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Study raises new concern about earthquakes and fracking fluids


By Sharon Begley

NEW YORK (Reuters) - 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.


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.




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.




The largest fracking-induced earthquake "was magnitude 3.6, which is too small to pose a serious risk," he wrote in Science.


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.




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."



Current federal and state regulations for wastewater disposal wells focus on protecting drinking water sources from contamination, not on earthquake hazards.

(Reporting by Sharon Begley; Additional reporting by Edward McAllister; Editing by Michele Gershberg and Xavier Briand)