Old mines likely cause of earth tremor

2013-11-19 00:00

AN earth tremor that registered four on the Richter scale and shook large parts of Johannesburg yesterday, was probably caused by old mines.

Spokesperson for the city’s Emergency Management Services (EMS), Robert Mulaudzi said they had not received any reports of damage or injuries related to the tremor.

The EMS did, however, receive many calls from concerned residents after the tremor, which shook Johannesburg just before 10 am.

Manager of the Council for Geoscience’s seismology unit, Michelle Grobbelaar, described the tremor as “quite a big guy”.

The preliminary analysis indicated the epicentre was near the University of Johannesburg in Auckland Park and measured four on the magnitude scale.

A tremor of this magnitude could make hanging pictures swing and create a sensation similar to vibrations caused by a large truck passing, according to the Modified Mercalli Intensity Scale. Such a quake could also cause windows and dishes to rattle, and could potentially cause stationary cars to rock slightly.

Grobbelaar said previously that after an earthquake, it was a rule of thumb that another tremor of similar magnitude could often be expected in the same region, but seismologists could not predict when this would happen.

Several geologists yesterday said it was not a quake, but a tremor probably caused by a stope in an old mine collapsing deep underground.

Dr Herman van Niekerk, of the department of Geology at the University of Johannesburg, yesterday said most of the mines under the city in the Langlaagte area were up to 100 years old. “When the structures in these mines age, it is only a matter of time before they collapse, which will cause shock waves and tremors,” he said.

Van Niekerk said Johannesburg was situated on the Kaap-Vaal craton, which is three billion years old and has a very stable crust that no longer suffers from earthquakes.

“Remember, most of the earthquakes we see everyday around the Earth happen on the edges of tectonic plates.”

He said although Johannesburg also had tectonic plates, their edges are about two billion years old. “They are very old and stable. In my opinion there is not really a possibility that one of these zones could have caused the tremor.”

Grobbelaar said the council would check if the cause of the tremor was mining related.

One Twitter user said: “There was a tremor in JHB about 20mins ago. Building shook for about a minute. I’m still shaking.”

Another tweeted: “Oh, so the tremor thing happened all over Joburg? Lol I thought it was just because of the construction in our office park.”

Quakes, or tremors, are the result of a stress build-up in the Earth’s crust. When the stress becomes too great, it is released in the form of a quake, usually on a geological fault line.

On Earthquake-report.com, Gauteng residents also related their experiences.

A Roodepoort resident wrote: “… The building shook and it felt like it swayed. Roof tiles and floor boards rattled quite a bit for about 10-15 seconds, very scary”.

Another posted: “Felt tremors for a few seconds. House shook, but no damage. Daughter called to say tremors felt in Sandton as well.”

According to the Council for Geoscience’s website, the first-recorded, damage-causing earthquake in South Africa occurred in Koffiefontein on February 20, 1912. It measured 6.2 on the scale.

One of the most destructive South African quakes recorded occurred in Ceres on September 29, 1969. It measured 6.3 on the local magnitude scale and was accompanied by landslides and rockfalls. Twelve people were killed.

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