The most destructive earthquake in South African history struck the Ceres area at 22H03 on the 29th of September 1969. Its magnitude was 6.3 on the Richter scale, and viii on he Mercalli scale. The shock was felt as far as Durban (1175Km). The earthquake was followed by a number of aftershocks, the most severe of which was on the 14th of April 1970. (5.7 on the Richter scale)
During the earthquake, even well-constructed brick houses were extensively damaged, and many adobe-type buildings were completely destroyed. Nearly all the roads in the area were cracked, pipelines were broken and tombstones fell. Fortunately none of the dams in the area failed, although the earth walls of some were cracked.
Extensive fires ravaged the mountains due to sparks caused by falling rocks and screeslides. The duration of the main shock was 15 seconds.
The Ceres – Tulbagh earthquake – Geological Background
A series of earth tremors centered on the Witzenberg Mountain range occurred from 1969-1970. The size and temporal spacing of the aftershocks indicated an earthquake “swarm”, which is characteristic of a tectonically heteregenous folded mountain belt. The spatial and temporal spacing of the earth tremors suggest a shallow tectonic failure along the ancient Saron-Groenhof lineament. It is estimated from the magnitude of 6.3 on the Richter scale, that the earthquake resulted from a displacement of 26 cm over a 20km length. The accumulation of forces over time will probably cause another earthquake in the future.