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Oklahomans are accustomed to scorching heat, driving thunderstorms and deadly tornadoes. But not earthquakes–until recently. From 1990 to 2008, the state never had more than 11 earthquakes a year that measured 2.0 or greater on the Richter scale. In 2013 there were 291. More than 250 earthquakes of at least that magnitude have struck in February 2014 alone. “It’s incredibly unusual,” says Austin Holland, an Oklahoma Geologic Survey seismologist.

Other normally calm states have experienced heavy seismic activity near oil and gas drilling sites in recent years, and suspicion has naturally turned to fracking, which uses underground explosives to extract hydrocarbons from subsurface wells.