The tremor was felt by office workers in Johannesburg but there were no immediate reports of damage or casualties.
The earthquake was centred six kilometres (3.7 miles) east of Orkney, which is 180 kilometres (112 miles) south of Johannesburg, at 10:22 GMT.
More than half an hour after it struck, no reports of casualties had been received by one of the country's largest emergency services ER24.
The tremor was also felt in neighbouring countries of Botswana and Mozambique.
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