An Intraplate Seismic Swarm in the Karoo Basin, South Africa, Highlights the Presence of Critically Stressed Faults in an Area of Proposed Shale Gas Exploration

Abstract Stable continental region seismicity is now recognized as a significant source of hazard and the combination of shale gas exploration and wastewater disposal increases the likelihood of failure along critically stressed structures. In this study, we describe a natural swarm of earthquakes that occurred within the Karoo basin, South Africa, in the vicinity of the crustal-scale structure associated with a major long wavelength magnetic anomaly. The cluster began suddenly in 2007, in an area of little prior seismicity, and is characterized by earthquakes with predominantly low magnitudes, however the largest event was ML 4.8. A short-term geophone deployment characterized the depth extent and trend of microseismicity, which runs west-northwest–east-southeast and spans the lower parts of the sedimentary cover down to, and perhaps into, the upper crystalline basement. Focal mechanisms suggest that the earthquakes involve left-lateral strike-slip motion, similar to the Mw 6.3 Ceres–Tulbagh earthquake that occurred in 1969. Ambient noise tomography reveals a prominent velocity transition concordant to the trend of the microseismicity within the sedimentary units. These 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. This agrees with worldwide observations that seismicity in stable continental regions often aligns with inherited basement structures. Thus, by carefully considering the depth extent of seismicity and its relationship with these structures it is possible to identify hazardous structures in advance of possible shale gas development.