Soweto tremor: Quakes are light and fewer, say experts

The Council for Geoscience has confirmed an earthquake occurred on Tuesday at about 8.54pm and the epicentre was located south of Johannesburg in Soweto.

According to the council, preliminary results show the earthquake registered a local magnitude of about 3.4 as recorded by the South African National Seismograph Network. 

The Wits School of Geoscience's Prof Raymond Durrheim said the epicentre lies on the property of Harmony Gold’s Doornkop Gold Mine west of Soweto.

Dr Herman van Niekerk, a senior lecturer in the department of geology at the University of Johannesburg, said the tremors occur regularly and are mostly related to mining activities.

“Sometimes these are felt during blasting, but these days they are mostly related to old mined-out areas where stope collapse occurs. This can be caused by changing water levels in the old mined areas, but could also be related to destabilising of old mining areas when illegal miners remove ‘pillars’, which are areas underground not mined to support the roof of a stope.

“Illegal miners remove these to access the left-behind ore. Small tremors can also be the result of the natural stress field in the crust of the Earth and the crust in south-eastern Africa is under stress due to the development of the east African Rift Valley,” Van Niekerk said.

He said there have been thousands of tremors in the Johannesburg area and they are usually only felt by people close to where they occur.

“There are about 100,000 tremors with magnitude 3 and 12,000 with magnitude 4 all over the world each year. In most cases, the tremors cannot be related to each other, apart from when they occur as aftershocks in the event of a large earthquake.

“They are seen as ‘minor’ earthquakes and might only cause damage to structures that are poorly constructed and do not conform to building regulations.”

Durrheim said earthquakes related to gold mining were first noted in the early 1900s as mines reached depths of several hundred metres.

“They occur when excavations grow large, and the weight of the overlying rocks exceeds the strength of the pillars and pre-existing weaknesses in the rock, such as ancient faults. Mines are carefully designed to reduce the likelihood of damaging and dangerous events, and we have seen great improvement in mine safety in the past 30 years,” he said.

He said the number of earthquakes related to gold mining has reduced in recent years as the amount of ore mined has diminished and mines have closed owing to the exhaustion of the ore.

“When I moved to Johannesburg in 1978 earthquakes occurred monthly and Gauteng residents were fairly accustomed to them. Now earthquakes that cause strong shaking occur much less often, maybe annually, and are surprising to many residents and somewhat paradoxically cause more alarm,” Durrheim said.

He said mining-related earthquakes are likely to persist for decades, even if mining ceases, as the rock mass settles and regains equilibrium. 

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