An earthquake study in the Lake Baringo basin of the central Kenya Rift

Abstract

A 100 × 80 km 2 earthquake recording network was operated for three months (January–March 1990) in the Lake Baringo region of the Kenya Rift Valley. Twenty-nine seismic sites were occupied by short-period stations over a region including the Elgeyo escarpment, the Kerio Valley, the Tugen Hills and Lake Baringo itself. Eighty local events of M L < 2.0 have been located within 50 km 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 about 5 km and occurring as swarm activity. Ninety percent of the events occurred at depths shallower than 12 km. The brittle-ductile transition zone in this area is determined at a depth of 12–16 km, similar to that in the Lake Bogoria area immediately to the south. A preliminary study of focal mechanisms of suitable events indicates WNW-ESE extension across the Lake Baringo basin and suggests the presence of sub-vertical rupture surfaces beneath the lake. These may be caused by the insertion of basic dykes into the upper crust, and the rupture process assisted by high temperature and pressure geothermal fluids. The inversion of local P-wave arrival time data for the upper crustal velocity structure beneath Lake Baringo identifies a low-velocity zone beneath surface geothermal activity, a relationship previously demonstrated for the Lake Bogoria region.