IMPLICATIONS OF THE DISTRIBUTION OF SEISMICITY NEAR LAKE BOGORIA IN THE KENYA RIFT

Previous seismicity studies in the Kenya Rift have not been able to determine accurately depths of earthquakes, nor have most of them determined epicentres precisely enough to allow correlation of the seismicity with particular surface features. We operated a small dense seismic array for 3 months near Lake Bogoria in the Kenya Rift with the aim of locating microearthquakes in 3-D. 572 earthquakes, 81 per cent with M(L) < 1.0, have been located. The majority of the events are associated with the larger older faults on the Rift shoulder rather than the young 'grid' faults in the centre of the Rift. Seismic activity in the central trough cannot be related directly to the surface faulting; we infer that it indicates the presence of deep buried faults. This possibility has important implications for extension estimates and models of the Rift. Most of the activity occurs at depths less than 12 km, and no normal activity is deeper than 16 km. There is a peak in seismic activity at a depth of 9-10 km and the cut-off depth for brittle failure is taken at 12 km. The depth distribution of these earthquakes is similar to that found in other intracontinental areas with similar heat flow which suggests that the crust beneath the Kenya Rift is of normal rheology.