In 2009, the South African National Seismograph Network (SANSN), operated by the Council for Geoscience (CGS), observed a dramatic increase in seismicity coinciding with the closure of the pumping stations at the East Rand Proprietary mines, in Gauteng.

CGS mining seismologist Denver Birch notes that the water flooding the mine void creates “a significant and costly environmental impact”, as large voids are interconnected and are located close to major urban centres in Johannesburg. He says that, based on a theoretical analysis, the flooding-induced seismic events that could occur are expected to fall within the range of magnitudes previously observed during mining.

The largest earthquakes associated with mining were a magnitude 5.3 event at the Stilfontein gold mine, in the North West, in 2005, and more recently, in 2014, a magnitude 5.5 event located near the Orkney gold mine, in the North West, caused serious structural damage.

“If such an earthquake were to occur in the densely populated urban area of Johannesburg, it could present major safety risks to properties on the surface in the vicinity of the old mines,” Birch warns.

He states that the occurrence of a flooding-induced event of a similar magnitude cannot be excluded and therefore should be regarded as one of the possible scenarios of the strong ground motion prediction. The nature of seismic events triggered by mine flooding, as well as the damage potential of surface ground motions associated with such events, needs to be monitored and investigated.

It is for these reasons that the SANSN is running a project that addresses the risks posed by fluid-induced seismicity in the greater Johannesburg area by focusing on accurate quantification of the most important factors that contribute to this hazard.

Birch elaborates that the related tasks were spread over a period of three years to improve the monitoring capacity around the Central Rand basin (CRB) of Johannesburg. The seismological unit of the CGS installed a dense seismic network, which has been in operation since the first quarter of 2010. “This high-accuracy network enables meaningful interpretation of the seismicity pattern.”

Tasks undertaken in this project include, but are not limited to, an evaluation of the ground motion and response spectra, including path and site effects, estimation of seismic source parameters and studies of the spatial and temporal evolution of the seismicity.

Birch reveals that the level of seismicity in the CRB does not show signs of decreasing with time, since the number of strong events with moment magnitudes above 1.4 are still as high as when monitoring began over five years ago.

He highlights that most events are located within historical mine boundaries. The seismicity pattern shows a strong relationship between the presence of the mining void and high levels of seismicity. Moreover, Birch points out that no seismicity migration patterns were observed outside the historical mining areas.

He comments that three-dimensional (3D) modelling showed that residual mining-induced stresses are being exploited by the water and also confirmed that the mined-out reef is controlling seismicity.

Birch says that strong lineaments in the seismicity pattern were observed that supported the results of the modelling. Over the past five years, the total cumulative seismic moment released in the CRB was 9 Nm × 1 014 Nm.

Fluid-Induced Seismicity

“This is equivalent to a single earthquake of magnitude 3.9, which is significantly less than the largest earthquake experienced during active mining. The temporal evolution of the inter-event time confirms that the fluid-induced seismicity follows a clustering pattern and is not random.

“Static stress drop heavily influences ground motion characteristics which, in urban areas, affects the risk assessment. “The observed static stress drop varied from 0.05 MPa to 10 MPa. It was found that large static stress drops could be associated with both small and large events,” he concludes.