A wave of earthquakes in Texas' Eagle Ford shale formation were likely caused by the extraction of large volumes of water and oil, new research suggests.

In Eagle Ford, oil is extracted using a process called hydraulic fracturing, or fracking. To release the oil, millions of gallons of chemically treated water and sand are pumped underground. From November 2009 through 2011 there was a series seismic events, many of which were too weak to be noticed at ground level.

A new study in the journal Earth and Planetary Science Letters reveals these small earthquakes are "associated with fluid extraction."

The findings suggest it's not so much the chosen method of extraction that is causing the problem, but rather the rate at which it is happening, The Wall Street Journal reports.

New wells are extracting nearly 600,000 barrels of oil a day, up from about 15,000 barrels a day in 2010, according to data from the Texas Railroad Commission, Bloomberg reports. A large amount of water is also taken out of the ground.

Given the scale at which oil is now being removed, enough liquids are being disturbed that rocks are settling and faults slipping, causing the small earthquakes, according to The Journal.

"We don't see any evidence that injection in the Eagle Ford appears to routinely cause earthquakes," Cliff Frohlich, co-author of the study and associate director of the Institute for Geophysics at the University of Texas, tells Bloomberg.

That conclusion contrasts the results of a 2012 study Frohlich conducted in the Barnett Shale in northern Texas. In that case, earthquakes appeared to be triggered by injecting wastewater underground.

Though Frohlich initially assumed he would experience similar results this go-around, "science is seldom that simple," he tells The Journal.

A possible explanation for the differing results is that oil and gas has been produced in the Eagle Ford region for much longer. Small earthquakes were first regularly noticed in 1970, Frohlich explains to The Journal.

Environmental groups, community members and an array of researchers have expressed concerns about the long-term risks of fracking and called for increased government oversight.

"This issue really boils down to effective risk management and continuously improving production processes," says Steve Everley, a spokesman for Energy in Depth, a group representing gas drillers.

"The industry has the tools to do just that, has been doing it, and will continue to do it well into the future," he tells Bloomberg.