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What role does deep-level mining play in seismic activity in South Africa?

CATALYST? Experts suggest that deep-level mining could play a role in triggering earthquakes

STUART CLAGUEWhere mining influences an active fault zone within such a crustal stress field, it is possible that it could trigger an earthquake through the sudden release of stress

RAY DURRHEIM Determining whether mining plays a role in seismic activity is a complex issue

MICHELLE GROBBELAAR Ingress of water into the mining voids remaining from the mining activities in the Witwatersrand area could also potentially increase the frequency of seismic activity

8th May 2015

By: Anine Kilian

Contributing Editor Online

  

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An earthquake with a magnitude of 5.5 shook Southern Africa in August 2014 and was felt as far apart as Botswana and Durban, in South Africa. One man was killed when a wall collapsed on him in Orkney, while 34 miners were injured in mines around Orkney and more than 600 houses were damaged in the North West.

Council for Scientific and Industrial Research (CSIR) Natural Resources and the Environment geophysicist Professor Ray Durrheim tells Mining Weekly that seismic events can take place in the absence of mining, but are rare in the regions that have similar strata and natural stress regimes near to, but outside, gold mining districts.

“I would use the term ‘mining related’ for the event that occurred in August last year because the rupture occurred beneath an area of extensive mining and did not extend beyond the mined-out area. Secondly, the rock surrounding the mine workings has been dewatered. This reduction in pore fluid pressure should help to stabilise a fault that was close to instability prior to mining,” he says.

Most mining-induced events have a focal mechanism, a description of the deformation in the source region that generates the seismic waves, indicating that the gravity is seeking to close the void. As a result, the movement is largely vertical. However, he notes that the motion of last year’s event was horizontal, which suggests that it is related to large-scale stresses associated with tectonic forces.

“There is overwhelming evidence that mining has induced and triggered earthquakes in countries that mine at great depths, as well as regions where natural stresses are high and the rock is weak,” Durrheim points out, noting that these countries include Australia, Canada, Chile, China, France, India, Poland, Russia, Sweden and the US.

Durrheim explains that seismologists and rock engineers worldwide meet to discuss these issues at the quadrennial Rockbursts and Seismicity in Mines Symposium, which was held in South Africa in 1982 and 2001, and most recently in Russia in 2013.

Mining major Exxaro principal of structural geology Stuart Clague states, however, that the stress field of the East African Rift system, which extends into Southern Africa, is the underlying origin of earthquakes in South Africa.

He argues that, although intense mining activity in the Witwatersrand goldfields produces numerous, detectable seismic responses, the fact that the earthquake of August 2014 was felt as far away as Botswana and Durban suggests that its origin was on a global tectonic scale, rather than stemming from mining activities only.

“Where mining influences an active fault zone within such a crustal stress field, it is possible that it could trigger an earthquake through the sudden release of stress. The resultant earthquake would, however, be smaller in magnitude than if the stress was allowed to increase until its ultimate natural release.”

Clague adds that the focus of the Orkney earthquake was determined at a 4.1 km depth, which is consistent with natural earthquakes triggered in rift-related stress environments, where focuses vary between 0 km and 70 km in depth.

He notes that there have been many other earthquakes in South Africa with epicentres that were quite some distance away from mining areas, such as the 6.3-magnitude Tulbach earthquake, in the Western Cape, in1969.

Council for Geosciences (CGS) seismologist Michelle Grobbelaar adds that ingress of water into the mining voids remaining from the mining activities in the Witwatersrand area could also potentially increase the frequency of seismic activity.

“Once mining in certain areas has stopped, there is a noticeable increase in the water within the mining voids,” she says. The phenomenon is very similar to the filling of a dam, with a noticeable increase in seismicity in and around a newly built dam once it has been impounded”, she adds.

Grobbelaar cites the Katse dam, in Lesotho, as an example. The faults become flooded with water and, therefore, lubricated, after which they start to move, she explains, adding that “the extra pressure from the huge body of water creates extra stresses once again”.

She points out that it has been observed that seismicity starts to increase whenever there is a change in water level within the mining voids. Thus, if the water level can be controlled or maintained at a particular level, the seismicity is expected to stabilise.

Research

Research institutions, led by the CSIR and Japan’s Ritsumeikan University, are in the final stages of a research project to gain a better understanding of mining-induced earthquakes.

Once the study is completed, it will assist the mining industry in being better equipped to deal with the ongoing challenge of keeping workers safe underground.

