THE DEPTH DISTRIBUTION OF SEISMICITY AT THE NORTHERN END OF THE RWENZORI MOUNTAINS: IMPLICATIONS FOR HEAT FLOW IN THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM IN UGANDA

Data from a six-month deployment of seismic stations around the northern end of the Rwenzori Mountains have been used to investigate the depth extent of seismicity and its implications for heat flow in the Western Branch of the East Africa Rift System in Uganda. Previous seismicity studies of the Western Branch in Uganda show earthquake nucleation at depths greater than or equal to 40 km suggesting that heat flow from the rift is not elevated. However, heat flow elsewhere in the Western Branch (e.g., western Tanzania and Lakes Kivu and Tanganyika) is elevated, similar to the elevated heat flow in the Eastern Branch of the East African Rift System in Kenya. To investigate further the depth extent of seismicity in the Western Branch, seismological clam were collected and analysed to obtain accurate hypocenters using both standard and double difference location algorithms. Focal mechanisms were also obtained to place constraints on the local stress regime. Results show that seismicity is concentrated along the major border faults of the Rwenzori hoist, suggesting an eastward dip on the Ruimi-Wasa Fault and a northwestward dip on the Toro-Bunyoro Fault. Fault motions are predominantly normal, with possible strike slip motions between the Ruimi-Wasa and Toro-Bunyoro Faults. The number of earthquakes increases with depth, peaking at 16 km and falling sharply by 22 km. Strength envelope models indicate that heat flow in the range of 54 mWm(-2) to 66 mWm(-2) is required to explain this result. A heat flow of 54 to 66 mWm(-2) is similar to heat flow in other mobile belts in East Africa away from major rift valleys, and therefore heat flow around the Rwenzori Mountains does not appear to be elevated. This finding does not preclude the possibility of a deep seated (i.e., mantle) thermal anomaly beneath the rift, which may not have had sufficient time to reach the surface.