CNN reports:

State leaders have ordered that four fluid-injection wells in eastern Ohio will be "indefinitely" prohibited from opening in the aftermath of heightened seismic activity in the area, an official said.

Ohio Department of Natural Resources Director James Zehringer had announced on Friday that one such well -- which injects "fluid deep underground into porous rock formations, such as sandstone or limestone, or into or below the shallow soil layer," the U.S. Environmental Protection Agency explains -- was closed after a series of small earthquakes in and around Youngstown.

Then on Saturday, a magnitude 4.0 earthquake struck that released at least 40 times more energy than any of the previous 10 or more tremors that had rattled the region in 2011.

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The New York Times notes that:

The latest quake, the 11th since mid-March, occurred Saturday afternoon and with a magnitude of 4.0 was the strongest yet. Like the others, it was centered near a well that has been used for the disposal of millions of gallons of brine and other waste liquids produced at natural-gas wells, mostly in Pennsylvania.

The waste, from the process called hydraulic fracturing that is used to unlock the gas from shale rock, had been injected under pressure into the well, which is 9,200 feet deep. Scientists had suspected that some of the wastewater might have migrated into deeper rock formations, allowing an ancient fault to slip. Similar links between disposal wells and earthquakes have been suspected in Arkansas and Texas.