Fracking and disposal wells create quakes that can be felt at the surface when shock waves or fluids release strain on a preëxisting fault. For example, high-pressure fluid can squeeze into and push apart a planar fault, freeing adjacent rock formations to slide past one another.

Such induced fault slips probably occurred at Youngstown, says Thomas Stewart, executive vice president of the Ohio Oil and Gas Association. But Stewart says induced quakes are rare events because well operators deliberately avoid drilling near known faults. Ohio's other 180 oil and gas wastewater wells have prompted few complaints, he notes. He adds that the Youngstown shakes hurt no one other than local gas producer D&L Energy, whose well was shut down by state regulators. "This guy's probably going to lose a $3 million to $4 million investment," says Stewart.

Cuadrilla Resources' geomechanical consultants also downplay the risk that its operations could induce damaging quakes greater than magnitude 3.0. Nevertheless, their report, authored by senior researchers at German geophysical consultancy Q-con and Dutch consultancy StrataGen Delft, recommends that Cuadrilla initiate fracking operations with less fluid than it employed at Blackpool. In addition, they call for underground seismometers to identify any problems early. Cuadrilla says it plans to implement the proposals.

McGarr at USGS says an early warning system is a good idea, and one in keeping with the seismic risk assessment protocol for well-blasting operations employed by geothermal-energy producers. He is less sanguine, however, about estimates of the maximum severity that earthquakes triggered by fracking and injection wells can reach, saying this question needs more science. That means the risk of anthropogenically inducing large, deadly quakes cannot be ruled out.