Earthquake: Moving tectonic rocks probable cause

GEORGE NEWS & VIDEO - Preliminary indications are that yesterday morning’s earthquake that sent tremors through the Little Karoo and George area could be as a result of movement of tectonic rock masses.

This is according to Eldridge Kgaswane, a senior scientist from the Council for Geoscience (CGS).

Kgaswane said the epicentre of the quake was roughly 60 km north of Oudtshoorn (33.355° South, 22.386° East).

Asked about the effect the seismic activity could have had on the Cango caves, he said, “In principle, it is possible for cracks or instability to occur in the caves, since they are in proximity to the epicentre. But this depends on other factors, such as soil conditions in the cave area and whether they could have possibly amplified the seismic waves from the epicentre or not.

“However, at this stage we haven’t received detailed reports from the Cango caves and nearby towns. Probably, we will have a team dispatched in the very near future to do a forensic seismic survey in the caves and the nearby towns and farms.”

He said the CGS has no record of any significant seismic activity in the past at this epicentre. However, along the same faultline to the west, there had been a significant earthquake in the Ceres-Tulbach area. Seismic activity in the Leeu-Gamka area which is just 100 km north of this epicentre has also been recorded in the past.

According to Kgaswane, it is possible that tremors of this morning’s quake could be felt as far as Mossel Bay, depending on the local soil or subsurface conditions.

The nearest seismograph station that recorded the tremor is just 150 km northwest of the epicentre. “We unfortunately do not have a seismograph station in the area or in close proximity. The area is seemingly rugged and mountainous and normally it is logistically difficult to put a seismic station in such areas.”

According to the book, ‘Geological Journeys – A traveller’s guide to South Africa’s rocks and landforms’ by Nick Norman & Gavin Whitfield, the fault system in which the epicentre is located, is one of the most extensive fault systems in the country, running from 100 km west of Port Elizabeth all the way to Tulbagh. This fault system “has shown all too dramatically that it is still active, not only by the movement beneath Ceres and Tulbagh in 1969, but also in other ways along its length in recent geological times”.

(Thanks to local retired geologist Deon Mocke who lent the George Herald his copy of the book.)

What to do in the event of an earthquake
Kgaswane said during an earthquake, one should follow the DCH (drop, cover and hold on) approach.

“If you happen to be inside the house or building during the earthquake, then the safest place would be under the table. If you are outside, then move away from buildings, bridges, and electric or phone lines. Fatalities in most cases happen due to falling objects. Earthquakes normally do not kill people, but buildings do.”