Of fissures and fault lines

Werner Strydom points to the main north-west orientated fissures.

Alarm bells have been sounded several times about the Milnerton Fault line which triggered an earthquake in Cape Town on December 4 1809, measuring 6.5 on the Richter scale.

It is the biggest recorded earthquake caused by that fault to hit the area.

According to geologist Werner Strydom, who lives in Melkbosstrand, earthquakes with a magnitude equal to or exceeding 6.0, are expected to rock the Cape Town area every 300 years.

Mr Strydom formerly worked for South Deep Goldmine near Westonaria, where his skills were put to use mapping gold-bearing strata deep below the surface of the earth.

When he was retrenched in 2013 due to a downturn in the mining industry, he moved from Gauteng to Melkbosstrand, where he continued his passion for researching and exploring the area around him.

In Cape Town, his last three years have seen him develop a deep interest in the fissures that run below the surface of the earth and the sea.

In 1809 there wasn’t much for an earthquake to destroy, but, should a quake of that magnitude hit the area today, it would be devastating, especially considering the high rise-buildings, size of the population and extensive infrastructure, says Mr Strydom.

“The entire West Coast area is located on a series of major westerly and north-westerly orientated fissures or fault lines and the Milnerton Fault may only be one of them.”

In layman’s terms, the fault is a crack in the crust of the earth along which movement occurs.

While he doesn’t believe there’s any reason to panic he says its important to educate the public about seismic hazards in the Cape Town area.

“It is of some concern to me that the fault lines along the West Coast could be reactivated if a major earthquake occurs along the Milnerton Fault or any other known active fault in the general vicinity.”

His research has revealed that those fissures in the ground, which likely include the Milnerton Fault line, formed some 650 million years ago when South America collided with Africa.

“The rocks at Melkbosstrand reveal a series of fissures, spaced from 30m to more than 1km apart. The fissures themselves are filled with cemented, crushed material and are between 5cm to half a metre wide,” says Mr Strydom.

His question is how strong the likelihood is of these fissures refracturing again, if another earthquake is triggered by movement along the Milnerton Fault.

“According to my research, the Milnerton Fault runs from False Bay in a north-westerly direction to the Cape Flats through the Milnerton area and somewhere in the region of Bloubergstrand goes into the sea, and by the time you get to Koeberg it’s about 8km offshore.”

He says he started to map the Melkbosstrand area about two years ago to maintain his field mapping skills.

Going back to the Milnerton quake, he says it was so severe that soil around the Rietvlei area liquified and started acting like sandy slurry bubbling above ground.

“When soils start behaving like liquid it causes havoc and was witnessed in the Mexico City Quake in 1985 where the area was built on the ancient lake bed of the Aztec capital of Tenochtitlan. When that earthquake hit, houses crumbled and havoc was caused by falling debris.”

But Mr Strydom says potential earthquakes are constantly monitored by the Council for Geoscience, and mini-tremors would be detected and early warnings issued.

The Milnerton Fault actually gave 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 the one in 1809. The epicentre was at Jan Biesjes Kraal, the site of today’s Royal Ascot housing estate in Milnerton.

At the time, 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.

“In many places, I was able to push into them, in a perpendicular direction, a switch to its full length, of three or four feet. By the people residing in the vicinity, I was informed, that they had observed these fissures on the morning of the 5th December, in some instances three and four inches wide, and that one person had been able to push the whole length of an iron rod used to fix curtains upon them, and that others had been able to do the same with whip handles of even ten feet in length.”

As recently as May 2009 a small quake was recorded in Cape Town from the geological fault.

While seismic experts have rated the earthquake risk as a rare, but a real risk, Mr Strydom simply says “forewarned is forearmed”.

* Sources: Cape Town Earthquakes: Review of the Historical Record, www.disaster.co.za/pics/Cape_Town_quakes.pdf, Cape Town Geoscapes, A geographical overview of Cape Town, South Africa” https://capetowngeoscape.wordpress.com/geological-hazards/