Cape Town quake zone alert

A geological expert has warned that recent seismic activity could indicate that a geological fault passing through Cape Town has "woken up" and could hold a "nasty shock" for Capetonians.

This emerged after the City of Cape Town insisted that a new 20-storey high-rise building in Table View be "earthquake resistant".

It has highlighted the fact that the suburb virtually straddles a major geological fault in the Earth’s crust. Developers of the new R140 million block of shops and flats said the building had to meet stringent earthquake resistance requirements.

Quake specifications apply throughout the city and structural engineers say all buildings will in future have to meet the building code. Essentially it means that concrete about three times the normal strength has to be used in construction.

"Previously these requirements were not actively enforced by the city but in recent times the city has insisted that buildings be earthquake resistant to a much greater extent," said Mike van Wieringen, geotechnical engineer and geologist.

Engineer Wayne Richie said the South Africa Standards Loading Code specified it was compulsory to build earthquake-resistant buildings in the area that stretches from Langebaan to Tulbagh and Rooi Els, an area identified as being "prone to seismic activity". He said authorities were now ensuring that developers followed specifications.

Years ago the former BP Centre skyscraper on the Foreshore was one of the few buildings in the city to be built to withstand strong earthquakes. The Koeberg nuclear power station further along the coast, built in the early 1980s, was also specifically designed to withstand powerful earthquakes.

Chris Hartenady, a former geology professor at the University of Cape Town and consultant to the International Strategy for Disaster Reduction of the United Nations, this week warned that the huge geological fault had shown signs of again becoming active.

He told Weekend Argus: "In 2003 an earthquake that measured 3.1 on the Richter scale was measured in Table View while smaller quakes are regularly picked up.

"This is not the first earthquake measured in the area. In 1809 a huge earthquake with its epicentre in the Table View area struck Cape Town, causing extensive damage.

"The sound of a loud explosion was recorded by a writer in Green Point who wrote that the quake hit about five seconds after he heard the explosion. This means he was about 25km from the epicentre.

"Geysers of water were forced through the surface of sand in the Table View area and ... people went out there to look at the small mud volcanoes that were formed by the spurting water.

"Such a phenomenon happens only at the epicentre of a quake."

He said in the two years following the 1809 quake, Cape Town was hit by many aftershocks.

"Of course in 1969 we had the quite large Tulbagh quake and smaller quakes are still regularly measured in that area.

"We have mapped a fairly large fault structure under the Big Bay area that runs out to sea and south-east through the Cape Flats area to the Pringle Bay-Rooi Els area. This is known as the Milnerton Fault.

"This is an old fault that was 'blast active' about 130 million years ago. Then it went dormant, but now it has become active and we don't know why.

"There is something dangerous here, something that needs to be studied. I have met the disaster management team of Cape Town and they are aware of the danger. We need to prepare, otherwise we could be in for a very nasty shock."