How and where is South Africa vulnerable

The Great Eastern Japan earthquake and tsunami left nearly 13 400 people dead and almost 5 000  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 changes of such an event occuring in South Africa?

So powerful was the one in March it was even recorded by seismograph stations in South Africa. The greater and lingering concern, however is the threat of nuclear meltdown at Fukushima that came in its wake.

The Japan quake, which measured 9.0 on the Richter scale and was the biggest ever to hit that industrialised island-nation has governments across the globe re-thinking their nuclear plans, yet here, in South Africa, cabinet has just approved plans to build additional new nuclear power stations at Bantamsklip, Thyspunt and a second reactor at Koeberg as part of Eskom's 20-year expansion plans.

Concerns, however were raised even before the Japanese quake.

Dr Chris Hartnady, research and technical director of the earth science consultants Umvoto Africa, warned in 2010 in an interview with the London Daily Express that the tectonic plates of the earth’s crust are increasingly active on the East African rift’s fault lines, and this 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.”

He singled out Durban as the area of the greatest concern in the event of an earthquake in South Africa in an interview with www.iolnews.co.za on 5 March.
Referring to severe earthquakes in Haiti and Chile, and another that struck Taiwan, 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.

‘It could be tomorrow’

The next one might be due any time soon.

In the South African Journal of Science, Harnady wrote that the cities of Durban and Pietermaritzburg are vulnerable to earthquake hazard along the Nubia-Somalia boundary.

The small earthquakes of July 2002, strung out along the Nubia-Somalia plate boundary, should serve as a wake-up call to policy-makers and those with hazard and disaster-reduction responsibilities, he added. (SA Journal of Science, September/October 2002).


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.

The Fault 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".

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



Coenie de Beer, a specialist scientist at the Council for Geoscience, told the Carteblanche-programme on M-Net on 27 March that the whole area, because it's sitting right beside Rietvlei, is on liquefiable soil, so one would expect that the worst effect of the seismic activity would have been here.
"The records that we have are mostly from a scientific traveller by the name of Wilhelm von Buchenroder who came around here at the beginning of the 1800s and experienced it.

“Then he also visited a farm by the name of Blaaubergvlei to the north of here and there he saw some sand volcanoes and liquefaction of the soil had occurred,” said De Beer.

Milnerton Fault

The Milnerton Fault runs through a built-up area, but evidence of its existence lies in the rocks on Blouberg beach. 

Dr Hartnady said the rocks are highly sheared and they are distinctively different in their style of deformation from those in Cape Town or those in the Tygerberg on the other side, and they are part of a fairly major shear-zone.
The fault runs eight kilometres off-shore past Koeberg, but nobody has ever seen it inland.

 The 1969 earthquake may have been the biggest so far, but between 1620 and 1971, the Western Cape experienced at least 50 quakes with readings higher than 4.0 on the Richter scale, according to De Beer.

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 maybe 10 000 years.

Earthquake swarm in Augrabies

According to an article on www.worldpress.com on 25 February 2011, the area of Augrabies has since February 2010 been experiencing an earthquake swarm. These are sequences of many minor earthquakes in a relatively short period of time.

A swarm may last for days, weeks, or months, but rarely more than two years.

They are differentiated from earthquakes succeeded by a series of aftershocks, by the observation that no single earthquake in the sequence is obviously the main shock.
As part of its national monitoring programme, the Council for Geoscience has been constantly monitoring the seismic activity in the Augrabies area.

The first recorded earthquake of the present swarm occurred during February 2010 but it was only when the population felt an earthquake measuring 3.7 on the local magnitude scale on 26 July 2010 that people became aware of this seismic activity.

Since then, at least five earthquakes exceeding magnitude 4 have occurred near Augrabies, the largest to date being magnitudes 4.2 and 4.9, events that occurred on 12 and 25 January 2011, respectively.

The Augrabies area is situated on the boundary of the Kaapvaal Craton (Archean), the Kheis orogenic belt (1 800 Ma) and the Namaqua-Natal orogenic belt (1000 Ma).  This area is highly deformed and there are numerous faults, shear zones, folds and other lineaments, which could provide weak areas in the crust.

It also lies on  the Hebron fault in Namibia, which is known to have been active during recent times. 

Earthquake epicentres tend to lie parallel to the Orange river in that area, on both sides of the river, and the structural maps indicate a number of faults striking parallel to this trend all along the river, according to www.worldpress.com (Source: 25th February 2011).

