Historical research by the Council for Geoscience (CGS) has found that the gold mining sector is not the sole cause of seismic events in and around Johannesburg but has, however, amplified the number and magnitude of seismic events, delegates were told earlier this month at this year’s CGS conference, held in Pretoria.

Following a 5.5-magnitude earthquake with its epicentre in Orkney – a gold mining town in the Klerksdorp district, in the North West – in 2014, that struck parts of Johannesburg, it was thought that historical mining activities had compromised the stability of the fault lines which the town overlays.

However, CGS seismicity researcher Nicolette Flint explained that seismic events in the former Transvaal, particularly in Johannesburg, had been recorded prior to the Witwatersrand Gold Rush, which started in 1886, suggesting that seismicity and earthquakes in Johannesburg had natural causes.

She noted that records of the first alleged mining-related seismic events dated back to 1898, albeit only one or two tremors reported each year. By 1906, the frequency of tremor and rock burst reports increased dramatically, becoming “a fact of life” for Johannesburg residents.

Following a government-decreed installation of a seismograph in Ophirton, Johannesburg, in 1910, seismic events were given greater prominence in the press, with every tremor having been reported from 1908 to 1918.

Prior to the installation of the seismograph, seismicity reports had no scientifically recordable mechanism against which they could be verified; therefore, no accurate sources existed for determining the scale and magnitude of those seismic events in Johannesburg.

Flint’s research found that, from 1840 to 1949, about 199 tectonic, or natural, seismic events were recorded in this manner across South Africa. From November 1907 to December 1950, about 700 mining-related seismic events were recorded in the Johannesburg area.

She concluded that determining the geographical distribution of tectonic and mining- related seismic events would enable future research to assess the threat of seismic events in areas of Johannesburg.