By Lenzy Krehbiel-Burton

TULSA, Okla. (Reuters) – One of the strongest earthquakes ever recorded in Oklahoma rattled a state where seismic activity has become a growing concern and sent tremors that were felt in 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.

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.

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. The mayor told Reuters he had yet to survey other parts of town, which has about 2,200 residents.

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

Oklahoma geologists have documented strong 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.

Last year the state recorded 585 quakes of magnitude 3 or greater, up sharply from 109 in 2013. Prior to 2008, Oklahoma averaged less than two a year.

The spike in earthquake activity 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.

(Reporting by Ian Simpson in Washington; Writing by Frank McGurty; Editing by James Dalgleish)