Moderate quake hits temblor-prone Oklahoma

TULSA, Okla., Sept 3 (Reuters) - One of the strongest earthquakes ever recorded in Oklahoma rattled a state where seismic activity has become a growing concern, sending tremors through six neighboring states, the United States Geological Survey said on Saturday.

The quake, which struck 14 km (9 miles) northwest of Pawnee in north-central Oklahoma at 7:02 a.m. CDT (1302 GMT), had a magnitude of 5.6, matching in strength a temblor that hit the state in 2011, the USGS reported on its website. There were no immediate reports of injuries.

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The earthquake, which had a depth of 6.6 km (4.1 miles), could offer fresh ammunition to environmentalists concerned about the side-effects of oil and gas production, which has been blamed for a spike in minor to moderate quakes in the region.

See how social media reacted to the quake:

"We just had an earthquake that shook our whole house!," wrote Gretchen Scott of Enid, Oklahoma, 60 miles west of Pawnee.

"Could actually hear it rumbling like thunder. Very freaky, and definitely not something I'd like to repeat soon!," she said on her Facebook page.

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 façade of an early 20th-century bank building had fallen into a downtown street, he said.

"We have had a spate of quakes over the last several years, but nothing like this," he said. "It was a long, sustained quake."

The likelihood of casualties and damage from the earthquake, was low, the USGS said. Most residences in north-central Oklahoma were resistant to earthquakes, it said on its website.

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.

Oklahoma is recording 2-1/2 earthquakes daily of a magnitude 3 or greater, a seismicity rate 600 times greater than before 2008, the Oklahoma Geological Survey (OGS) said.

The spike has put Oklahoma at the center of a national debate over whether wastewater disposal from oil and gas production triggers earthquakes. The state's economy depends heavily on energy production, accounting for one of every four jobs there.

The water at issue is extracted from the ground along with oil and gas, separated and re-injected into deep wells.

The drilling technique known as hydraulic fracturing, or "fracking," generates large amounts of wastewater. But the OGS report said fracking is responsible for only a small percentage of the total volume of wastewater injected into disposal wells.

Zachary Reeves, a seismologist with the USGS National Earthquake Information Center in Golden, Colorado, said the agency had received reports of the Oklahoma quake from South Dakota, Wisconsin, Kansas, Missouri, Arkansas and Texas.

"It's a relatively large quake for the area. The central U.S. doesn't tend to get a lot of five-plus earthquakes."

He said it was the third magnitude 5 quake in the state since 2011, and there were a couple of dozen or so 4s or bigger in Oklahoma last year.

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