A swarm of small earthquakes within the Karoo Basin in South Africa has revealed a critically stressed fault that could be perturbed by potential shale gas exploration in the area, according to a new report in Seismological Research Letters.
The analysis by Benjamin Whitehead of the University of Cape Town and colleagues concludes that the Karoo microseismicity occurred along a buried fault that may extend through sedimentary layers to the crystalline bedrock, which would increase its vulnerability to stresses produced by shale gas exploration.
Previous research suggests damaging earthquakes can occur even in relatively seismically stable, intraplate regions such as South Africa if regional faults are stressed by hydraulic fracturing, wastewater disposal and other similar techniques used in unconventional oil and gas exploration.
The earthquake swarm near Leeu Gamka is in a part of the southern Karoo Basin undergoing strategic environmental assessment before potential shale gas development. To learn more about this swarm, which began suddenly in 2007, the researchers used data collected from an array of geophones, along with previously collected geophysical data.
"Our local array deployment enabled us to characterize and interpret this seismicity in a more rigorous way, to identify the location and scale of the structure on which the earthquakes occurred, and to determine that this structure was likely to be hydraulically connected to the depths which could be effected by unconventional shale gas development," said Whitehead.
Although regional networks initially discovered this earthquake cluster (the largest at magnitude 4.8), the new analysis confirmed a depth, direction and extent for the swarm of low magnitude earthquakes. Whitehead and colleagues also showed that the earthquake swarm is associated with a region in the crust where seismic waves abruptly change their velocity, which helped to further delineate the fault structure.
The researchers were able to image Rayleigh wave group velocities using ambient seismic "noise" collected by the array to image seismic structures, even though the array was not configured for this purpose. "I think this is a nice example of successfully reusing data for something it was originally not intended for," Whitehead explained.
Together, the results "suggest that a critically stressed fault extends from the proposed target for shale gas exploration down to the basement, and so the necessary preconditions for significant induced seismicity may be met," the authors write.
University of Cape Town researchers continue to deploy geophone arrays to better understand the seismic hazard in South Africa, Whitehead said.
"Interesting questions include whether some of the currently observed clusters of low-magnitude seismicity may be long-lived aftershock sequences for larger earthquakes, associated with the repeated activity on long-lived weak fault zones, or if very ancient stresses are being released and seismicity is likely to migrate to other regions once they are released locally," he noted.
There are large fault scarps in southern Africa, indicating that large (magnitude 7 or larger) earthquakes may be rare but do occur, he added. "Knowing where and how often these larger events occur can help engineers, regulators and policymakers in the region make more informed decisions."
The research team has made its data freely available, Whitehead said. "Our group has been involved in this research area for some time and continues to be open to advising regulators and shale gas developers, if development is to proceed."
This study is part of an upcoming SRL Focus Section on intraplate earthquakes.
