Mining Earthquakes

“When the artificial tremors and earthquakes stop, the natural ones have a chance to take their place.”

The discovery of gold in the Witwatersrand area of Johannesburg in the late nineteenth century has subsequently led to extensive mining activity that is oftentimes characterized by frequent small quakes and tremors caused by the detonation of explosives within the mines. In July of 2021, seismic activity caused by these detonated explosives around the Benoni and Boksburg area in Johannesburg was recorded at 3.5 on the local magnitude scale—an alternate seismic measurement scale to

the Richter’s scale

based on the relative proximity of a region to the earth’s crust—which qualified it as an earthquake and not a tremor.
        Although most earthquakes are the result of tectonic plate movement,which is a natural phenomenon, it has often become difficult to differentiate between tectonic-induced and mining-induced seismic activity in the region. The voids created from mining activity occasionally result in minor tremors and shakes throughout the Witwatersrand area.
        In the areas where mining activities have stopped, there is a noticeable increase in the subterranean water level as it finds the voids and gaps, similar to the filling of the dam. The higher water level creates additional pressure, exerting excess stress onto the rocks causing them to move underneath, a phenomenon that has been recorded as a precursor to increased seismicity.

Image source: Mining related seismicity of magnitude 4.0 and above in the gold mining regions of South Africa.