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.
“We here at the geological survey feel the interpretation that best fits the current data is that the Prague earthquake was the result of natural causes,” OGS seismologist Austin Holland said. “All the data is consistent with a natural earthquake. The data does not require it to have any cause other than the natural accumulation of stress and that release.”
The geological survey pointed out that Oklahoma has experienced more than 10 magnitude 4.0 or greater earthquakes since the previous record 5.0 El Reno earthquake in 1952.
“This is statistically consistent with the Gutenberg-Richter relationship, which describes the distribution of earthquakes of differing magnitude over time,” the survey wrote in a statement.
The survey also found that water injections began in the area in 1955 and that activity increased until about 2005 and has remained relatively consistent since that time.
“Some researchers have observed that the earthquake activity did not increase over time as injection increased, but rather occurred in a distinct ‘swarm' more typical of a natural event,” the survey said.
The survey also called for further study into the 2011 earthquakes and other seismic activity.
“We have a lot to learn in this arena,” Holland said. “No scientist can say this is how we tell a triggered earthquake from a natural earthquake. There isn't a rigorous scientific finding at this point. Clearly the way we're going to get there is through a lot more data and a lot more research on the subject.”
Keranen said she is not surprised by the differing opinions.
“It's great that we're having this debate and discussion overall,” she said. “That is generally how science progresses: one person states a hypothesis based on good data, and others agree or disagree. I welcome that discussion.”
The journal Geology is published by the Geological Society of America, which has called for public policy changes aimed at reducing greenhouse gasses that many scientists believe contribute to climate change. The Oklahoma Geological Survey, on its website, said its most important objective is “to maximize the ultimate recovery of the energy resources that Nature so generously has bestowed upon us.”
Three outside scientists contacted by The Associated Press said the researchers made a strong case for a likely man-made cause.
“I think they made the case that it is possible; it's probably even more than possible,” said Steve Horton, director of the Center for Earthquake Research and Information at the University of Memphis. “They have a very reasonable conclusion.”
CONTRIBUTING: Business Writer Paul Monies and The Associated Press