Major quake in SA just a matter of time?

By Time of article published Mar 5, 2010

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By Fouzia van der Fort

Staff Reporter

A major earthquake in South Africa is a real possibility, but there is no way of predicting when it might occur.

Dr Chris Hartnady, research and technical director at earth science consultancy Umvoto Africa, singled out Durban as the area of greatest concern in the event of an earthquake on the continent.

But Cape Town is not risk-free.

Referring to severe earthquakes in Haiti and Chile, and another that struck Taiwan on Thursday, he said such events occurred when the tectonic plates of the earth's crust moved, slid, sheared and ground against each other.

In Africa, the boundary or fault line between the Nubia and Somalia plates runs from the Andrew Bain Fracture Zone in the Indian Ocean, starting underground at Port Shepstone, up through KwaZulu-Natal and Lesotho into Mozambique and all the way through north-eastern Africa.

The intervals between major earthquakes of a magnitude greater than seven ranged between 500 and 1 000 years, he said. In some parts of the East African rift system, the last major quake might have occurred 1 000 years ago.

The next one might be due any time soon.

"It is not a question of if, but when, and it could even be tomorrow," Hartnady said.

The complicated system of plate boundaries in Africa was active and moving.

"Big lakes in Africa could ultimately become small oceans, but the rate of the rift growing is very slow. The strain in northern Malawi is growing," Hartnady said.

When the quake would strike could not be determined, but the segments in the boundary zone between the Nubia and Somalia plates were active.

South Africa's biggest recorded earthquake struck in 1969 in the Ceres area.

The worst damage was in the Great Winterhoek valley, Ceres, Tulbagh, Wolseley and Prince Alfred Hamlet.

In an article on Umvoto's website written late last year, Hartnady said earthquakes were a "rare but very real threat for Cape Town".

He wrote that the epicentres of all the quakes reported in and around Cape Town through the centuries were "thought to lie along a structure that geologists from the former Atomic Energy Corporation called the Milnerton Fault, which runs in a south-easterly direction from about 8km offshore of Koeberg, beneath the Milnerton area and probably across the central Cape Flats and the north-eastern part of False Bay".

Michelle Grobbelaar, manager of the seismology unit at the Council for Geoscience, said there had been a lot of debates about whether recent major quakes around the world were related.

Research had shown that the number of earthquakes measuring more than 6.0 on the Richter scale had not increased worldwide, Grobbelaar said.

"Since there has been an increase in the number of good quality seismograph stations throughout the world, many of the smaller earthquakes which previously were not recorded are now being monitored and noted," she said.

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