Scientists have, for the first time, linked hundreds of earthquakes across a broad swath of Oklahoma to a handful of wastewater wells used by the fracking industry.

The research, published in the journal Science on Thursday, said about one-fifth of the quakes that helped turn Oklahoma into the earthquake capital of America were caused by just four wells.

Oklahoma has had about 240 magnitude 3.0 or higher earthquakes just since the start of the year. The state now has twice the number of 3.0 earthquakes as California.

Before 2008, when the oil and gas boom got underway, the state averaged about one a year.

The researchers from Cornell University and other institutions traced a large number of earthquakes through 2012 to just four wells, south-east of Oklahoma city.

Those wells were pumped with significantly higher volumes of fracking wastewater and chemicals than the thousands of other disposal wells in the state.

The findings were the first to show such waste wells can trigger earthquakes up to 40kms away from the injection site.

They are bound to further deepen the controversy surrounding fracking, which has vastly expanded America's oil and natural gas production, but with rising consequences for health, safety and the environment.

Another Cornell-led team this week found that 40% of the fracked wells in north-eastern Pennsylvania were at risk of leaking methane into groundwater and air.

The researchers said faulty cement casings could be responsible.

Earthquakes in Oklahoma between 1976-2014. Earthquakes are magnitude > 1 from the NEIC catalog (10). Black lines are faults (26–28). Photograph: /Science journal Photograph: Science journal

In Thursday's study, researchers found a suite of wells around Oklahoma city, which collectively were pumped with nearly 5m barrels a month of waste, caused the swarm of earthquakes.

“These really big wells have the biggest impacts on the system,” said Geoffrey Abers, a professor at Columbia University's Lamont-Doherty Earth Observatory, and one of the authors of the study.

The biggest of the disposal wells was pumped with up to 1.6m barrels a month of fracking waste.

“The earthquakes themselves seem to occur on small discrete faults. As the pressure builds up in the sedimentary formation that they are pumped into ... They put that fault over the edge by jacking up the pore pressure.”

He ruled out a natural explanation for the spike in earthquakes. “This many earthquakes over and over again is not really something we have seen in a natural system,” he said.

Instead, the researchers found the earthquakes over the last five years were triggered by a relatively small number of the thousands of injection wells drilled across the state to dispose of the mix of water and chemicals used by drillers to flush oil and gas from layers of rock.

The researchers said the four Oklahoma city wells raised underground pressures, triggering a swarm of earthquakes across nearly 2,000 square kilometres.

In some instances, the earthquakes were more than 30km from the disposal site – much farther than researchers had expected.

The first earthquakes known to be caused by a fracking waste disposal well, occurred in Youngstown, Ohio. Scientists registered at least 109 earthquakes after the injection well came into operation in December 2010 until it was shut down a year later, following an earthquake that registered a magnitude of 3.9.

Those earthquakes were localised, however, the researched noted.

In their study, earthquakes travelled great distances from the disposal sites. There was also a time lag.“This is a situation where the pumping starts months or a couple of years before the earthquakes are observed at all,” Abers said.

The researchers found the areas of underground pressure continually expanded, increasing the likelihood of encountering bigger faults, and the risks of triggering higher-magnitude earthquakes.

The Oklahoma regulator has no rules limiting the pressure or volume of fracking waste that can be pumped into such disposal wells.

The authors refused repeated requests to identify or discuss the four high-volume wells responsible for a large volume of the earthquakes.

However, records maintained by the Oklahoma regulator indicate a number of disposal wells with significantly higher volumes of fracking waste than the thousands of other such wells across the state.

The biggest such well was owned and operated by New Dominion LLC, according to the Oklahoma Corporation Commission.

At least four other high-volume wells belonged to a now bankrupt company, Beard Oil.

Herb Mee, Beard's president, told the Guardian the company filed for bankruptcy in October 2012, and the wells have been out of operation since December 2012. He noted earthquakes had increased this year - well after his disposal wells shut down.

“We've got earthquakes every day, but they are much worse now than they were then,” he said. “We don't have any operations. If they were trying to pin anything on us, they are barking up the wrong tree.”

Jack Money, a spokesman for New Dominion, said the findings were "irresponsible" and based on "certain false assumptions", and that the company was seeking legal counsel.



In an emailed statement, Money said the company had not had enough time to study the findings but "an initial review reflects it is premised on certain false assumptions".



The statement added that the company operated its four wells in the Oklahoma city area safely and in co-operation with state regulators.



The statement added: "At best these incorrect assumptions are irresponsible".

The Oklahoma Corporation Commission, which is charged with overseeing the safety of oil and gas operations, said it could not yet comment on the findings.