Quake triggered by mine activity – expert

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IOL st main welkom1.JPG Independent Archives Workmen clear away debris from a flattened Ford Cortina, one of the vehicles trapped under tons of cracked masonry when an earthquake struck Welkom in the 1970s.

Johannesburg - An earthquake expert has warned South Africans to be alert to the possibility of quakes.

This follows Tuesday’s quake that originated at the small mining town of Orkney in the Free State.

It was felt as far afield as Botswana, Zimbabwe, Swaziland, Lesotho and KwaZulu-Natal.

The quake, measuring 5.5 on the Richter scale, left one person dead in Orkney.

“South Africans should not be worried, but they must be aware of the possibility of earthquakes,” said Dr Chris Hartnady, technical director of earth science consultants Umvoto Africa in Cape Town.

“They must always be on the alert and be prepared, especially the emergency services.”

In 2010, Hartnady warned that South Africa was “almost certain” to be hit by an earthquake.

Writing in a UN newsletter at the time, he said it was not a question of if, but when.

“The consequences would be so expensive in terms of mortality and economic cost that the risk of being ill-prepared is unacceptably high,” he was quoted as saying in 2011.

Africa needed to be aware of earthquakes so as to be prepared to cope with disasters, he wrote at the time.

He said Tuesday’s quake was triggered by the mining activities around Orkney.

“In this particular instance, it is probably a mining-triggered earthquake, caused by mining activities in the old mines in the areas of Klerksdorp.

“We will call them triggered earthquakes, related to the past and current mining activities. Such quakes occur because the excavation of the mine at such deep levels changes stress in the earth’s crust.”

Hartnady said Tuesday’s earthquake was the biggest recorded resulting from mining activities.

“In 2005, a similar one damaged the Sterkfontein mine. This one is the biggest. We all have to be prepared.”

The Sterkfontein earthquake measured about 5.3 on the Richter scale and left two miners dead and damaged buildings.

South Africa has been hit by earthquakes over the years, some measuring almost the same as that of Tuesday’s or even higher.

The biggest quake recorded in South Africa’s history was the one that struck Ceres near Tulbagh in the Western Cape on September 29, 1969.

It measured 6.3 on the Richter scale and its reverberations were felt across the country.

A seismic event, at 5.2 magnitude, struck another the mining town of Welkom, also in Free State, on December 8, 1976, leaving four miners dead.

Another that hit Welkom on September 26, 1990, had a magnitude of 4.2 on the Richter scale, resulting in two deaths and five injuries.

Quakes – each measuring 4 on the scale – were felt in areas of Gauteng in November last year and in Limpopo in December.

* The November quake lasted about six seconds and had its epicentre at the University of Johannesburg in Auckland Park.

It was felt in parts of northern and southern Joburg.

* The quake on December 2 struck near Bela-Bela in Limpopo and was noted as far afield as Parkhurst and Randburg in Joburg, Hartbeespoort Dam in North West, Witbank and KwaMhlanga in Mpumalanga and Soshanguve near Pretoria.

Hartnady said that the coastal regions of Cape Town and KwaZulu-Natal were generally most likely to be hit by natural earthquakes because of the rapid spread of the East African Rift fault lines.

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