Geophysical constraints on intraplate deformation in southern Africa

The hazard posed by large intraplate earthquakes is relatively well-known in the GlobalNorth, but Sub-Saharan Africa is poorly represented in compilations of such events, and thehazard they may pose in this region is not well understood. Much of southern Africa is anunusual example of an intraplate region undergoing predominantly extensionaldeformation, complicating comparisons with otherwise similar regions. Here we present thelocations of moderate magnitude instrumentally-recorded seismicity, as well as eight majorpaleoseismic fault scarps across South Africa, Namibia and Botswana. We focus on regionsgenerally considered to be stable, and compare these data to available aeromagnetic andseismic tomographic datasets. Major events are primarily focussed on either the boundariesof the cratonic cores in the region, or in the vicinity of large igneous complexes, suggestingthat variations in large-scale lithosphere rheology provide a first-order control on theiroccurrence. Aeromagnetic lineaments, associated with Jurassic-Cretaceous normal faults orancient shear zones within mobile belts, are associated with almost all of the majorpaleoseismic ruptures, and appear to control fault bends and terminations. Significantdifferences in strike over relatively short length-scales suggest the orientation of the faultsare controlled by crustal anisotropy rather than variations in stress orientation. Some of thescarps are likely to be associated with M7+ events, suggesting that such events can occur instable regions experiencing extensional stresses. The association with major crustalstructures likely explains their great length, relative geometric simplicity and unexpectedlylarge magnitudes, despite limited recent brittle offset. While intraplate events are relativelypoorly studied in southern Africa the excellent preservation potential of landscape, and therarity of extensional events in other comparably stable regions, mean that this region hasexcellent potential to increase our understanding of these phenomena.