More tremors rocked the Stilfontein area in North West on Wednesday night as rescue workers battled to reach 42 miners trapped underground after an earthquake.

"It's about the eighth tremor since the big one," said operational manager of UASA trade union Johnny Hanekom by 7pm.

DRDGold mine company spokesperson James Duncan said no contact had been made with the trapped workers since the first of the tremors N which experts said measured 5.0 on the Richter scale.

Rescue teams were working to unblock tunnels that had caved in, trapping the miners 2.4 kilometres below the ground at Hartbeesfontein gold mine.

Duncan said the process was a manual one, and could take long.

Meanwhile, 23 miners have been confirmed injured.

"Their conditions are still being assessed," Duncan said.

Duncan said 3&nbsp200 miners were underground when the first four tremors shook the area.

By 7pm, 3 158 were safe, of which 2 380 had clocked out and reached the surface.

Duncan said the others were ascending to the surface in lifts with varying carrying capacities.

DRDGold operates eight shafts in the area.

The 42 trapped miners were all stuck at shaft five.

Mine workers reaching the surface were immediately being taken home, Duncan said.

The earthquake was "a secondary effect" of mining activity, an expert said.

"This is probably a re-activation of an existing fault line. No one is to blame," said Ian Saunders, project leader of the SA National Seismograph Network at the Council for Geoscience in Pretoria.

Police said about 40 people in Stilfontein town suffered slight injuries, and a number of buildings had to be evacuated.

Klerksdorp municipality spokesperson Wendy Sokupha said a disaster management team had been set up to co-ordinate relief activities.

She said the team would decide whether to provide temporary accommodation once structural engineers had completed inspecting damaged buildings.

About 60 families had been left homeless, emergency services spokesperson Mesh Letanta said on Wednesday night.

Sokupha said four blocks of flats had been found uninhabitable.

"The schools will all be open tomorrow. They have been inspected and they are fine."

Most of the shops and banks would also be open on Thursday.

Sokupha said many residents volunteered to house those whose homes had been damaged by the quake, and only six families remained to be housed by 7pm.

"Two blocks of flats had to be evacuated because some walls went down. We have also had reports of broken geysers, broken windows and huge cracks in houses," Sokupha said.

"At the... shopping centre, the ceilings are down and there are broken windows. Children at the high school and primary school were let off early today because the schools were damaged."

The North West health department said the injured were being treated at various hospitals in the area.

Duncan said the mines had seismic monitoring systems which monitor earth movement on an ongoing basis.

This system registered four "fairly large" events between 12.15pm and 12.22pm.

"We are still pinpointing the exact magnitudes. There were a number of smallish events afterwards," said Duncan.

Saunders said the earthquake's epicentre was believed to be around Klerksdorp.

He said such a measure was "quite serious" for South Africa, which is an aseismic country (not prone to earthquakes).

The largest recent earthquake to have hit South Africa - in the Tulbagh area in the Western Cape in 1969 - had measured 6.1 on the Richter Scale.

Saunders said any seismic events above three on the scale were considered earthquakes. Below that, they were regarded as micro-seismic activity.

The tremor was felt as far as Johannesburg. - Sapa