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On Saturday night, and again on Sunday morning, many Capetonians felt tremors and heard noises related to seismic activity.

Saturday saw a 6,2 earthquake strike around 1 600 kilometres south-east of South Africa, at 7:10PM, followed by a second, weaker earthquake (2,5 on the Richter scale) at 8:41PM, in far closer proximity to Cape Town.

The tremors felt in parts of the city on Saturday night were most likely due to the second, smaller earthquake.

Just after 9AM on Sunday, a 2,3 earthquake was recorded, with the epicentre around six kilometres north of Durbanville.

First of all, for those disappointed that they missed out on the tremors, here’s Dr David Khoza, executive manager of Applied Geosciences at the Council for Geosciences, speaking to EWN about why some areas were seemingly unaffected:

You could be sitting 20 km away from the epicentre and you might not feel anything, and someone sitting on the other side, 20 km away from the epicentre will feel the tremors. And that purely depends on the rocks over which propagate.

So if they are propagating over soft sand, for example, even if a shallow earthquake could cause quite significant damage. But if they are propagating over very hard rock, you might not feel as much as the other person on the other side.

Then let’s get to why these seismic activities are relatively common, and shouldn’t be cause for concern.

News24 spoke with Jasper Knight, a professor in physical geography, and a geoscientist at the School of Geography, Archaeology and Environmental Studies at Wits.

He said that these events are to be expected, and that we are generally protected from larger earthquakes due to a number of factors:

[Knight] explained that larger earthquakes in southern Africa are uncommon and that they don’t often result in much damage.

“We don’t experience the kind of damage that many other parts of the earth experience,” Knight said.

He said this may be due to the fact that South Africa is in the middle of one of the plates, which, in effect, protects it from larger earthquakes as it tends to occur on the boundaries of the plates.

As a result, South Africa is most usually confronted by the creeks and groans (smaller tremors) of the African plate moving around.

The most notorious earthquake in the Cape’s history, the 1969 Tulbagh earthquake, claimed 12 lives, but that is very much a rarity.

Still, experts say we should be “alert and prepared”, so next time you feel some vigorous tremors, maybe consider the course of action you’ll take other than posting to Facebook.

Do you really want your final words to be “OMG so shook. Did anyone else feel that” followed by five cry-face emojis?

Some of the fears related to the aftermath of the earthquakes had to doe with the risk of tsunamis, whilst others voiced concerns about Koeberg nuclear power station.

On that note, you’ll be pleased to know that tsunamis did not batter the coastline, and also that Koeberg was built to higher earthquake and tsunami specifications than Japan’s Fukushima.

Below, IOL speaking with nuclear physicist and former South African Nuclear Energy Corporation (Necsa) chair Dr Kelvin Kemm:

“Koeberg is designed to withstand 7.5 on the Richter scale. The earthquake in the ocean is reported as only 6.2, so Koeberg could withstand a ‘quake’ of over 10 times larger than that,” he said.

When Koeberg was built, all the sand on site was removed down to the bedrock, Kemm added.

“A mixture of sand and cement was then laid about 5m deep. Then about 3m of concrete was laid on top of that. Then on top of the concrete the anti-earthquake system was built.

“It is a set of Bronze-Beryllium plates which can move sideways. The concrete and steel foundation for the reactors was then built on top of this ‘plate chamber.’

If an earthquake was to strike, the metal plates allow for movement, which adds a layer of safety.

“The nuclear operating defences in the Koeberg reactor system are so advanced that the slightest radiation release that could occur would be detected by a multiplicity of defence systems,” Kemm added.

Well, that’s certainly good news.

As for us Capetonians – fear not, we will rebuild.

[sources:ewn&news24&iol]