The magnitude 5.7 earthquake that struck in central Oklahoma in 2011 likely was caused by oil and natural gas drilling activity, according to a report co-authored by University of Oklahoma seismologist Katie Keranen.
The report published in Tuesday's edition of the journal Geology contradicts the findings of the Oklahoma Geological Survey — also based at the University of Oklahoma — which has said the earthquake likely was a “naturally occurring event.”
“There appears to be a strong correlation between the wells that are injecting wastewater in Lincoln County and the earthquakes in 2011,” Keranen told The Oklahoman on Tuesday.
In November 2011, central Oklahoma was rocked by a 5.0 earthquake, followed several hours later by a 5.7, the largest ever recorded in Oklahoma. Keranen's team found that oil and natural gas activity likely caused the first quake, which triggered the larger temblor.
The study focuses on injection wells, which are also called disposal wells and are used to pump water and other fluids recovered from oil and natural gas wells deep below the surface.
Small to moderate earthquakes have been connected with injection wells in Ohio and other parts of the country, but in most cases, those earthquakes occurred within weeks or months after the injection wells became operational.
The injection wells in Lincoln County, however, were used for at least 17 years before the 2011 tremors.
The findings challenge a previous theory that the man-made earthquakes occurred after months of injections.
“After decades instead of months of pumping, we think there's strong geological evidence to support that this is a different type of induced earthquake,” Keranen said. “The pressure built up over time rather than rapidly.”
Thousands of injection wells dot the Oklahoma landscape, including many that date back before statehood.
While most Oklahoma wells are unlikely to cause earthquakes, Keranen said important lessons can be learned from the Lincoln County wells.
“There are a few wells that because of the geology of where they are injecting, they are high risk,” she said. “We know this happens not just in Oklahoma but in other areas as well.”
While most rock around injection wells in Oklahoma tends to release pressure quickly, the Lincoln County wells do not, she said.
“The permeability was low here, so it was not able to diffuse away as is with most wells,” she said. “It also is very near the fault. I think in general, everybody agrees that injecting near a fault strongly increases the risk of earthquakes. That's a difference here.”
Despite the findings of Keranen's team, the Oklahoma Geological survey has reached a different conclusion.