WASHINGTON (AP) -- An unusual and widely felt 5.6-magnitude quake in Oklahoma in 2011 was probably caused when oil drilling waste was pushed deep underground, a team of university and federal scientists concluded.
That would make it the most powerful quake to be blamed on deep injections of wastewater, according to a study published Tuesday by the journal Geology. The waste was from traditional drilling, not from the hydraulic fracturing technique, or fracking.
Not everyone agrees, though, with the scientists' conclusion: Oklahoma's state seismologists say the quake was natural.
The Nov. 6 earthquake near Prague, Okla., injured two people, damaged 14 houses and was felt for hundreds of miles in 14 states, according to the U.S. Geological Survey. It was the largest quake in the central part of the country in decades and largest in Oklahoma records, experts said.
The study by geophysicists at the University of Oklahoma, Columbia University and the USGS says that a day earlier there was a slightly smaller quake in an old oil well used to get rid of wastewater, right along a fault line. That smaller quake triggered the bigger one, and a third smaller aftershock.
The location of the tremors right at the spot where wastewater was stored, combined with an increased well pressure, makes a strong case that the injections resulted in the larger quake, they said.
This area of Oklahoma had been the site of oil drilling going back to the 1950s, and wastewater has been pumped into disposal wells there since 1993, the study authors said. Water and other fluids used for drilling are often pumped more than a mile below ground.
The report said there was a noticeable jump in the well pressure in 2006. USGS geophysicist Elizabeth Cochrane described the pressure increase from injections as similar to blowing more air in a balloon, weakening the skin of the balloon
"We have a lot of evidence that certainly leads us to believe" the quake was caused by the injections, said Cochrane, a study co-author.
The evidence isn't as complete as other smaller earthquakes that have been linked conclusively to injections of waste, such as those in Arkansas, Colorado and Nevada, said co-author Heather Savage of Columbia.
But with the quake at the "right place" at the well, the increased pressure and the other smaller quakes across the region triggered by injections, "it becomes compelling," she said.
A National Academy of Sciences study last year documented 60 small injection induced quakes in the United States in the last 90 years, mostly in California, Texas, Colorado, Oklahoma and Ohio.
In a statement, the Oklahoma Geological Survey said the interpretation that best fits the data is the quake "was the result of natural causes" but needs further study. The state officials cited new 3-D seismic data, a time lag between injection and the quakes, and the orientation of the faults to say it was natural not induced.
Just being in the right place isn't proof enough, said Austin Holland, seismologist for the state agency. There are few places in Oklahoma where you can have an earthquake that's not near an injection well, he said.
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."