Just four wastewater wells in Oklahoma – where energy companies dump water after completing the hydraulic fracturing process – have caused scores of earthquakes this year, some 30 km from the site, according to a new study by top US universities.

The report, published in Science magazine, focused on the Midwestern state, which has produced 45 percent of the country’s magnitude 3 or bigger seismic shocks in the past five years – with the numbers rising rapidly to match the intensification of fracking activities in the area.

While hydraulic fracturing, or fracking, – which involves pressuring rock formations with liquid until they crack, and then extracting the oil and gas within – poses an inherent risk of earthquakes, according to the authors, the biggest culprits were the wastewater wells, where the liquids used for fracking are pumped, once a reservoir is opened.

“The disposed fluids are capable of contributing to the seismic activity,” Katie Keranen, a geophysics professor at Cornell, and the lead author of the study, told The Oklahoman newspaper.

“These wells are capable. That doesn’t exclude anything else from contributing, but we have no reason to think these are tectonic. They don’t match tectonic activity in other areas. It does seem these are just linked to wastewater. Our research focuses on wastewater and shows it is sufficient.”

Because of a paucity of real data from the thousands of sites in Oklahoma, the team, which included a member from the US Geological Survey, used a three-dimensional model of the underground formations in the area, and then combined that with real liquid injection rates in the area.

There are more than 4,500 active disposal wells in the state, but scientists found that of those, four large wells collectively responsible for pumping 5 million barrels – the equivalent of more than 200 Olympic swimming pools – of liquid into the ground each month, exceed a threshold beyond which disproportionately large earthquakes can happen.

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

“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.”

The researchers have not named the four sites, though they are located around Oklahoma City. This is particularly alarming, as the academics say that the impact of the earthquake in their model was as far as 22 miles away (the previous limit was thought to be about, and also could happen even after the disposal well was shut down.

This is a situation where the pumping starts months or a couple of years before the earthquakes are observed at all,” said Abers.

New Dominion, which operates the biggest wastewater wells in the state, has dismissed the simulation used to draw the conclusions.

"An initial review reflects it is premised on certain false assumptions," said company spokesman Jack Money. "At best these incorrect assumptions are irresponsible."

The oil and gas industry – which directly or indirectly employs 1 in 6 in the formerly lagging state – has also urged caution.

“As an industry, we’ve been saying we need more data and we need to work with regulators and others to help determine what is causing the significant increase in seismic activity,” said Chad Warmington, president of the Oklahoma Oil and Gas Association. “But to unequivocally link it to wastewater injections, I think there still needs to be more research.”

Between 1967 and 2000, there was an average of 21 earthquakes of a magnitude greater than 3.0 – considered strong enough to be noticed - in Oklahoma. Last year there were over a hundred, and this year there have been more than 200.

State industry regulator, the Oklahoma Corporation Commission, said it would not comment on the findings until it had a chance to examine them carefully.