Fears that several small earthquakes in the US state of Ohio may have been triggered by the disposal of wastewater from "fracking" has prompted the closure of an injection well -- and questions about the oil and gas extraction technique.

The latest and strongest earthquake, which had a magnitude of 4.0, occurred on New Year's Eve near the well operated by D&L Energy in Youngstown, in a region of northeastern Ohio that has seen a boom in fracking activity at a major shale formation known as the Utica Shale.

The state had quietly shut down operations at the Youngstown injection well after a 2.7 magnitude quake on December 24, but then extended the moratorium to a five-mile (eight-kilometer) radius around the well after the larger quake on December 31, industry officials said.

"As a precautionary measure we've reached agreement with the well's owner to halt injections until we are able to further assess and determine any potential links with recent seismic events," the state's natural resources director James Zehringer said.

"I consider the link to be persuasive," John Armbruster, a seismologist with Columbia University's Lamont-Doherty Earth Observatory who studied the quakes, told AFP on Tuesday.

"In three dimensions, the earthquake is about a kilometer from the bottom of the well."

After measuring the latest of 11 earthquakes, Armbruster's team told state regulators that "the link between them seems pretty good and they decided to shut down the well, which was their responsibility."

A spokesman for the Ohio Oil and Gas Association said the suspension was the right thing to do until the event was better understood.

"We also believe that while we may have a suspicious event here, nonetheless we do not have a direct link. That needs to be established by using good scientific measurements," said Tom Stewart, the association's executive vice president.

Ohio's oil and gas industry has a lot riding on the development because the huge oil shale formations in Ohio are believed to hold potential reserves of up to 5.5 billion barrels of oil and 15 trillion cubic feet of natural gas.

Hydraulic fracturing, or "fracking," a process by which high-pressure injections of water, sand and chemicals are used to blast through rock to release oil and gas trapped inside, is considered key to unlocking that potential.

The wastewater that comes back out of those wells are then disposed of in separate injection wells.

Stewart pointed out that there are 180 water injection wells in operation in Ohio, taking in seven million barrels of wastewater a year, and problems have been rare since they began in the mid-1980s.

"In my view, if there is a link -- if being the operative word -- you have this proverbial needle in a haystack where this well system may be touching some kind of fault system, or something similar to that, underneath the basement rock, causing those types of temblors," he said.

"But these wells have been in operation since the 1980s and the track record is superb. It is the preferred method for managing the waste stream," he said.

But little is known about the potential side effects of such operations on a larger scale, and while earthquakes have been rare, they are not unprecedented.

A large cluster of earthquakes in Arkansas last year prompted authorities there to suspend operations at two water injection wells, and seismologists at Southern Methodist University found links to quakes near an injection well in the Fort Worth-Dallas area in 2009.

The Youngstown injection well entered in operation in December 2010, and soon after, in March, the earthquakes began.

By the end of last year, 11 earthquakes had occurred, all in virtually the same location and at the same depth, ranging in magnitude from 2.1 to 4.0.

They caused no injuries and only minor damage, but they were especially surprising in that the area had little history of seismic activity.

"My view is you cannot predict what is going to happen when you start operating one of these injection wells, because the earthquakes that are happening are earthquakes you are triggering, sitting, waiting to happen and you've given these earthquakes a slight extra push," Armbruster said.

"In order to predict what a given well is going to do, it's like predicting an earthquake. And no one has predicted a method for predicting earthquakes."