Pumping wastewater underground may rock Oklahoma. Vast quantities of water left over from oil and gas extraction and then injected into disposal wells may have set off a surge of earthquakes that has shaken the state since 2008.
And disposal wells don’t just trigger quakes nearby. Tremors can rattle the ground up to 35 kilometers away — much farther than scientists had previously thought, researchers report July 3 in Science.
The new study is the most definitive to link Oklahoma’s rocketing earthquake numbers to fluid injection, says seismologist Steve Horton of the University of Memphis in Tennessee.
In 2008, a series of small earthquakes began ripping through the land near Jones, a tiny town about 30 kilometers northeast of Oklahoma City. Some of these quakes, known as the Jones swarm, were strong enough to crack sidewalks and sheetrock. And unlike typical earthquakes, which tear along a single fault, these swarms of tremors spread scattershot through a huge expanse of the state, says study coauthor Katie Keranen, a geophysicist at Cornell University. “They’re like a swarm of mosquitoes buzzing all over in a big cloud,” she says.
And they’ve kept coming and coming. Before 2008, only one or two magnitude 3 quakes rumbled through Oklahoma per year. So far in 2014, the state has had more than 200. “We’ve never seen anything like this before,” Keranen says.
To get fuel out of the ground, drilling engineers have to pump out water too, she says. And that vast amount of water isn’t exactly fresh: It’s more like brine. Oil and gas companies get rid of the brine by injecting it into rocks via disposal wells at other sites.
Scientists had suggested that wastewater dumping could set off quakes (SN: 8/10/13, p. 16). But in this case, there was one snag: Most wells sat far away from the Jones swarm. People argued that the wells were just too distant to cause the quakes, Keranen says. She wasn’t so sure.
Her team created a computer simulation to estimate how far wastewater travels underground and the pressure it creates within the rocks around 89 wells. Too much pressure can stress the faults threading throughout Oklahoma’s crust, forcing the ground to shudder.
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Then Keranen and colleagues analyzed ground movement data to pinpoint the origin of thousands of Jones’ swarm quakes that took place from 2010 to 2013. According to the simulation, the pressures spreading from the wells to these earthquakes’ origin sites were high enough to kick-start the quakes.
What’s more, the main culprits were just four high-volume disposal wells. “We’re not saying that every disposal well is likely to cause earthquakes,” says Keranen. “But there’s a handful that are just absolutely anomalous — disposing of millions of barrels of water a month.”
She thinks the findings could help the oil and gas industry pinpoint problem wells and guide future operations.
But even the magnitude 5.6 earthquake that jolted Oklahoma in 2011 — the largest in the state’s history and one that scientists later linked to wastewater injection — didn’t prompt new regulations, says geophysicist Arthur McGarr of the U.S. Geological Survey in Menlo Park, Calif. “I think it will take some bigger earthquakes before anything is likely to change.”
The new study probably won’t convince industry scientists that wastewater injection induces earthquakes, Horton says. Still, he adds, the question is an easy one to answer: “All you’ve got to do is turn off the wells. If the earthquakes stop, they’re induced,” he says. “It’s that simple.”