Seismicity studies in the East African Rift, Kenya.

An analysis of the microseismicity distribution within Kenya from temporary recording networks shows the activity to be primarily associated with the Kenya Rift and the Nyanza Trough with increased activity in areas of recent volcanic and geothermal activity. A 100km x 80km earthquake recording network was operated for 3 months (Jan-Mar 1990) in the Lake Baringo region of the Kenya Rift Valley. Twenty-nine stations were deployed over a region including the Elgeyo escarpment, the Kerio Valley, the Tugen Hills and the Baringo basin. Eighty local events of ML< 2.0 were located within 50km of the network. These events are situated within the central part of the rift, showing some association with the main rift faults but mainly clustering beneath Lake Baringo at a depth of ~5km and occurring as swarm activity. 90% of the observed activity occurred at depths shallower than 12km indicating that the brittle/ductile transition zone in this area is between 12-16km, which is similar to that in the Lake Bogoria area. Studies of the focal mechanisms of suitable events indicates a WNW-ESE extension direction across the Baringo basin and the presence of subvertical tensional fractures beneath the lake, which are suggested to be caused by the propagation of basic dykes into the crust. The inversion of local P-wave arrival-time data, for the upper crustal velocity structure beneath the Lake Bogoria and Lake Baringo areas, identified velocity lows associated with geothermal areas, a velocity high beneath the Goituimet basaltic lavas and a 10% velocity contrast across the Bogoria-Emsos-Legisianana fault. A model for the lithospheric structure beneath the Baringo basin is proposed with partial melt in the upper mantle beneath the rift, a dyke intruded lower crust with localised intrusions into the mid-upper crust in areas of geothermal and volcanic activity.