Fracking has become more common due to a surge in U.S. oil and gas production and has been linked to an increase in small to moderate induced earthquakes.
Geologists have known for 50 years that injecting fluid underground can increase pressure on seismic faults and make them more likely to slip. The result is called an "induced" quake.
Now seismologists at Columbia University say they have identified three quakes — in Oklahoma, Colorado and Texas — that were triggered at injection-well sites by a major earthquake a long distance away.
The process of fracking uses vast amounts of water to crack open rocks and release natural gas or to bring up oil and gas from standard wells.
"The fluids (in wastewater injection wells) are driving the faults to their tipping point," said Nicholas van der Elst of Columbia's Lamont-Doherty Earth Observatory in Palisades, New York, who led the study. It was funded by the National Science Foundation and the U.S. Geological Survey.
Fracking opponents' main concern is that it will release toxic chemicals into water supplies, said John Armstrong, a spokesman for New Yorkers Against Fracking, an advocacy group.
But "when you tell people the process is linked to earthquakes, the reaction is, 'What? They're doing something that can cause earthquakes?' This really should be a stark warning," he said.
Fracking or hydraulic fracturing, which has been shown to cause earthquakes, is explained.