Powerful earthquakes thousands of km away can trigger swarms of minor quakes near waste water-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 US Environmental Protection Agency conducts a study of the effects of fracking, particularly the disposal of waste water, which could form the basis of 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 called an “induced” quake.
A recent surge in US 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 a major earthquake a long distance away.
“The fluids [in waste water 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 US Geological Survey.
Fracking opponents’ main concern is that it will release toxic chemicals into water supplies, said John Armstrong, a spokesperson 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 US 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 arriving from faraway quakes are therefore enough to set off a swarm of small quakes in a process called “dynamic triggering”.
Once these 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 waste injection, a 5.7 tremor in Prague on 6 Nov 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 waste water 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 tremors in Trinidad, Colorado, near wells where waste water used to extract methane from coal beds had been injected.
On 22 Aug 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 tremor.
Long-distance triggering is most likely where waste water wells have been operating for decades and where there is little history of earthquake activity, the researchers write.
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 waste water disposal wells focus on protecting drinking water sources from contamination, not on earthquake hazards.