Johannesburg - THERE’S a rule of thumb in seismology: history repeats itself. So, sometime over the next few days, earthquake hunters predict that one, two, or possibly three quakes could rattle the country again.
“I expect there would be at least two or three magnitude 4 events coming up within the next week or two,” predicts Dr Chris Hartnady, the technical director of earth science consultants, Umvoto Africa.
In 2010, it was Hartnady who first warned, in a UN newsletter, that South Africa was “almost certain” to be struck by an earthquake. It was not a matter of if, but when.
When it struck on Tuesday, the country experienced the largest magnitude quake in the past decade, measuring 5.5 on the Richter Scale. With its epicentre in Orkney in the North West, the earthquake left one person dead and scores of homes damaged. The tremors were felt as far afield as Botswana and Mozambique.
“Very often one finds in quake areas the main shot delivers a damage blow to some buildings, but then quite often it’s the smaller aftershocks that finish them off,” Hartnady says. “That’s the danger. There are damaged buildings. If people reoccupy them too soon, before the aftershock sequence is closed, that’s a risk.
“I would recommend earthquake engineers go around and assess the structural state of affected buildings. There are not many of those, if any, in this country,” he said.
Seismologists at the Council for Geoscience told the Saturday Star on Friday that they had “lost count” of the aftershocks recorded since Tuesday.
Michelle Grobbelaar, who heads the seismology unit at the Council for Geoscience, agrees more quakes are on the way. “We can expect more. Definitely.
“The rule of thumb in seismology is that history repeats itself. We can get those magnitudes (of 5.5) coming back. We can’t say when it will happen but we can expect another one,” she said.
Grobbelaar says in mining areas like the North West, there are more than 80 percent seismicity. “In places like Carletonville, there are earthquakes daily, but they probably wouldn’t… make us raise our eyebrows.
“We’ve been monitoring these aftershocks in the Orkney/Klerksdorp area… Sometimes they’re as regular as a minute apart. Most are too small in the area too feel. We foresee they will continue in the area for the next couple of days or weeks.”
Grobbelaar is involved in a new three-year research project with disaster management and the City of Joburg to determine “hotspots” across Joburg.
“This is where one could expect an earthquake and where we could expect the ground motions to be amplified. These results could feed into building codes,” she said.
Deep mining, says Hartnady, is creating stress field disturbances in the earth, together with the accumulated effects of decades of mining, which “has the potential to trigger earthquakes on faults below the mining level and that seems to be what happened”.
Grobbelaar says that the council is working closely with disaster management “for better advice on what to do and how to react” as South Africans are “not hardened earthquake survivors”.
Saturday Star