Hydraulic fracturing in western Canada can prime faults for earthquakes that strike months after fracking ceases, reports a new study published this week in Science. Although it has long been known that the injection of wastewater into disposal wells can trigger earthquakes by increasing pore pressure and destabilizing fault lines, rarely has fracking itself been identified as the source of tremors. Typically, fracking involves injections into impermeable rock layers that inhibit the spread of fluid and increase pore pressure. Looking at seismic records near Fox Creek, in northwest Alberta, where there are six drilling sites, researchers found an intermittent set of induced earthquakes between December 2014 and March 2015, clustered around fracking operations. The majority of seismicity occurred during fracking as the elastic response of the rock caused mounting stress. However the largest quake, which had a magnitude of 3.9, struck on 23 January 2015—2 weeks after fracking had been completed. The researchers believe that a limited recovery of fracturing fluids—one well retrieved only 7% of its fluids—pressurized a fault that extended down to the crystalline basement, resulting in the series of quakes over several months. In the future, they say, drillers should take account of such risks, especially when they fail to recover fracking fluids.