THE DEPTH DISTRIBUTION OF SEISMICITY AT THE NORTHERN END OF THE RWENZORI MOUNTAINS: IMPLICATIONS FOR HEAT FLOW IN THE WESTERN RIFT, UGANDA
Open Access
- Author:
- Tugume, Fred Alex
- Graduate Program:
- Geosciences
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Andrew Arnold Nyblade, Thesis Advisor/Co-Advisor
Andrew Arnold Nyblade, Thesis Advisor/Co-Advisor - Keywords:
- Heat flow
seismicty - Abstract:
- In this study, data from a 6-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, terrestrial heat flow measurements for the Western branch in Tanzania are high (~109mWm-2), similar to high average heat flow in the Eastern branch of the East African Rift System in Kenya (100mWm-2). To investigate further the depth extent of seismicity in the Western branch, seismological data were collected and analyzed 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 horst, suggesting an eastward dip on the Ruimi-Wasa Fault and a northwestward dip on the Toro-Bunyoro Fault. Fault motions are predominantly normal, with strike slip motions between the Ruimi-Wasa and Toro-Bunyoro Faults. The observed depth distribution of seismicity peaks at 16 km depth and extends to 31 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 54-66 mWm-2 is similar to heat flow in other mobile belts in East Africa away from major rift valleys, and is much lower than the high heat flow reported for Eastern branch in Kenya or the Western branch in Tanzania. 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 simply may not have had sufficient time to reach the surface.