismograms recorded by the KRISP85 (Kenya Rift International Seismic Project 1985) Lake Bogoria earthquake network and inverted using a modified version of Thurber's 3‐D iterative simultaneous inversion program. The results are consistent with the inversion of teleseismic traveltime residuals from the same network and correlate with the mapped surface geological features. There is a high‐velocity anomaly located at depth beneath the Goituimet volcanic region in the southwest of the study area which may be associated with basic igneous intrusives or metamorphism. There is a low‐velocity anomaly in the north of the study area which can be correlated with the geothermal activity around the southern shores of the lake. The main feature is the variation in velocity across the Legisianana–Emsos–Bogoria fault, with velocities up to 5 per cent higher than the average to the east and up to 2.5 per cent lower to the west.