Until 2008, Oklahoma typically had one or two earthquakes of magnitude 3.0 or greater per year, according to the U.S. Geological Survey; since the start of 2015, the state has averaged 2 of this strength or greater per day.
Other normally calm states have experienced heavy seismic activity near oil and gas drilling sites in recent years, and suspicion has naturally turned to tracking, which uses underground explosives to extract hydrocarbons from subsurface wells. There is no proof yet that fracking itself can cause earthquakes. But Cornell geophysicist Katie Keranen says “the evidence is strong” that Oklahoma’s tremors are the result of a related practice: disposing of drilling wastewater by injecting it into the ground.
Holland says oil and gas extraction are not solely behind the state’s increased seismic activity. “We have felt earthquakes caused by hydraulic fracturing,” he says. “But we know that can’t be the cause of all of them because we have earthquakes happening where no hydraulic fracturing has been occurring—Denver nicks