Did mining cause SA quake?

Cape Town – An inquiry should be held into Tuesday’s 5.5 quake to determine the cause and to find out if mining activity triggered the event, a South African seismotectonics expert said.

Dr Chris Hartnady, the research and technical director of Umvoto Africa, said the US Geological Survey (USGS) had published online records showing that a 4.9 quake, possibly a precursory fore-shock, occurred close to Tuesday’s epicentre on June 15.

He said there were possibly many smaller events between June 15 and August 5, but the global network of the USGS is generally not able to locate South African earthquakes with a magnitude less than 4.5.

The local South African National Seismographic Network (SANSN) operated by the Council for Geoscience is capable of accurately locating micro-earthquakes of magnitude much less than 3, but unfortunately it no longer makes its seismographic data freely available online, according to Hartnady.

What happened between quakes?

“It would be very interesting to know what happened between June 15 and August 5,” Hartnady told Fin24. “The clustering pattern of small events in space and time might give an indication of how the big event was triggered.”

“We need a special commission like the one they set up after the 5.3 earthquake on 9 March 2005 earthquake destroyed a shaft near Stilfontein, about  9km north-east of the two recent Orkney events,” he said. “The 2005 quake was also large enough to be picked up by the global network.

“In the case of some earthquake clusters elsewhere in the world, a pattern called Accelerating Seismic Release (ASR) is recognised as a potential precursory anomaly leading up to the occurrence of large earthquakes. If such an ASR pattern developed near Orkney in the few weeks leading up to the 5.3 quake, it should be evident in catalogue records of the Council for GeoScience (CGS) in Pretoria. Where detected in advance ASR may be useful for short-term earthquake forecasting purposes.

“Data from South Africa’s seismographic network is no longer released to the public in near-real time,” he said. “They are considered as commercial property of the CGS, which in my view is outrageous.”

Readers react

Readers asked what impact mining had on seismic activity. Fin24 user Majaji said the continuous “digging of the earth was causing the earth to be unstable”.

Majaji  wrote: “Why is there no responsibility for those who mine in keeping the underground stable after mining? I know that some will argue that it is not true that mining is causing the earthquakes. Why are we upsetting mother earth? All in the name of profit.”

Fin24 user Shannon Adrianne said the earthquake was caused by creeping acid ground water due to mining. “We are seeing an increase in earthquakes where they are fracking in countries where there were no previous reports of earthquakes. Let this be a lesson for our plans to frack up the Karoo!”

The quake

Hartnady said that the Orkney earthquake occurred in the upper, brittle layer of the Earth’s crust, with a focal depth of 12km according to more sophisticated estimates by the USGS. About 10cm of sudden horizontal slip occurred along a near-vertical fault surface with a length of about 10km and probably an East-West orientation.  

Micro-earthquake tremors are happening all the time in mining areas. “Active mining, such as still occurs in the Klerksdorp-Orkney-Stilfontein-Hartebeestfontein (KOSH) goldfield, triggers seismic events or rock bursts close to the working face,” he said. “They kill miners if they are not careful.”

“The Vaal Reef is offset by faults, which miners have to forecast and they mine through those faults, he said. “Those faults are weak and highly stressed. It’s possible for changes in stress patterns to trigger other events some distance from the mining face or along the fault line.”

“The tectonic strain produced by the driving forces of African plate motion, builds up very slowly over  millennia leading to a state of fracture criticality, where faults in the crust are everywhere close to the point of failure,” he said. “Therefore a small trigger event in the mine can propagate along a fault, which then suddenly gives way and slips to release the stored elastic energy. That’s what I think happened.”

In other others like the West, Central and East Rand areas, where deep mining has ceased, there may now be an increase in seismicity due to rising water levels,” he said. “This is because of the lack of pump-station maintenance and flooding in disused mines, causing potentially unstable faults to be weakened by rising fluid pressures in fractured zones.”