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In April and May this year, two small earthquakes struck the UK near the town of Blackpool. Suspicion immediately fell on hydraulic fracturing, known as fracking – a controversial process to extract natural gas by fracturing the surrounding rock. A report has now confirmed that fracking caused the earthquakes.

New Scientist looks at what happened, and whether fracking is likely to cause more earthquakes.

When and where did the earthquakes happen?
A magnitude-2.3 earthquake occurred on 1 April, followed by a magnitude-1.5 quake on 27 May. Both occurred close to the Preese Hall drilling site, where Cuadrilla Resources was using fracking to extract gas from a shale bed.

Initial studies by the British Geological Survey (BGS) suggested that the quakes were linked to Cuadrilla's fracking activities. The epicentre of the second quake was within 500 metres of the drilling site, at a depth of 2 kilometres. Less information was available on the first quake, but it seems to have been similar.

The link with fracking has now been confirmed by an independent report commissioned by Cuadrilla, Geomechanical Study of Bowland Shale Seismicity, which states: "Most likely, the repeated seismicity was induced by direct injection of fluid into the fault zone."

The two geologists who wrote the report ran detailed models to show that the fracking could – and most likely did – provoke the quakes.

How did the fracking cause the earthquakes?
Fracking works by injecting huge volumes of water into the rocks surrounding a natural gas deposit. The water fractures the rocks, creating dozens of cracks through which the gas can escape to the surface.

The UK quakes were not caused by the violent rupturing of the rocks, as you might expect, but by the presence of water. This lubricates the rocks and pushes them apart, allowing them to slip past each other. "It's a bit like oiling the fault," says Brian Baptie of the BGS.

Seismologists have not been able to find the fault that moved, probably because it is tiny. Baptie says the surface area of the fault is likely to be just 100 metres by 100 metres, and that the rocks moved by about 1 centimetre – the seismological equivalent of a needle in a haystack.

So should we expect lots more earthquakes from fracking?
It's difficult to say. Fracking has been going on in the US for decades, and has become much more common in recent years, yet evidence that it causes earthquakes has so far been elusive. "This is one of the first times felt earthquakes have been associated with fracking," Baptie says.

The Cuadrilla report says the earthquakes occurred because of a rare combination of circumstances: the fault was already under stress, was brittle enough to fracture and had space for large amounts of water that could lubricate it. The report says this is unlikely to happen again at the Preese Hall site.

Baptie is not so sure. He says small faults are probably common in deep rocks, but go undetected because of their size. "It seems quite possible, given the same injection scheme in the same well, that there could be further earthquakes," he says.

Cuadrilla is proposing to monitor seismic activity around its fracking site. If earthquakes begin to occur, it could reduce the flow of water into the well, or even pump it back out, preventing the bigger quakes. Baptie says such monitoring is now necessary to avoid further quakes at fracking sites.

Are these earthquakes dangerous?
Not particularly. Magnitude-2.3 earthquakes can shake the ground enough for people to notice, especially if they occur close to the surface, but damage is normally limited to objects falling off shelves.

According to Baptie, the UK gets an average of 15 magnitude-2.3 earthquakes every year, so the quakes produced by the fracking are not out of the ordinary. http://www.newscientist.com/article/dn21120-how-fracking-caused-earthq
uakes-in-the-uk.html

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