Newly discovered fault near Leeu-Gamka raises questions about shale gas development

For a region whose farms and towns could be directly affected by shale gas decisions, the discovery reinforces a simple principle: development should not move faster than the science needed to assess it.

A previously hidden geological fault beneath the Karoo Basin has been identified by researchers from the University of Cape Town recently. This discovery adds an important new consideration to South Africa’s long-running debate over shale gas exploration in the region.

The fault appears to be critically stressed and capable of producing earthquakes under the pressure already present within the Earth’s crust. Researchers say this does not mean that a major earthquake is imminent or that shale gas development must necessarily be abandoned. It does, however, mean that far more detailed seismic monitoring may be needed before exploration proceeds. The UCT study, published in Seismological Research Letters, examined an unexpected cluster of earthquakes near Leeu-Gamka in the Western Cape.

An earthquake swarm in a quiet region

The Leeu-Gamka earthquake swarm began in 2007 in an area previously considered relatively quiet in seismic terms. Researchers examined at least 66 earthquakes recorded between 2007 and 2022, ranging in magnitude from 0.7 to 4.8.

An earthquake swarm differs from a typical sequence in which one large earthquake is followed by smaller aftershocks. Instead, numerous earthquakes occur within a relatively confined area and period without one event necessarily dominating the sequence.

To understand what was happening beneath Leeu-Gamka, researchers deployed a temporary network of 23 seismic stations. They then used the information gathered by the network to locate the earthquakes and examine the underground structure responsible for them.

The analysis revealed that the earthquakes were occurring along a fault running from west-northwest to east-southeast. Activity was detected at depths of between approximately two and 12 kilometres.

This depth is important because it indicates that the fault passes through the sedimentary layers of the Karoo Basin that have been considered for shale gas exploration. The deepest earthquake activity may also extend into the older crystalline rock beneath the basin.

The earthquakes were naturally occurring

Lead author Benjamin Whitehead, who completed the research as part of his PhD at UCT, has emphasised that the earthquakes were natural. They were not caused by hydraulic fracturing or any existing shale gas operation.

No commercial shale gas extraction was taking place in the area when the earthquake swarm began. The research therefore provides evidence of the Karoo Basin’s natural seismic behaviour before possible future development.

That distinction is essential. The study has not shown that hydraulic fracturing has caused earthquakes in the Karoo. It has shown that a naturally active and critically stressed fault already exists beneath part of a potential shale gas region.

According to the researchers, understanding this baseline is necessary because certain underground industrial activities can alter pressure around existing faults. International studies have found that hydraulic fracturing and, more commonly, the underground disposal of wastewater can trigger seismic activity where faults are already close to slipping.

What this means for shale gas exploration

Hydraulic fracturing involves injecting water, sand and chemicals into underground rock formations at high pressure. This creates fractures through which trapped gas can move and be recovered.

The presence of a fault does not automatically make hydraulic fracturing impossible. Faults are found throughout the Earth’s crust and most do not produce damaging earthquakes. The concern is whether a particular fault is active, how much stress it carries and whether injected fluids could reach it.

The Leeu-Gamka fault appears to share characteristics with geological structures associated with induced seismic activity in shale gas regions elsewhere in the world. It is critically stressed, extends through the targeted sedimentary rocks and may reach the crystalline basement below.

This discovery strengthens the argument by Karoo communities that environmental assessment cannot be limited to questions about groundwater contamination and surface disruption. Seismic conditions must also be understood before drilling and fluid injection are considered.

A deeper geological structure

Researchers found that the fault coincides with the Beattie Magnetic Anomaly, a large geological feature extending across southern Africa. The anomaly has interested scientists for decades because of its unusual magnetic characteristics deep beneath the surface.

Whitehead suggests that it may represent an ancient zone of weakness in the Earth’s crust where tectonic stress can become concentrated. If so, the Leeu-Gamka activity may not be an isolated curiosity but evidence of a deeper structure that deserves investigation across a wider area.

South Africa is much less seismically active than countries situated along major tectonic plate boundaries. However, earthquakes can still occur within comparatively stable continental interiors. The damaging Ceres-Tulbagh earthquake of 1969 remains the country’s clearest modern example.

Researchers call for expanded monitoring

The research team has not called for shale gas development to be stopped outright. Instead, it recommends expanding seismic monitoring across the Karoo before exploration begins.

This would allow scientists to identify other hidden faults and establish the region’s normal level of earthquake activity. Without that baseline, it could become difficult to determine whether future seismic events were natural or connected to exploration.

Monitoring could also help authorities decide where additional precautions are required and where drilling or wastewater disposal may present unacceptable risks. For a region whose farms and towns could be directly affected by shale gas decisions, the discovery reinforces a simple principle: development should not move faster than the science needed to assess it.