Will the city be hit by a series of earthquakes in the next five years? asks Nawaal Deane
There is a real threat of significant earthquakes and tremors affecting the Johannesburg-Pretoria region in the near future if deep mining projects being planned in Carletonville go ahead, according to research commissioned by Hannover Reinsurance Company Africa.
Records of earthquake and tremors kept by the South African Council for Geoscience in Pretoria indicate that mining activity accounts for up to 98% of all quakes and tremors in the country. “It is clear that if you live in a mining area you will experience tremors on a weekly, if not daily basis,” says Nicholas Davies, a statistical analyst at Hannover Re Africa who was involved in the seismic research project.
The African sub-continent does not face the massive naturally occurring earthquakes that rock California, Japan and central Europe. In 1995 the Japanese city of Kobe was hit by the largest naturally occurring earthquake since 1923, with a magnitude 6,9 on the Richter scale, resulting in massive damage to property and loss of life. But there is still a risk of naturally occurring earthquakes here. Cape Town, for example, is likely to face a damaging seismic event within the next 10 to 15 years due to natural geotectonic forces, caused by the movement of the Earth’s plates relative to one another.
According to the research, South Africa could experience another earthquake similar to the 1969 Ceres- Tulbagh one, which reached a magnitude of 6,3 and left 12 people dead and caused huge insured damage. Dr Andrzej Kijko, a leading seismologist at the South African Council for Geoscience, calls the risk of serious earthquakes “not negligible”.
The predominant seismic risk faced by South Africa and, in particular, the Johannesburg-Pretoria region, relates to mining. “In the not too distant future … mining activity is likely to give rise to seismic events that are going to impact not just on the mining areas but quite possibly central Johannesburg, in a very material way,” says Davies.
Intensity of the expected level will certainly result in some damage to buildings and construction. “In fact, as was shown by the 1992 tremor originating in Carletonville, significant damage was experienced as far afield as Johannesburg and Pretoria,” he says.
But the research warns that it is the proposed “ultra-deep” mining in the Carletonville region that poses the highest risk to South Africa. The country’s deepest gold mine is just more than 3 000m, but the ultra-deep mining programme will take mine shafts to a depth of 5 000m.
“This mining will certainly lead to increased seismic activity,” says Davies. “Calculations suggest that an event of magnitude 5,5 on the Richter scale or 5,6 can be expected once those depths are reached. Damage to buildings in Johannesburg could exceed anything we have seen before in this area.”
When questioned on why the mining houses had not taken note of this risk, Davies said: “As far as I can see, for every seismologist who identifies such a risk there will be another who will approach it from another angle and discount it.”
Davies did not want to speculate on when these quakes could occur but said there are theories that they could happen in the next five years.
The research has not yet significantly affected insurance rates. “To date we have not seen premiums being increased to accurately reflect seismic risk in this country. Unfortunately, it seems that it will take a serious seismic event before heed is paid to the warnings and then, of course, it’s too late,” he says.