Earthquakes in South Africa

September 5th, 2014, Published in Articles: PositionIT

 

In light of the 5 August 2014 M5,6 earthquake, the Council for Geoscience held a panel discussion at the CSIR on the likeliness of similar earthquakes reoccurring in South Africa. Earthquakes do not only damage buildings, but are often accompanied by fires and flooding. They also pose serious risk to vulnerable areas such as the Cape region where the Koeberg power station is situated. The panel consisted of Dr. Ray Durheimm, Prof. Andrzej Kijko, Michelle Grobbelaar, Prof. Herbert Uzoegbo and Prof. Hiroshi Ogasaware.

Earthquake CSIR

Prof. Herbert Uzoegbo, Prof. Andrezej Kijko, Michelle Grobbelaar, and Dr. Ray Durheimm.

According to Grobbelaar, the 5 August earthquake was the largest mine-related seismic incident since 2005, and occurred at 5 km below the surface. The measured acceleration of the slip-event (as opposed to collision event) was 0,2 G, and within 10 minutes of the initial event more than 100 minor aftershocks (the largest of which was M3,8), was measured and mapped. Grobbelaar explained how the shockwave travels and what destruction it causes depends on the geology on which a building has been built.

Dr. Durheimm explained that it is not expected that a seismic event larger than M6,8 to occur in South Africa. Structural engineer, Prof. Herbert Uzoegbo, said that earthquakes up to M4 are unlikely to cause structural damage. He said that earthquakes occurring at shallow depths emit a relatively high frequency, which affects low-rise structures the most as they respond more than higher buildings to high frequencies, and vice versa for higher building and deeper earthquakes. Also, buildings with most of their support walls built perpendicular to the direction in which the shockwave travels are the strongest, while walls parallel to the shockwave are the weakest.

Prof. Kijko explained that one increment on the magnitude scale represents a 32 times stronger seismic event. The depth of the event is also important; Seismic activities at shallow depths are usually mining related, while seismic activities at 10 km or deeper imply tectonic related incidents. Mining creates stresses and weaknesses in rock formations, and can trigger seismic activity. He also stated that there exists a continued chance of a repeated event in mining-related earthquakes, even decades after mining has stopped in an area. Prof. Kijko believes that it is possible for fracking to influence the occurrence or regularity of earthquakes, as has been observed before, though he said that this depends on the location’s geology.

Prof. Hiroshi Ogasaware gave a brief presentation about monitoring earthquakes, and spoke about the SA-Japan research collaboration project JST-JICA SATREPS. The observation program operates 87 projects in 41 countries, with more than 100 sensors installed across South Africa alone.