How and where is South Africa vulnerable? {writer: Fanie Heyns}

The eastern Japan earthquake and tsunami left nearly 13 400 people dead and almost 5 000 people injured when it struck on 11 March 2011. Just three weeks earlier, an earthquake hit the Canterbury area in New Zealand and left 172 people fatally injured. What are the chances of such an event occurring in South Africa?

So powerful was the March quake in Japan, that it was even recorded by seismograph stations in South Africa.

However, the greater and lingering concern is the threat of a possible nuclear meltdown at Fukushima Nuclear Power Plant in the wake of the disaster.

Dr Chris Hartnady, research and technical director of the earth science consultancy, Umvoto Africa, warned in 2010 in an interview with the Daily Express that the tectonic plates of the Earth’s crust are increasingly active on the East African Rift’s fault lines, which could pose an earthquake threat to South Africa with increasing certainty.

He was quoted as saying: “A major earthquake disaster in the region is inevitable because wide areas of southern Africa are affected by the slow, southward spread of the East African Rift System. It is 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.”

Durban most vulnerable

In an interview with www.iolnews.co.za on 5 March, Dr Hartnady singled out Durban as the area of greatest concern in South Africa.

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 northeastern Africa.

The intervals between major earthquakes of a magnitude greater than seven ranged between 500 and 1 000 years, said Dr Hartnady.

In some parts of the East African Rift System, the last major quake may have occurred 1 000 years ago. And the next one may be due any time soon.

In the South African Journal of Science, Dr Hartnady wrote that the cities of Durban and Pietermaritzburg were vulnerable to earthquake hazard along the Nubia–Somalia boundary.

Durban has reason for concern because of the potential for a repeat of the 1932 Cape St. Lucia earthquake (magnitude 6.3), but in a location much closer to the city.

‘Rare but very real threat’

In an article on Umvoto’s website written late last year, Dr Hartnady stated that earthquakes were a “rare, but very real threat for Cape Town”.

He added 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.

“The Fault runs in a southeasterly direction about eight kilometres offshore of Koeberg, beneath the Milnerton area, and probably across the central Cape Flats and the northeastern part of False Bay.”

Today, a shopping mall in Milnerton Ridge stands on the spot where a house on the Jan Biesjes Kraal farm was completely demolished by a quake measuring 6.1, which struck Cape Town in 1809.

The Milnerton Fault runs through a built-up area, but evidence of its existence lies in the rocks on Blouberg Beach. Dr Hartnady said these rocks are highly sheared and distinctively different in their style of deformation from those in Cape Town or those in the Tygerberg area on the other side, and they are part of a fairly major shear zone.

Between 1620 and 1971, the Western Cape experienced at least 50 quakes with readings higher than 4.0 on the Richter scale, according to Coenie de Beer, a specialist scientist at the Council for Geoscience.

Dr Hartnady said other faults in the Southern Cape are known, and fairly recently it was established that there had been prehistoric earthquakes of between magnitude 7.0 and 7.5.

But the probability of a big earthquake in the region is very low. He put the probability down to the order of once perhaps in 10 000 years.

Minor earthquakes in Gauteng

In 2010, four very minor earthquakes were monitored in the Gauteng area.

Asked by Service how significant these were, and whether there is a possible threat of major earthquakes in South Africa, Michelle Grobbelaar, manager of the seismology unit at the Council for Geoscience, said Johannesburg is situated in the Central Rand Gold mining area, which has experienced more than 900 seismic events since 1985.

The magnitude ranged from M1 to M4.

But monitored seismic activity in the Rand revealed reduced earthquake activity and magnitude since 2006.

The Council for Geoscience has noted a slight increase in the frequency over the past year and has installed seismographic stations in the area to monitor the activity.

South African hot spots

The Council for Geoscience’s earthquake hazard map shows areas of high hazard in KwaZulu-Natal, Lesotho/Free State and Western Cape areas.

But risk is defined as the product of hazard and vulnerability: If there are no vulnerable people and buildings in an area of high hazard, there is no risk.

Hence, the southern Free State (Zastron, Fauresmith and Koffiefontein) has relatively high hazard – in the South African context – but low risk in the technical sense.

“Earthquakes in South Africa are, however, possible because they happen all the time, and because several large historical and prehistoric events are well documented.”

Asked about areas of concern, Grobbelaar said that as a general rule of thumb, if an area has experienced an earthquake of a certain magnitude in the past, it will experience another earthquake of similar magnitude again.

She added that one could expect another earthquake in the Wolseley/Tulbagh area. “This area is a natural hot spot for earthquakes due to the geology and faults in the area,” she noted.

‘Early warning devices’

Much has been done to monitor seismic activity in the country and to be proactive in warning communities about possible disaster.

Grobbelaar said the South African National Seismograph Network (SANSN) consists of 26 stations scattered throughout South Africa and is operated by the Council for Geoscience.

The stations are equipped with state-of-the-art equipment and monitor the seismicity in and around South Africa. “A number of the seismometers at the stations are the best in the world and are considered the ‘Rolls-Royce’ of seismometers,” said Grobbelaar.

“The Council for Geoscience adheres to the New Manual of Seismological Observatory Practice, which is an internationally accepted guideline for monitoring seismicity.”

A full report and maps can be found on the “Service” magazine website at www.servicepublication.co.za.