Powerful earthquakes thousands of miles away can trigger swarms of minor quakes near wastewater-injection wells like those used in oil and gas recovery, scientists reported on Thursday, sometimes followed months later by quakes big enough to destroy buildings.
The discovery, published in the journal Science by one of the world's leading seismology labs, threatens to make hydraulic fracturing, or "fracking," which involves injecting fluid deep underground, even more controversial.
It comes as the U.S. Environmental Protection Agency conducts a study of the effects of fracking, particularly the disposal of wastewater, which could form the basis of new regulations on oil and gas drilling.
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 an "induced" quake.
A recent surge in U.S. oil and gas production - much of it using vast amounts of water to crack open rocks and release natural gas, as in fracking, or to bring up oil and gas from standard wells - has been linked to an increase in small to moderate induced earthquakes in Oklahoma, Arkansas, Ohio, Texas and Colorado.
Now seismologists at Columbia University say they have identified three quakes - in Oklahoma, Colorado and Texas - that were triggered at injection-well sites by major earthquakes a long distance away.
"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 proponents reacted cautiously to the study.
"More fact-based research ... aimed at further reducing the very rare occurrence of seismicity associated with underground injection wells is welcomed, and will certainly help enable more responsible natural gas development," said Kathryn Klaber, chief executive of the Marcellus Shale Coalition.
Quakes with a magnitude of 2 or lower, which can hardly be felt, are routinely produced in fracking, said geologist William Ellsworth of the U.S. Geological Survey, an expert on human-induced earthquakes who was not involved in the study.
The largest fracking-induced earthquake "was magnitude 3.6, which is too small to pose a serious risk," he wrote in Science.
But van der Elst and colleagues found evidence that injection wells can set the stage for more dangerous quakes. Because pressure from wastewater wells stresses nearby faults, if seismic waves speeding across Earth's surface hit the fault it can rupture and, months later, produce an earthquake stronger than magnitude 5.
What seems to happen is that wastewater injection leaves local faults "critically loaded," or on the verge of rupture. Even weak seismic waves from faraway quakes are therefore enough to set off a swarm of small quakes in a process called "dynamic triggering."
"I have observed remote triggering in Oklahoma," said seismologist Austin Holland of the Oklahoma Geological Survey, who was not involved in the study. "This has occurred in areas where no injections are going on, but it is more likely to occur in injection areas."
Once these triggered quakes stop, the danger is not necessarily over. The swarm of quakes, said Heather Savage of Lamont-Doherty and a co-author of the study, "could indicate that faults are becoming critically stressed and might soon host a larger earthquake."
For instance, seismic waves from an 8.8 quake in Maule, Chile, in February 2010 rippled across the planet and triggered a 4.1 quake in Prague, Oklahoma - site of the Wilzetta oil field - some 16 hours later.
That was followed by months of smaller tremors in Oklahoma, and then the largest quake yet associated with wastewater injection, a 5.7 temblor in Prague on November 6, 2011.
That quake destroyed 14 homes, buckled a highway and injured two people.
The Prague quake is "not only one of the largest earthquakes to be associated with wastewater disposal, but also one of the largest linked to a remote triggering event," said van der Elst.
The Chile quake also caused a swarm of small temblors in Trinidad, Colorado, near wells where wastewater used to extract methane from coal beds had been injected.
On August 22, 2011, a magnitude 5.3 quake hit Trinidad, damaging dozens of buildings.
The 9.1 earthquake in Japan in March 2011, which caused a devastating tsunami, triggered a swarm of small quakes in Snyder, Texas - site of the Cogdell oil field. That autumn, Snyder experienced a 4.5 quake.
The presence of injection wells does not mean an area is doomed to have a swarm of earthquakes as a result of seismic activity half a world away, and a swarm of induced quakes does not necessarily portend a big one.
Guy, Arkansas; Jones, Oklahoma; and Youngstown, Ohio, have all experienced moderate induced quakes due to fluid injection from oil or gas drilling. But none has had a quake triggered by a distant temblor.
Long-distance triggering is most likely where wastewater wells have been operating for decades and where there is little history of earthquake activity, the researchers write.
"The important thing now is to establish how common this is," said Oklahoma's Holland, referring to remotely triggered quakes. "We don't have a good answer to that question yet."
Before the advent of injection wells, triggered earthquakes were a purely natural phenomenon. A 7.3 quake in California's Mojave Desert in 1992 set off a series of tiny quakes north of Yellowstone National Park in Wyoming, for instance.
Now, according to the Science paper, triggered quakes can occur where human activity has weakened faults.
Current federal and state regulations for wastewater disposal wells focus on protecting drinking water sources from contamination, not on earthquake hazards.