The preliminary results of the Council for Geoscience’s (CGS’s) seismic microzonation model for Johannesburg are expected to be released by the end of March, CGS microzonation project manager Michelle Grobbelaar tells Mining Weekly.
The seismic microzonation model for Johannesburg project, which started in 2014, aims to map areas that could experience amplified seismicity in the event of an earthquake, and this can feed into disaster management plans to mitigate damage to areas with the greatest sensitivity.
The plan will also enable the City of Johannesburg to mobilise emergency response teams more effectively by directing them to the most sensitive areas, while avoiding areas that could delay them.
Grobbelaar explains that areas with a signifi- cant amount of soil above the bedrock are particularly at risk of amplified ground motion/vibrations, and this phenomenon is known as the Mexico City effect, which refers to an earthquake that occurred more than 350 km away from the Mexican capital.
“Ordinarily, [an earthquake] would not have any, or very little, effect on an area that far away. However, parts of Mexico City were devastated, owing to the ground motions/vibrations of the earthquake being amplified in areas of the city that were located on an old lake bed with signifi- cant amounts of sediment.
She points out that the microzonation model will aid the City of Johannesburg in effective town planning through the avoidance of placing sensitive infrastructure in areas that might be affected. The model will also identify areas that will require existing infrastructure to be reinforced.
The CGS is calculating the expected amplitudes of ground vibrations from seismic events in Johannesburg, which will provide engineers with the data necessary to design solutions to enable buildings and infrastructure to withstand seismic events.
Grobbelaar points out that, previously, seismicity had been declining and eventually stabilised for several years in areas of Johannesburg where there are abandoned mines. However, the CGS recorded increased seismicity in these areas following the closure of the last operational mine, and thus the cessation of pumping of water, in the area at the end of 2008.
“We noticed that the abandoned mines in the Johannesburg area were starting to fill with water, which was initially an environmental threat. However, it is well documented that whenever a large body of water is trapped in an area, water seeps into the ground and essen- tially lubricates the faults, resulting in increased seismic activity,” she explains.
Grobbelaar highlights that, during the filling of the Katse dam, in Lesotho, which started in 1996, there was a significant increase in the number of seismic events in the surrounding area.
“Originally, it was theorised that liquid- induced seismic events in the abandoned mines would have the same magnitude as seismic events caused during mining activities,” she points out, adding that new theories, which state that liquid-induced seismic events are more closely linked to natural phenomena, are beginning to challenge traditional thought.
Grobbelaar explains that the magnitude of an earthquake can be estimated by measuring the length of a fault line, which forms part of the CGS study to determine the magnitude of potential earthquakes in Johannesburg.