Not long after two mild earthquakes jolted the normally steady terrain outside Youngstown, Ohio, last March, geologists quickly decided that hydraulic fracturing operations at new oil-and-gas wells in the area had set off the tremors. Now a detailed study has concluded that the earthquakes were not isolated events, but merely the largest of scores of quakes that rattled the area around the wells for more than a week. The study, published this week in The Bulletin of the Seismological Society of America, indicates that hydraulic fracturing, commonly called fracking, built up subterranean pressures that repeatedly caused slippage in an existing fault as close as a half-mile beneath the wells. The number and intensity of fracking-related quakes have risen as the practice has boomed. In Oklahoma, for example, quakes have increased sharply in recent years, including the state’s largest ever, a magnitude 5.7 tremor, in 2011. Both state and federal […]