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Koeberg sitting alongside fault line

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Cape Town - Koeberg nuclear power station is sitting alongside a geological fault which gave rise to an earthquake in 1809 that was the same magnitude as last month’s earthquake in Christchurch, New Zealand.

The Milnerton fault lies about 8km offshore of Koeberg.

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Koeberg Power Station near Melkbos as seen from the air. Photo: Sam Clark

New Zealand’s earthquake, which killed 145 people, measured 6.3 on the Richter scale. Cape Town’s earthquake 200 years ago, which had its epicentre in present-day Milnerton, is estimated to have measured between 6.3 and 6.5.

Several earthquakes in Cape Town have been recorded from this fault, including a small one in May 2009.

Seismic experts have rated the earthquake risk as “rare, but a very real threat for Cape Town”.

The government intends doubling South Africa’s nuclear power capacity and, in spite of the earthquake risk, Koeberg has been selected as one of the sites for more nuclear stations.

Eskom says it is aware of the fault line, but has built Koeberg to withstand an earthquake of 7.

According to a paper published by the Council for Geoscience in Pretoria, the Milnerton fault has given rise to several earthquakes, the first of which occurred “very close to Cape Town” in 1620. There was another in 1811, but the strongest was on December 4, 1809. The epicentre was at a place called Jan Biesjes Kraal, the site of today’s Royal Ascot housing estate in Milnerton.

A visiting naturalist, Wilhelm von Buchenroder, recorded the events five days after the earthquake. He wrote: “Near the Kraal I found rents and fissures in the ground, one of which I followed for about the extent of a mile. In some places they were more than an inch wide.”

Liquidification of sediments occurred in Milnerton as it did in Christchurch.

Chris Hartnady, an international expert on geotectonics and technical director of the Umvoto science consultancy in Muizenberg, wrote on the company’s website that he estimated the size of the 1809 quake to be 6.5.

The website states: “Earthquakes are a rare but a very real threat for Cape Town... increased public awareness of this hazard is needed.”

John Rogers of UCT’s geology department said the sediments surrounding Koeberg were the same as those at Christchurch. When shaken, sand grains in both sediments moved, releasing water from between them, which rose to the surface, as seen at Christchurch. This liquidification was seen at Blouberg in 1809, Rogers said.

He said because of the Milnerton fault Eskom dug out these sediments to get to hard bedrock before building Koeberg. They had then laid a 6m foundation of cement, placed pillars on this with neoprone rubbers on top to absorb vertical movement. Above that they put a metal plate designed to allow sideways movement. Koeberg was built on top.

Eskom spokesman on nuclear matters, Tony Stott, said the design would mean Koeberg could withstand an earthquake of 7. “It is designed as if the epicentre of the earthquake were right under Koeberg, although the Milnerton fault line is 8 to 9km away,” Stott said.

But geologist Nik Wullschleger believes Eskom should not be taking this risk. “(Japan) shows in the final event anything can happen, despite risk assessments.” - Cape Times

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