The Ceres earthquake of 1969 remains the most destructive earthquake in South African history. The earthquake occurred on the 29 September 1969 in the Ceres/Tulbagh area and registered an alarming 6.3 on the Richter scale, stronger than that of the 1809 Cape Town earthquake which destroyed a local Milnerton Farm. The Ceres/Tulbagh earthquake was followed by a series of aftershocks, the strongest of which was a magnitude 5.7 aftershock that occurred a few months later, on the 14th of April 1970. There were reports of the shock of the original 6.3 magnitude earthquake being felt from as far as Durban in Kwazulu Natal, over 1000km away. It is also estimated to have caused a displacement of 26 cm over a distance of 20km.
While the Milnerton earthquake of 1809 wasn't too far behind the Tulbagh earthquake on the Richter scale (by estimates), the damage was far less because of the population density at the time being far less than it was in 1969. Structural damage in Tulbagh was extensive with in excess of 70% of the buildings suffering damages and over half of the local population being left homeless, there was also a number of damaged homes in the Ceres area. Damage was not confined to homes and buildings though, majority of the local roads in the area were left with large cracks - large fires were even ignited when electricity lines and boxes were damaged. Much of the Tulbagh community had to leave the area while cleanup and reconstruction was performed on the damaged roads and houses, though many of the houses damaged during the earthquake were never rebuilt.
The damage wasn't confined to property though, during the 15 second long earthquake eleven people lost their lives.
Geological investigation into the earthquake suggested that the main earthquake along with the aftershocks that followed were part of a 'swarm'. It is suspected that the earthquake was caused by a shallow tectonic failure along the Saron-Groenhof lineament, which is consistent with the type of tremors that occurred in the Witzenberg Mountain range during the 1969 - 1970 period.
The Western Cape is more prone to naturally occurring earthquakes than any other province in the country and it's only a matter of type before another one hits. During the 1809 quake damage was minimal due to the lack of developed areas, during the Tulbagh earthquake it was a similar scenario, while the town of Tulbagh was more developed than early 19th century Cape Town, there'd no doubt that if (or when) a magnitude 6+ earthquake hits the Cape Town area, damage will be extensive. While you may hear talk of Cape Town being overdue for an earthquake, the reality is that you can't tell when an area is likely to receive an earthquake (other than foreshocks and other noticeable geological changes). So while it's very true that Cape Town could be hit by a 'big one' at any time, it's certainly not something one should expect to happen soon.