Cape Town was shook this weekend, when citizens across the city experienced three rare earth tremors between Saturday night and Sunday morning.
What’s the difference between an earthquake and a tremor?
The Council for Geosciences, earthquakes and tremors are determined according to the magnitude of the event. Within the South African context, any seismic event – such as the ones experienced this weekend – with a magnitude lower than 4.0 is considered a tremor. Those above are termed earthquakes.
What exactly shook Cape Town?
According to EWN, a magnitude 6.2 earthquake struck the ocean approximately 1,600 kilometres southeast of South Africa. Later that evening, there was a magnitude 2.5 tremor just north of Durbanville. The following morning, another magnitude 2.3 tremor was felt in the same area.
Despite the coincidental timing of the large off-shore earthquake and the Durbanville tremor, The Council for Geosciences’ Dr David Khoza told EWN that it was “unlikely that Cape Town experienced the effects of the bigger earthquake off the south coast” and that instead, the tremors felt in Cape town were related to the Durbanville event.
“The 6.2 magnitude event basically happened a little bit earlier and because it was so far away, over 1,600 kilometres, it was highly unlikely that we felt the event, so we’re attributing it to the much smaller event that happened just north of Durbanville.”
Why did/didn’t I feel the tremors?
Though many were shocked by the tremors over the weekend, the Western Cape is actually not unfamiliar with this kind of seismic activity. Whether you’ve experienced them or not just depends on where you are and the geology surrounding you.
“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 [they] propagate”, says Dr Khoza.
“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.”
Should we be worried?
There were no reports of injury or damage on account of the Cape Town tremors, and Dr Khoza stresses that they resulted in no imminent threats to public safety.
“There is no reason to panic. We have seismic monitoring stations all over the country, monitoring these incidents immediately when they happen.”
As for those who were concerned that the off-shore earthquake may lead to a tsunami, Dr Khoza says there is no immediate cause for concern. “In this case, the Indian Ocean Tsunami Warning Centre, of which we participate in, did not issue any tsunami warnings for this earthquake”, he said.
Regarding the future, Dr Khoza says that while we cannot predict when earthquakes will occur, they are constantly being monitored.
“To say that it is more likely to happen in the future, we don’t know, because we can not predict, all we are doing is monitoring the current events. When they happen we observe then and we try to issue alerts as and when they come through”.