The Aswa Shear Zone is a long-lived, northwest-southeast trending fault system crossing northern Uganda and extending into South Sudan and Kenya, running for several hundred kilometres. It is an ancient Precambrian shear zone rooted deep in the African crust that has been repeatedly reactivated over geological time.
Today the Aswa lies obliquely across the developing East African Rift and influences how rifting propagates, acting as a structural grain that younger faults must interact with. It shows predominantly strike-slip character, and its inherited weakness helps localise deformation and seismicity in an otherwise stable interior region.
The shear zone is associated with a diffuse belt of moderate earthquakes across northern Uganda and neighbouring areas, reflecting slow ongoing reactivation. While it does not produce the great quakes of active plate margins, it is an important control on the seismotectonics of the northwestern branch of the East African Rift.