Scientists say wastewater injection from hydraulic fracturing was linked to a magnitude-5.7 earthquake that struck the US state of Oklahoma.  http://www.bbc.co.uk/news/science-environment-21952428  The practice risks causing seismic events and contaminating groundwater. The study in Geology shows that “induced seismicity” can occur years after wastewater injection begins. The new study adds to an increasing body of evidence that the injection of wastewater from fracking – rather than the wellhead injection that extracts gas or oil – is correlated to an increase in seismic events. “necessitate reconsideration of the maximum possible size of injection-induced earthquakes, and of the time scale considered diagnostic of induced seismicity”. “there’s something important about getting unexpectedly large earthquakes out of small systems that we have discovered here, the risk of humans inducing large earthquakes from even small injection activities is probably higher”