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One of the strongest earthquakes ever recorded in Oklahoma rattled the area northwest of Pawnee on Saturday, fuelling growing concern about seismic activity linked to fracking, a federal agency said.
The magnitude 5.6 quake, which was felt from South Dakota to Texas, prompted the closure of some 35 wastewater disposal wells in the area, officials said.
There were no immediate reports of injuries in Pawnee, where about 25 percent of the residents are Native Americans. Damage in the town appeared to be minor, and the Pawnee Nation declared a state of emergency for its area.
The earthquake, which was only 4.1 miles deep, could fuel concerns about the environmental impact of oil and gas drilling, which has been blamed for a massive spike in minor to moderate quakes in the region.
Oklahoma has been recording 2-1/2 earthquakes daily of magnitude 3 or greater, a seismicity rate 600 times greater than before 2008, the Oklahoma Geological Survey (OGS) said.
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Oklahoma's economy is heavily dependent on energy production, which accounts for one of every four jobs in the state.
Oklahoma geologists have documented links between increased seismic activity in the state and the injection into the ground of wastewater from oil and gas production, according to a report from a state agency last year.
The drilling technique known as hydraulic fracturing, or "fracking," also generates large amounts of wastewater. The OGS report said fracking is responsible for only a small percentage of the total volume of injected wastewater.
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