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March 26, 2013

Big quake in 2011 likely man-made

WASHINGTON — 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 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 the 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.

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