Oklahoma rocked by 5.6 magnitude earthquake

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 energy production, a federal agency said.

The magnitude 5.6 quake, occured at 1202 GMT and was felt from South Dakota to Texas.

The state Corporation Commission immediately ordered the closure of 35 wastewater disposal wells in the area, Governor Mary Fallin announced on her Twitter account.

There were no immediate reports of any casualties or injuries in Pawnee but several of the town's buildings suffered damages.

Pawnee Nation of Oklahoma has declared a state of emergency in the city where about 25 percent of the residents are Native American.

Releasing a statement on the website, Pawnee Nation said emergency operations are continuing in the city.

"You heard it before it happened," Pawnee resident Jasha Lyons Echo-Hawk said. "Watching my drawers all shake out and my headboard rattle, it felt like I was watching 'Paranormal Activity.' It felt like I was in a movie."

Pawnee Mayor Brad Sewell said the tremor lasted nearly a minute, far longer than previous ones that lasted only a second or two. Part of the facade of an early 20th-century bank building fell into a downtown street, he said.

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. A smaller quake of 3.2 magnitude occurred nearby on Thursday.

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.

One source of wastewater is from hydraulic fracturing, or fracking, the process of shooting water mixed with sand and chemicals deep into the earth to crack rock formations and bring up oil and natural gas trapped inside.

The process has unlocked massive amounts of oil and gas in the United States over the past decade.

But along with the oil and gas come large quantities of brackish water, which is disposed of by injecting it into separate wells that are dug as deep as a mile (kilometer) below ground.

The unnatural addition of the water can change pressure along fault lines, causing slips that make the earth shake, experts say.