A spike in seismic activity over seven years, a flurry of reports, and then last week, more earthquakes. Last Wednesday night, in an area where drilling for oil and gas occurs, a community in northwestern Oklahoma was hit with two quakes that registered 4.7 and 4.8 on the Richter scale, and 22 more tremors were logged throughout the area over a 13-hour period. The panhandle state experienced 585 quakes in 2014. In 2015 there were 842, according to data from the National Earthquake Information Center (NEIC), a government agency in Colorado. 

“That’s almost a millennium’s worth of earthquakes in two years,” said George Choy, a seismologist with NEIC, in an interview with the Guardian. “When you see that you suspect something is going on.”

According to the agency, the practice of disposing of toxic wastewater from the drilling process deep underground destabilizes fault lines in the bedrock, adding to problems caused by the high-pressure injection of water, sand, and chemicals, or hydraulic fracturing, also known as fracking, The Guardian reports. Choy, of NEIC, tells The Guardian that the amount of wastewater being produced in Oklahoma, which he estimates at 200 million barrels a month, puts the state at risk. “Water is finding its way to the underground faults and there is always that possibility of a big earthquake. We are certainly concerned,” he said.