Last month, Mining Weekly reported that the research project was funded by Japanese research programme the Science and Technology Research Partnership for Sustainable Development (SATREPS), and drew on more than a century of studies on mining-related and tectonic earthquakes by South African and Japanese researchers.

Durrheim notes that the research, titled “Observational studies in South African mines to mitigate seismic risks”, is a five-year project that started in 2010.

“We are in the final stages of this collaborative Japanese–South African research project, which is the largest and most ambitious mine seismology project anywhere, any time. We decided to do this as mining-induced earthquakes pose a risk to workers in deep mines, such as South African gold mines, while large earthquakes that occur near plate boundaries and occasionally in stable continental regions, such as Japan, also pose a risk to the public,” he says.

The research project entails installing acoustic emissions sensors, accelerometers, strainmeters and controlled seismic sources at three research sites – that of mining group Sibanye Gold’s Cooke 4 shaft, gold producer Gold Fields’ Hlanganani shaft, and AngloGold Ashanti’s Moab Khotsong gold mine. These devices will monitor the deformation of the rock mass, the accumulation of damage during the earthquake preparation phase and the propagation of the rupture front.

In addition, a ten-station surface array of accele- rometers has been installed and is operated by the CGS on the West Rand, in Johannesburg.

“The data that has been collected is being integrated into measurements of stress, in-stope closure and strong motion, as well as data that was recorded by seismic networks.”

Durrheim says new insights into the physics of earthquakes have been gained, and technolo- gies have been developed or adapted to assess seismic hazards and mitigate rockburst risks.

He points out that some of the objectives of the research project that have been achieved thus far include the drilling of 80 boreholes, with a combined length of about 2.8 km, at the three research sites; rock mechanical testing of core samples; installing closure meters and strong motion sensors in stopes; and collecting, processing and analysing data.

The data collected by the surface seismic array is analysed by seismologists at the CGS using a Kinemetrics Antelope system supplied by SATREPS, Durrheim says, adding that SATREPS also supplied sophisticated automatic location software that was used to successfully delineate the aftershock activity of the 2014 earthquake in the North West.

“The surface accelerograms, together with the SATREPS underground straingrams and in situ stress information, will contribute to revealing the mechanism of an earthquake in detail, and the network will continue to be operated by the CGS after the termination of the project.”

Further, new insights into rock failure processes in deep and overstressed mines have been gained, Durrheim says. He notes that it is “still difficult to predict whether forerunners that reliably occur before mining-induced seismic events will be identified”.

He adds that progress has also been made in the development of several new technologies that will mitigate the risk posed by rockbursts, including the compact conical borehole overcoring stress-measurement technique.

“About 20 South African technicians, practitioners and scientists have been trained to use the technique for routine monitoring and research in South African mines.”

A suite of tools to map the vulnerability of the hanging wall to strong motion has been created at the CSIR, as well as an autonomous robotic platform that will eventually be able to gather information, scale the hanging wall and install support without exposing humans to harm.

Durrheim says the research project has largely gone according to plan over the past four-and-half years, despite many uncertainties and challenges, including the occurrence of earthquakes, as well as changes in mining plans because of variable ore grades and business factors such as fluctuations in the gold price and industrial action.

Challenges

Clague cites that determining whether an earthquake was triggered by natural or anthropogenic causes in a naturally seismically active area where mining takes place can be suggested only by circumstantial evidence such as the coincidence of mining and faulting at the depth of the earthquake focus.

“Seismic events could be reliably attributed to mining only where localised tremors were experienced in mining areas within stable crustal plates,” he says.

Meanwhile, Durrheim notes that determining whether mining plays a role in seismic activity is a complex issue because the physics of the seismic activity, which usually originates at some point of weakness, comes into play. Rocks, he points out, are unlike engineered materials, such as steel or concrete, because they are usually heterogeneous, discontinuous, anisotropic and inelastic.

“We need knowledge of the mechanical properties of the rock and the position and properties of the many flaws and weaknesses within the original rock mass, as well as those induced by mining. We also need information regarding the stress state and deformation of the rock mass as mining proceeds.” This, says Durrheim, requires constant monitoring using a range of sensors.

“We need to improve our understanding of the physics of earthquakes, how instabilities nucleate and are triggered,” he concludes.

Edited by Martin Zhuwakinyu
Creamer Media Senior Deputy Editor

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