A study published Tuesday in the scientific journal Geology links the magnitude 5.6 earthquake that hit Oklahoma in November 2011 to natural gas drilling operations nearby that were using an extraction technique called fracking, which requires massive quantities of wastewater be injected into underground reservoirs.

The quake struck between Tulsa and Oklahoma City just before 11 p.m. on Sunday, November 6, 2011, causing serious damage to homes and even buckling a highway. Tremors were reportedly felt in 17 states -- as far away as Illinois -- and dozens of subsequent aftershocks kept Oklahomans up all night.

Researchers at the University of Oklahoma and Columbia University noticed that the quake was one of five greater than magnitude 5.0 that occurred in the continental interior of the U.S. in 2011. Studying the aftershocks of the Oklahoma quake, they discovered that the rupture plane of the quake, or the site where the tension built up and then violently shifted, was within 200 meters of fracking injection sites and nearly one meter from the surface.

Interestingly, the study's findings show that while most seismic activity linked to newly installed fracking wells is minor, tension between fault planes can build for years, resulting in unexpected interactions with wastewater sites.

In this case, researchers said 18 years worth of fluid injection in sealed oil wells "lowered effective stress on reservoir-bounding faults," causing several fault planes to give way in sequence after the initial quake hit, which is why the quake was so substantial.

Study co-author Geoffrey Abers told the BBC that his findings mean "the risk of humans inducing large earthquakes from even small injection activities is probably higher" than previously thought.

Opponents of fracking have pointed to its potential to trigger earthquakes for years, but studies on the matter remain elusive despite mountains of anecdotal evidence. Even Tuesday's study in Geology notes that many uncertainties remain, and the Oklahoma Geological Survey says it's still possible the event was a natural phenomenon.

It's not just fracking that poses a seismic threat, however: carbon capture methods used to produce so-called "clean coal" also require injecting massive amounts of dirty water into underground reservoirs. A 2012 report by the National Research Council found that "may have the potential for inducing larger seismic events."


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