Although the number of mining-related earthquakes had diminished, large events continue to occur from time to time.
Seismology expert prof Ray Durrheim told Sowetan on Monday that the amount of ground-shaking energy released by mining-related earthquakes was related to the extent and depth of mining.
On Sunday morning, a large part of Gauteng was left trembling after an earthquake that measured 4.4 in magnitude. The council for geoscience (CGS) confirmed that the earthquake’s epicentre was in Boksburg, Ekurhuleni, a few kilometres outside East Rand Proprietary Mine.
“Mining-related earthquakes occur daily, but most are small and events that are felt by people on the surface are now rare in Johannesburg, Soweto and Boksburg because no deep mining is taking place in the area,” he said.
He said during the 1970s and '80s, mine tremors were felt several times a week. However, since 2000, there have been about four events with a magnitude greater than four [4] in the East Rand area.
“We dig shafts, tunnels, and stopes to mine rock containing useful and valuable minerals and metals [e.g. gold, platinum, coal, diamonds and copper]. Mining of the ore creates voids. The weight of the overlying rocks is carried by pillars of unmined ore and the abutments of the excavations.
“Sometimes the weight exceeds the strength of the rock, and the pillar or abutment [the edge of the excavation] suddenly fails. The elastic energy stored in the rock is suddenly released, causing the rock to shake violently.
“Sometimes the roof and walls of the excavation collapse. This might injure or kill mine workers,” he said.
Durrheim said the shaking could also cause damage to structures on the surface of the Earth.
He said mining-induced earthquakes were experienced in other districts where there are deep mines, such as Carletonville, Klerksdorp and Welkom.
“Many of the deep gold mines are now exhausted and closed, so the number of mining-related earthquakes is now a lot smaller than when mining was at its peak in the 1970s and 80s.
“For example, the East Rand Proprietary Mine (ERPM) near Boksburg started in 1893 and closed in 2008. It reached a depth of almost 3,600m and yielded about 43m ounces of gold.”
Durrheim also said the largest mining-related event ever to occur struck the town of Orkney in the North West in August 2014. It had a magnitude of 5.5 and was felt 500km from its epicentre.
In that earthquake, one person died and at least 21 were injured. Thousands of miners were also trapped underground and homes damaged.
“We use a logarithmic scale to measure the ‘size’ of earthquakes, meaning that the amount of seismic energy it released was more than 30 times greater than that released by the magnitude 4.4 earthquake that occurred on Sunday 11 June near Boksburg,” Durrheim said.
“We have developed many techniques to make mining safer. These include techniques to support and reinforce the rock mass, making it less likely to fail violently and methods and machinery to mine remotely, removing miners from the danger zone.”
Durrheim said after the Sunday earthquake, residents can “certainly expect to feel some aftershocks during the next few weeks”.
“Usually, the biggest aftershock is about 1 unit of magnitude smaller than the main shock, so it is unlikely they will exceed magnitude 3.4 or cause any damage to buildings. It is unlikely, but not impossible, that we will experience an event of similar or greater size in the near future. It will likely take decades or even centuries for the Earth to reach equilibrium again and [for] earthquakes related to past mining to cease altogether.
“However, there is still a lot of gold underground, though mostly at a relatively low grade. If the price increases or technology improves significantly, we could see the industry revive.”
Residents likely to feel aftershocks of earthquake
Less quakes after closure of deep gold mines
Although the number of mining-related earthquakes had diminished, large events continue to occur from time to time.
Seismology expert prof Ray Durrheim told Sowetan on Monday that the amount of ground-shaking energy released by mining-related earthquakes was related to the extent and depth of mining.
On Sunday morning, a large part of Gauteng was left trembling after an earthquake that measured 4.4 in magnitude. The council for geoscience (CGS) confirmed that the earthquake’s epicentre was in Boksburg, Ekurhuleni, a few kilometres outside East Rand Proprietary Mine.
“Mining-related earthquakes occur daily, but most are small and events that are felt by people on the surface are now rare in Johannesburg, Soweto and Boksburg because no deep mining is taking place in the area,” he said.
He said during the 1970s and '80s, mine tremors were felt several times a week. However, since 2000, there have been about four events with a magnitude greater than four [4] in the East Rand area.
“We dig shafts, tunnels, and stopes to mine rock containing useful and valuable minerals and metals [e.g. gold, platinum, coal, diamonds and copper]. Mining of the ore creates voids. The weight of the overlying rocks is carried by pillars of unmined ore and the abutments of the excavations.
“Sometimes the weight exceeds the strength of the rock, and the pillar or abutment [the edge of the excavation] suddenly fails. The elastic energy stored in the rock is suddenly released, causing the rock to shake violently.
“Sometimes the roof and walls of the excavation collapse. This might injure or kill mine workers,” he said.
Durrheim said the shaking could also cause damage to structures on the surface of the Earth.
He said mining-induced earthquakes were experienced in other districts where there are deep mines, such as Carletonville, Klerksdorp and Welkom.
“Many of the deep gold mines are now exhausted and closed, so the number of mining-related earthquakes is now a lot smaller than when mining was at its peak in the 1970s and 80s.
“For example, the East Rand Proprietary Mine (ERPM) near Boksburg started in 1893 and closed in 2008. It reached a depth of almost 3,600m and yielded about 43m ounces of gold.”
Durrheim also said the largest mining-related event ever to occur struck the town of Orkney in the North West in August 2014. It had a magnitude of 5.5 and was felt 500km from its epicentre.
In that earthquake, one person died and at least 21 were injured. Thousands of miners were also trapped underground and homes damaged.
“We use a logarithmic scale to measure the ‘size’ of earthquakes, meaning that the amount of seismic energy it released was more than 30 times greater than that released by the magnitude 4.4 earthquake that occurred on Sunday 11 June near Boksburg,” Durrheim said.
“We have developed many techniques to make mining safer. These include techniques to support and reinforce the rock mass, making it less likely to fail violently and methods and machinery to mine remotely, removing miners from the danger zone.”
Durrheim said after the Sunday earthquake, residents can “certainly expect to feel some aftershocks during the next few weeks”.
“Usually, the biggest aftershock is about 1 unit of magnitude smaller than the main shock, so it is unlikely they will exceed magnitude 3.4 or cause any damage to buildings. It is unlikely, but not impossible, that we will experience an event of similar or greater size in the near future. It will likely take decades or even centuries for the Earth to reach equilibrium again and [for] earthquakes related to past mining to cease altogether.
“However, there is still a lot of gold underground, though mostly at a relatively low grade. If the price increases or technology improves significantly, we could see the industry revive.”