For decades, the Karoo has sat at the centre of South Africa’s shale gas debate. Supporters have argued that exploiting underground gas reserves could strengthen the country’s energy security and stimulate economic development. Opponents have warned about water contamination, biodiversity loss, and the risks associated with hydraulic fracturing.
Recent research from the University of Cape Town (UCT) suggests that another issue deserves closer attention.
Scientists based in the Department of Geological Sciences have identified evidence of a previously hidden fault beneath the Karoo Basin that appears to be critically stressed and capable of generating earthquakes. Their findings suggest that the geological conditions associated with induced seismicity in other shale gas regions worldwide may also exist beneath parts of the Karoo.
The study, published in Seismological Research Letters, examined an earthquake swarm near Leeu Gamka in the Western Cape that began unexpectedly in 2007 in an area previously considered relatively quiet seismically. Since then, at least 66 earthquakes have been recorded – including one with a magnitude of 4.8.
Using a temporary network of 23 seismic stations and advanced subsurface imaging techniques, the researchers found that the earthquakes occurred along a west-northwest to east-southeast fault at a depth range of 2km to 12km.
The depth range indicates that this fault extends through the Karoo Basin, the sedimentary rock layers under consideration for shale gas exploration. The deepest earthquakes may even extend into the underlying crystalline basement.
“Our study shows that critically stressed faults are already present beneath parts of the Karoo,” says UCT PhD graduate and the study’s lead author, Benjamin Whitehead. “These structures are capable of generating earthquakes under the current stress regime, which means they should be carefully identified and monitored before any future shale gas development.”
Whitehead conducted the study as part of his PhD and is now affiliated with the UiT The Arctic University of Norway. He stresses that the earthquakes observed in the Karoo were natural and not caused by hydraulic fracturing. However, he says that international experience shows that wastewater injection and shale gas operations can reactivate pre-existing faults when geological conditions are favourable.
Studies from the US, Canada and the UK have demonstrated that fluid injection can increase earthquake risk where critically stressed basement faults extend into the rock formations targeted for shale gas extraction. The Karoo fault identified in this study appears to share several of those characteristics.
The fault also coincides with the Beattie Magnetic Anomaly, a major geological structure stretching across southern Africa that has long attracted scientific interest because of its unusual properties. Whitehead believes this may represent a deep crustal zone of weakness capable of concentrating tectonic stress.
He says the new findings should inform future planning rather than halt development.
“Our study provides the kind of geological evidence needed to make informed decisions about future exploration,” Whitehead says. “Better seismic monitoring before development begins can be used to characterise baseline seismic activity and can help identify areas where additional precautions may be necessary.”
Based on the findings, Whitehead and his PhD supervisors, Dr Diego Quiros and Associate Professor Alastair Sloan, recommend expanding seismic monitoring across the Karoo before any shale gas exploration proceeds.
Although South Africa experiences relatively few earthquakes compared with tectonically active regions, Whitehead says rare but damaging intraplate earthquakes have occurred before – including the 1969 Ceres-Tulbagh earthquake.
“The findings indicate that similar geological structures may remain active beneath parts of the Karoo,” he says.