Image caption Many critics have questioned the impact of hydraulic fracturing sites like this one on tectonic plates

New research suggests that fracking is not a significant cause of earthquakes that can be felt on the surface.

UK scientists looked at quakes caused by human activity ranging from mining to oil drilling; only three could be attributed to hydraulic fracturing.

Most fracking events released the same amount of energy as jumping off a ladder, the Durham-based team said.

They argue that the integrity of well bores drilled for fracking is of much greater concern.

The research is published in the Journal of Marine and Petroleum Geology .

Hydraulic fracturing is not really in the premier league for causing felt seismicity... Prof Richard Davies, Durham University

In recent years, hydraulic fracturing has become a significant means of recovering oil and gas that is too tightly bound into rock formations to be recovered by normal drilling.

Fracking, as it is called, utilises a mixture of water, sand and chemicals pumped underground at high pressure to crack open sedimentary rocks and release the fuels within.

Earth movers

But opponents of fracking have long been concerned that the process could induce earthquakes such as the one that occurred near a shale gas operation in Lancashire in 2011.

Now researchers from Durham University's Energy Institute say that the pumping of fracking liquid does indeed have the potential to reactivate dormant fault lines. But they say that compared to many other human activities such as mining or filling reservoirs with water, fracking is not a significant source of tremors that can be felt on the surface.

Earthquakes There are thousands of earthquakes each day, most too small to be detected without equipment

Earthquakes are usually caused by the motion of tectonic plates over the viscous mantle beneath

The Shaanxi earthquake of 1556 is the deadliest on record - killing nearly one million What causes earthquakes?

"We've looked at 198 published examples of induced seismicity since 1929," Prof Richard Davies from Durham told BBC News.

"Hydraulic fracturing is not really in the premier league for causing felt seismicity. Fundamentally it is is never going to be as important as mining or filling dams which involve far greater volumes of fluid."

The researchers detailed just three incidences of earthquakes created by fracking - one each in the US, the UK and Canada. The biggest at Horn River Basin in Canada in 2011 had a magnitude of 3.8.

"Most fracking related events release a negligible amount of energy roughly equivalent to, or even less than someone jumping off a ladder onto the floor," said Prof Davies.

What has been shown to cause bigger seismic activity is the underground injection of oil-drilling waste water. Recent research in the US has linked this to a 5.7 magnitude earthquake in Oklahoma in 2011. This isn't an issue in the UK as the practice of injecting waste water underground is banned by EU legislation.

If oil and gas exploration companies want to reduce the risk from fracking completely, the key thing according to Prof Davies is not to drill too close to tectonic faults.

Image caption One of the few recorded episodes of an earthquake being caused by fracking took place in Lancashire

By using 3D seismic imaging he says, these problem could be identified and the risk of quakes avoided. However this technology is not a legal requirement at present and is likely to be resisted as it will increase drilling costs.

Cement issues

Of greater concern to Prof Davies is the long term threat posed by the well bores that are drilled to allow fracking to take place. There have been concerns that over time the cement that is used to line the wells may give way under pressure.

"I think there are some good research questions about the long term integrity of well bores - it has been shown in the US and Canada that a percentage have not been cemented properly and are going to leak or do leak - that's what I would focus on," he added.

This view is echoed by Robert Jackson, professor of environmental sciences at Duke University in the US, who has published research on ground water contamination through hydraulic fracturing.

"If I were to emphasise one thing it would be well integrity. There are many examples of well integrity issues, we know that a certain number of wells leak through time. Some people might say 5 % but one study suggested as many as half of all wells have sustained casing pressure, suggesting there is something wrong," he said.

Prof Jackson says that transparency is key to successfully developing the shale gas industry. With new incentives from the Government announced in the budget to boost the recovery of shale gas, the likelihood was the fracking industry was going to expand rapidly in the UK.

According to Prof Jackson this would have implications for people all over the country.

"One thing that will change in the UK, is that gas extraction is no longer in the North sea it is in people's neighbourhood," he said.

"It is an industrial activity in people's backyards. It means trucks, dust, noise - people aren't used to that in the UK."

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