Explainer: Earthquakes in South Africa

Earthquakes can be scary — stillness and calm preceding a sudden, violent shaking. Often, they can be devastating, as was the case in Türkiye in February, when an earthquake measuring about 7.8 on the Richter scale left more than 50 000 people dead.

By comparison, the quake that hit Johannesburg last Sunday measured about 5.0, making it a mid-sized one. It hit about 17km underground, which is relatively shallow.

The quake struck the East Rand, traditionally a mining region. Mining areas where there are large underground excavations can lead to stressors that can cause earthquakes. This is according to the Council of Geoscience, which is carrying out studies to determine what caused last weekend’s earthquake.

Expert Ray Durrheim wrote in The Conversation in 2019 that “some of South Africa’s gold and platinum mines are so deep that the great stresses around excavations exceed the strength of the rock, which may rupture suddenly”.

“These can cause shaking strong enough to damage nearby excavations,” Durrheim added.

He also said rock bursts pose a risk to mineworkers and nearby communities. These can happen anywhere where mining is deep enough.

The historical mining in the East Rand is being investigated as a possible cause of the recent quake.

The KwaZulu-Natal region has much seismic activity because of the presence of tectonic plates, which are the outer layers of the earth’s crust broken into large pieces.

The Council for Geoscience believes Gauteng is more stable in terms of its tectonic activity than KwaZulu-Natal or even Cape Town but this is not cast in stone. Last weekend’s earthquake could have been the result of tectonic activity elsewhere which resulted in pressure releasing in Gauteng. It could also be that the historic mining in the area led to a change in earth pressures or even that the tremor was caused by fluctuating water levels.

The geoscience council and Earthquakelist.org, a resource for earthquakes happening across the planet, both warned that aftershocks should not be ruled out, although they believed these were unlikely and would probably be small if they did occur.

Mining

Being a mining city, Johannesburg is no stranger to tremors which is why many residents thought the shaking felt on Saturday was probably just one of them. 

Durrheim and his colleagues analysed an earthquake in 2005 which occurred in Klerksdorp. Measuring 5.3, the quake killed two people and injured 58. It was felt in the nearby town of Stilfontein, as well as Johannesburg and a mine close to Klerksdorp. 

The study found that “the magnitude 5.3 event and its aftershocks can be ascribed to past mining”.

“The event was caused by rejuvenated slippage on an existing major fault, with the extensive mining activities in the region contributing most of the strain energy.

The chance of these events being solely due to natural forces is considered to be extremely small,” it added.

A crucial finding noted in the paper was that statistical analysis showed there were greater chances of damaging seismic events in areas where there had been gold mining. These types of seismic events become more probable as mining continues to happen and need to be monitored, the study said.

“Some buildings in gold mining districts are considered to be vulnerable to damage, and even collapse, during seismic events, posing safety and financial risks.”

Earthquake size

A normal assumption would be that the size of the earthquake determines the damage it does. But this is not true. According to Durrheim: “Earthquakes don’t kill people; buildings do.” Often, it is infrastructure that is not equipped to deal with earthquakes which falls and causes deaths.

Africa is not known to have big earthquakes. There have only been four that have measured above 7.0, including a 7.3 event that hit Tanzania in 1910. Algeria and Morocco are high on the risk list, as well as countries on the East African rift. 

An example of a mid-sized earthquake that caused widespread damage was one that struck in Morocco in 1960. It measured at 5.7 but killed about 13 000 people, making it the deadliest quake to hit Africa and one of the deadliest globally in terms of lives lost. 

This ties into the argument that infrastructure must be able to cope with earthquakes.

Durrheim wrote that disaster managers; emergency first responders; town and regional planners and architects and engineers have key roles to play. 

Forming and enforcing building regulations and training emergency services and the public about what to do in the case of a quake are the best ways to mitigate risks associated with earthquakes.