Geophysical constraints on intraplate deformation in southern Africa

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