Minor earthquakes in Gauteng

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

Asked how significant these were and whether there are a possible threat of major earthquake in South Africa, Michelle Grobbelaar, manager of the seismology unit at the Council for Geoscience, said the Johannesburg area is situated in the Central Rand gold mining area, which had experienced more than 900 seismic events since 1985. The magnitudes ranged from M1 to M4.

But monitored seismic activity on the Rand revealed reduced earthquake activity and magnitude since 2006. The Council for Geoscience has noted a slight increase in the frequency the past year and has installed seismographic stations in the area to monitor the activity.

South Africa has a low incidence of large earthquakes since it is considered an intra-plate region. Earthquakes do occur in the country with the largest number occurring in the gold and platinum mining areas, Grobbelaar told Service magazine.

Hartnady said to Service that earthquakes in Gauteng are mostly related to mining-induced seismicity, due to the readjustment of stresses around the active mining face.
“Recently, there have been some events in old, mined-out areas of the Central Witswatersrand near Soweto, and these are being linked to the current flooding of the mine void with water.”

SA hotspots and map

Hartnady said one should distinguish between risk and hazard. The Council for Geoscience publishes an earthquake hazard map, which shows areas of high hazard in KwaZulu-Natal, Lesotho-southern 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 Southern Free State (Zastron-Fauresmith-Koffiefontein) has relatively, in the SA context, high hazard, but low risk in the technical sense.

Asked whether South Africans should be concerned about what happened in Japan and New Zealand, she said generally the great distance between the events excludes the idea about a link.

“However, there are theories that state that if there is movement along one plate boundary, it will affect other plates because the plates are interlinked.
“This theory is again hotly debated, and has thus far not been proven.”

Hartnady said that earthquakes, both natural and mining-induced, occur in South Africa almost every day. It is just that most are too small to be felt and are only known from the local seismographic record.

Earthquakes of magnitude 4 and above are recorded by regional stations elsewhere in Africa and federations of seismological agencies.

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

Asked about areas of concern, Hartnady highlighted Natal and the Western Cape, because these zones of relatively higher hazard have large populations in big cities, like Durban and Cape Town:

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.
Cape Town has reason for concern because of the possible repeat of the 1809 Milnerton earthquake.

There could be another Tulbagh-quake

Asked about areas of concern, Grobbelaar said 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.

“The question is when. The concern is whether it was taken into account when infrastructure was developed and constructed.

“Seismic hazard assessments deal with this aspect and provide the engineers guidance with respect to the size of earthquakes to design for, especially sensitive structures such as dams, power plants and pipelines.”

Grobbelaar said one can expect another earthquake in the Wolseley/Tulbach area. “This area is a natural hotspot for earthquakes due to the geology and faults in the area.”

Perspective

Prof Abraham Rozendaal, professor in economic geology at the department of earth sciences at the University of Stellenbosch, told Service that South Africa is located on a very stable part of the crust and earthquakes are a rarity.

“If it is in Gauteng, it could be related to bumps from the gold mines, sink hole collapsing or simply adjustments that are erosion related.

“Historically, the Western Cape/Boland has been active along the Worcester Fault and has caused damage in the Tulbagh area. The risk compared to active zones on global scale, such as the Ring of Fire, is minimal.”

In  the Eastern African rift valley (the boundary or fault line between the Nubia and Somalia plates) are tectonically active zones which reflect divergent plate margins with active volcanism -- referred to as continental rifting.

They indicate the breaking up of a crustal zone margins and plate movement. Along this active rift zone there will be tectonic activity such as earthquakes.

“The crustal plates in that area are moving very slowly, that is 2.7 centimetre per year, compared to up to 15 centimetres in other areas. The tectonics are also tensional in the rift valley, compared to compressional in areas such as Japan.

“In simple terms, the plates in the African rift are moving away from each other, whereas in Japan they are moving towards each other and colliding,” said Prof Rozendaal.

‘Early warning devices’

Much has been done to monitor seismic activity in the country and to be proactive in warning communities about a possible disaster.
Grobbelaar said to Service the South African National Seismographic Network (SANSN) consists of 26 stations scattered throughout South Africa and is operated by the Council of 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.
“The Centre of Geoscience adheres to the New Manual of Seismological Observatory Practice which is an internationally accepted guideline for monitoring seismicity,” said Grobbelaar.

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