As U.S.-based production of oil and gas has boomed over the last decade, millions of gallons of chemical-laden wastewater has been pumped and stored deep underground.
It turns out, however, that this disposal method—popular for fracking waste—is causing a spike in the number of earthquakes across the country, according to a new set of maps from the U.S. Geological Survey. Seven million Americans are now at risk from man-made quakes, particularly in Oklahoma, Kansas, Texas, Colorado, New Mexico and Arkansas. “My first thought was actually, ‘Holy crap, Oklahoma is redder than California,’” USGS geologist Susan Hough told The Washington Post about seeing the maps for the first time. Unlike California, however, most of these states don’t have earthquake-ready structures, experts said, so communities are having to update building codes and purchase new insurance.