SA not immune to US-size earthquakes

2013-11-24 10:40

South Africa is not famous as an earthquake hotspot. But the country is not immune and an earthquake can be every bit as devastating as a Los Angeles quake, seismologists in South Africa are quick to point out.

The stable tectonic setting of South Africa means that large earthquakes have been comparatively rare. Most destructive earthquakes are of tectonic origin, and occur at the boundaries of the earth’s major plates.

But low-intensity tremors are a daily occurrence in South Africa, thanks to our mining legacy, the majority of which rumbles by unnoticed.

Like the earthquake that rocked Johannesburg’s world this week, most seismic events are associated with the deep gold-mining areas.

The Council of Geoscience says most seismic events in the region measure below five and originate in the mining areas.

Research has shown seismicity has increased in parts of the country where the mines reach greater depths. This is to be expected with the yearly growth of the mining industry, scientists have warned.

Mining-related seismicity typically starts when mines reach depths of between 800m and 1 000m as the rock stresses increase.

According to Dr Chris Hartnady, technical director of Umvoto Africa – an earth sciences consultancy firm – South Africa is fairly susceptible to triggering earthquakes with human activity, such as mining and building the Katse Dam in Lesotho.

He said South Africa was a “critically stressed state” that was “on the edge of failure”. He said that even though we were not on the boundary of major tectonic plates, South Africa was a type of hub around which the major tectonic plates, the Nubian and Somalian plates were rotating.

“So you don’t actually need much to trigger earthquakes. There is almost a last straw that breaks the camel’s back,” he said. He explained that there was an area offshore on the east coast that was being squeezed – “kind of like a nut in a nutcracker”.

He said this activity was giving rise to a stress anomaly, making that part of South Africa susceptible to breaking up and causing the big earthquake that everyone fears.

That can happen anywhere, any time, Hartnady said.

“One can never say, where and when, it is pretty unpredictable. It can happen next year or in a 1 000 years,” the seismologist said.

Hartnady says in the bigger scheme of things, geographical changes were happening slowly, moving at about one to two millimetres every year. That amounted to about one metre every 1 000 years.

He said there would be a big quake at some stage, whether it is on our lifetime, no one can predict.

Two earthquakes in recent history may have measured 7 to 8 on the Richter scale.

“The 1932 off St Lucia’s coast may have been such an earthquake, it happened offshore. The earthquake in the 1800s offshore from (Port) Shepstone, might have been another.”

Because of seismic research done for the new nuclear station at Thyspunt in the Eastern Cape, scientists were able to establish that about 11 000 years ago an earthquake measuring 7.4 hit near Uniondale.

South Africa’s most devastating earthquake, near Tulbach, however, remains a bit of a mystery to scientists.

“The truth is, we don’t really have an answer of why it happened,” said Hartnady. “One theory that is that it was caused by the Table Mountain mountain range’s aquifers (water-bearing rock) in an old fault structure and that it might have been a hydro-seismic event.”

This week’s Johannesburg quake is also thought to be linked to water – the flooding of old mines by dirty acid water. Geologists are fairly sure that the quake had something to do with the old abandoned mines underneath Johannesburg.

The council said since mines around Gauteng started shutting down, seismic activity has doubled. The rising water is suspected to destabilise the old mines, leading to rockfalls and cave-ins.

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