Cape Town and the 12 Apostels from above in South
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  • It's only a question of when another earthquake will hit Cape Town, an expert says.
  • And when it does, it will be a big one. 
  • That likely quake would cost Cape Town about R10 billion in property damage, of which a big portion could be at the Greenpoint stadium.
  • For more stories go to www.BusinessInsider.co.za.

Cape Town is likely to experience another major earthquake sooner or later – and recent seismic activity in the vicinity of the city could be an indicator of rising tectonic stresses. That’s the view of Professor Andrzej Kijko, director at the University of Pretoria’s Natural Hazard Centre.

The Council for Geoscience (CGS) says more than 80% of seismic events recorded in South Africa are mining related and occur in areas with significant deep-level mining activity, such as the North West and Northern Cape provinces. However, the south-western Cape is one of the regions with the highest level of tectonic-origin seismic activity in the country. In fact, the province has been the site of two of the most devastating earthquakes in South African history – the 1969 Tulbagh earthquake and the 1809 Milnerton quake, both of which measured around 6.3 on the Richter scale.

“While we cannot say exactly when an earthquake will occur, what we can say is that once an earthquake happens in a particular area it will happen again at a similar magnitude,” says Kijko. “It’s only a question of when.”

The epicentre of the 1809 Milnerton quake is thought to have been near the site of the old Ascot Racecourse where the majority of structural damage occurred, and which lies about 15km from the Cape Town CBD. The origin of the quake was likely due to seismic activity along the Milnerton Fault which starts about 8km offshore of the Koeberg Nuclear Power station and runs in a south-easterly direction beneath Milnerton towards the Cape Flats.

“I am not very worried about Koeberg, which was very cleverly designed and can withstand a very strong earthquake,” says Kijko. “I am worried rather about Cape Town.”

Of particular concern is the Cape Town stadium in Greenpoint.

According to a 2014 paper by Kijko, A Smit and N Van De Coolwijk, it is not clear what calculated seismic load the stadium was designed to withstand. In the same paper, titled “A scenario approach to estimate the maximum foreseeable loss for buildings due to an earthquake in Cape Town”, provision was made for "standard" seismic loading of 0.1 g. Nevertheless, the paper found that should a “worst case scenario” earthquake occur in Milnerton measuring 6.96 on the Richter scale, roughly 19.3% of the stadium would be damaged, a loss that equates to R849.2 million based on its original construction cost. If 16.7% of the stadium were damaged then losses would amount to R734.8 million. (Of course, building costs have risen since the stadium was first completed in time for the 2010 FIFA World Cup so actual reconstruction costs would be higher.)

“We did our calculations based on the cost of the whole structure,” says Kijko. “By calculating what percentage of the stadium would likely be damaged in the event of an earthquake in Milnerton at a particular magnitude, you can calculate the estimated value of the damage.”

The paper goes on to say that an earthquake with a magnitude of 6.3 on the Richter scale, the same as the estimated magnitude of the 1809 Milnerton quake, could result in an overall loss to the insurance industry of at least R10 billion for insured structures located between 10km and 50km from the epicentre of the earthquake.

See also | SA’s worst ever earthquake was in the Western Cape – but it wasn’t typical

“Our goal is not to cause panic but to raise awareness among people of the seismic risks,” says Kijko.

Interestingly, Kijko says he believes there may well be a link between the 6.2 magnitude earthquake detected on Saturday, 26 September in Cape Town (which originated 1,600km offshore) and the subsequent 2.5 and 2.9 magnitude quakes that occurred later that weekend. This differs from the stance by the Council for Geoscience which stated that the seismic events were unrelated.

“Nothing happens without reason,” says Kijko. “Earthquakes are caused by the presence of stresses. It is very likely that the magnitude 6.2 event triggered the subsequent events that were felt in Cape Town.”

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