Johannesburg - The most recent earthquake in Orkney caused less damage than one almost a decade ago, also in North West.

This was despite the fact that it was a larger quake on the Richter scale.

Professor Herbert Uzoegbo, of the Wits School of Civil Engineering, said this was due to the quake being a tectonic quake, which means it was triggered by mining, as opposed to a solely mining-related quake.

He said tectonic quakes had different frequencies to mining-related quakes, causing different types of damage.

The quake in 2005 measured 5.3 on the Richter scale, whereas the recent quake in Orkney measured 5.5.

There was also a second small quake measuring 4.6 near Carletonville last week.

Although South Africa is not on a major plate divide, there are cracks in the plates that can still cause quakes, and mining aggravates this.

Professor Ray Durrheim, of the Wits School of Geosciences, said that if one compared parts of Klerksdorp in which mining had occurred with parts where it had not, there was between 500 and 1 000 times more seismic activity in the mining areas.

He said although the rate of mining had slowed, a wave of seismic activity could be triggered as the water table rose after mines were closed.

Higher levels of seismic activity could last a few decades, before settling down.

Durrheim said recommendations had been made and enforced since the 2005 quake. This included better monitoring of seismic activity, particularly around mining areas. He said they had also worked with their Japanese colleagues to have a closer look at the physics of quakes and improve safety.

Uzoegbo said we should start taking seismic activity into account when building new structures.

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The Star