Earthquakes in region of Prague, Oklahoma, from Nov. 5, 2011, through Dec. 4, 2011. Red indicates 2.2 magnitude, up through magenta representing the 5.6 magnitude quake. Credit: Flickr/KellyMcD

Earthquakes have become more than 10 times more common in normally quiescent parts of the U.S., such as Ohio and Oklahoma, in the past few years. Given the simultaneous uptick in fracking—an oil and gas drilling technique that involves fracturing shale rock deep underground with the use of a high pressure water cocktail—it's common to suspect a link. There might be one, but the real culprit behind the largest earthquake in Oklahoma's recorded history is not what goes down but what comes up with the oil: wastewater.

Oklahoma has long benefited from a robust oil industry. One of the side effects of oil production is that a lot of water flows back to the surface with the petroleum. That flowback water must be disposed of, because it is laced with all kinds of contaminants the liquid solvent has picked up during its long residence deep underground, ranging from trace amounts of radioactive elements to lots of salt.

In Oklahoma and in much of the rest of the country, the most common burial ground for such wastewater—whether we're talking oil or gas—is a disposal well back underground. Oil producers in central Oklahoma had been using this approach for 18 years when a swarm of powerful earthquakes rumbled across the countryside starting on November 5, 2011. The biggest temblor, a magnitude 5.7 felt as far away as Milwaukee, was linked to pumping yet more wastewater down old oil wells in the vicinity. (The wastewater pumping there continues despite the quakes.)

According to a new study published online March 26 in Geology, the earthquake was indeed caused by filling up the old oil cavities with water until there was simply too much pressure on the surrounding rock. Records showed that after years of requiring little pressure to dump the wastewater, oil operators recently have had to actively pump the water down the old wells to overcome a more than 10-fold increase in underground pressure, which peaked at 3.6 megapascals, or 525 pounds-per-square-inch. That's because the volume of wastewater pumped down had exceeded the volume of oil extracted, suggests the team of researchers from the University of Oklahoma, Columbia University and the U.S. Geological Survey. That increased pressure then caused the rock to jump along a pre-existing fault, known as the Wilzetta Fault.

Similar wastewater quakes have struck from Ohio to California—and abroad in the past few decades. And with the rapid expansion of fracking for oil and natural gas for contributing an ever-growing volume of wastewater, unregulated dumping down disposal wells could lead to similar quakes elsewhere unless new treatment methods can be found. Or oil and gas operators could be required to avoid dumping near known faults. Operators also could provide a record of fluid volumes and the pressures they encounter deep underground—a potential warning sign. If the new research is correct, the earthquake near Prague, Okla., now stands as the largest earthquake ever recorded as a result of fluid injection.

And the Wilzetta Fault remains under pressure from local dumping despite the recent earthquake, which buckled pavement and destroyed 14 homes. Our fossil fuel addiction means there's a lot of wastewater to get rid of and a lot of questions about whether it can be safely dumped underground.