All your earth tremor questions answered

So what actually is an earth tremor and why do we feel them so far away from the original earthquake?

Randburg Sun chatted to the University of the Witwaterand Professor Ray Durrheim, who is the South African Research Chair in Exploration, Earthquake and Mining Seismology.

What is a tremor: An ‘earth tremor’ is simply a shaking of the earth that is strong enough to be felt by people.

Causes of tremors: Explosions, the movement of trains and heavy trucks, the vibrations caused by machinery, volcanic eruptions, and tectonic forces within the earth. Earth tremors occur when the force exerted on the rock mass exceeds its strength.

How do we in Joburg feel the effects of an earthquake in Botswana: Simply because the earthquake source was powerful enough, and not so very far away. Two hundred earthquakes with magnitudes greater than 6 occur every year. They are really rare in southern Africa because we are a great distance from the boundaries of the tectonic plates.
So when they happen, [Joburgers] go into a panic or are surprised.

Here is a list of important historic tectonic events and events happening in South African mining towns:


What caused the tremor on 3 April: With the current information [studies are ongoing] it is likely that the event that occurred near Stilfontein is related to the extensive mining that has taken place in the region over the past century.

Can we expect more tremors to occur soon: All earthquakes are followed by aftershocks which will continue for weeks and months after the main shock as the rocks settle into a new equilibrium. The largest aftershock is usually at least one unit of magnitude smaller than the main shock, so the shaking should be considerably less.
Basically, there are still mini aftershocks happening but are simply too small for human beings to feel.

At what measurement (magnitude) should we get worried: Slight damage can occur to buildings close to the epicentre when magnitudes exceed 5. Serious damage can occur if the magnitude exceeds 6.

For the detailed explanation of tremors and what Prof Durrheim had to say, catch your latest copy of Randburg Sun next week Thursday (13 April).

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Ashtyn Mackenzie

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