Event Waveform Data for Leeu Gamka, Karoo, SA

Event waveform data for Leeu Gamka, RSA, with event origin time in the name of the file.Manuscript: Seismic swarm in the Karoo Basin, South Africa, highlights the presence of critically stressed faults in an area of proposed shale gas explorationAbstract: Stable continental region seismicity is now recognised as a significant source of hazard and the combination of shale gas exploration and wastewater disposal dramatically increases the likelihood of failure of critically stressed structures. Here, 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 the Beattie magnetic anomaly. The cluster began suddenly in 2007, in an area of little prior seismicity, and is characterised by earthquakes with predominantly low magnitudes, however the largest event was M_L4.8. A short-term geophone deployment characterized the depth extent and trend of microseismicity, which runs WNW-ESE 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. 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.