An earthquake measuring 2.9 on the Richter scale has rocked the UK's only active fracking site near Blackpool.

The tremor at the Lancashire shale gas site was recorded at about 8.30am on Monday and is stronger than those that forced owners Cuadrilla to suspend test fracking in 2011.

It is the fourth quake to hit the area since last Wednesday.

How bad were the quakes?

On Wednesday there was a magnitude 1.55 tremor, on Friday night there was one measuring 1.01 and on Saturday there was a magnitude 2.1 quake and yesterday's was the largest one yet at 2.9, with a depth of 1.2 miles.

People reported their walls and furniture shaking during the tremors, and hearing a "loud roar" for the duration.

Cuadrilla, the oil and gas production company that own the site, said it understood the quake "caused concern for local people" but added: "it is worth noting that this event lasted for around a second and the average ground motion recorded was 5mm per second".


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What's causing the quakes?

Cuadrilla said it was investigating the tremor and no fracking was being carried out at the time.

The Oil and Gas Authority have suspended fracking for more than the required 18 hours while it assesses the seismic activity.

What is fracking?

Fracking, or hydraulic fracturing, is the process of drilling down into the earth before a high-pressure liquid is directed at the rock to release the gas inside - gas which is being considered by governments as a potential energy source.

Backers, including Westminster, claim accessing the fossil fuel through fracking could reduce reliance on imports, secure supplies, help cut carbon emissions and create jobs.

But opponents say it can cause earthquakes, irreparably damage the countryside and keep the UK hooked on fossil fuels instead of focusing on renewable energy to help tackle climate breakdown.

Drilling for shale gas is still at an exploratory phase. However, reserves of shale gas have been identified across large swathes of the UK, particularly in northern England. More than 100 licences have been awarded by the government, allowing firms to pursue a range of oil and gas exploration activities in certain areas.

The Scottish Government has a moratorium in place until further research is completed into fracking's environmental impact.

Campaigners against the invasive gas-extraction technique are calling on Holyrood to place a full-legislative ban on the process so the more fragile policy ban can't be overturned by future governments.


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Should we be worried?

In 2011, Cuadrilla suspended test fracking operations near Blackpool after earthquakes of 1.5 and 2.2 magnitude hit the area. It resumed last October after campaigners failed to get an injunction.

A subsequent study found it was "highly probable" that shale gas test drilling triggered the tremors.

Environmental campaign group Friends of the Earth said in 60 days of fracking last year there were 57 tremors in Lancashire and that it cannot be carried out without triggering earthquakes.