Earthquakes in Oklahoma are up more than a hundredfold in recent years, and a new study spies a pretty clear link between the shaking and the fracking that has given the state's economy a huge boost. Researchers took a close look at four specific sites where wastewater from oil and gas extraction was injected into the ground and found that the process could be linked to swarms of quakes in areas up to 20 miles away from the sites, reports the BBC. The four wells examined have been pumping four million barrels of water a month to a depth of around two miles underground. At one site linked to the wells, a small town called Jones, there have been more 2,500 earthquakes greater than magnitude 3.0 since 2008—a fifth of the total in the central and western US during that period.
"It really is unprecedented to have this many earthquakes over a broad region like this," a study co-author tells Scientific American, explaining that wastewater injection can cause quakes by sending out waves of fluid pressure, causing faults miles away to slip. "Most big sequences of earthquakes that we see are either a main shock and a lot of aftershocks or it might be right at the middle of a volcano in a volcanic system or geothermal system. So you might see little swarms but nothing really this distributed and this persistent," he says. (In Texas, several small towns troubled by quakes are considering banning fracking.)