Earthquake, tremor or seismic activity: what is happening at the Lancashire fracking site?

The Preston New Road fracking site
The Preston New Road fracking site
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As fracking is halted for the second time, we look at what exactly is happening at the Preston New Road site.

Fracking in Lancashire was halted early yesterday after a seismic event with a magnitude of 0.4 on the Richter Scale was detected close to where the energy firm is fracking for gas in shale rock.

READ MORE: Fracking halted after tremor near Blackpool
This is the sixth such event that has been detected since fracking began at the site last week, although they have all been extremely small, well below the threshold that can be felt by people living nearby.

The British Geological Society keeps an ongoing count of seismic activity in Britain on their web page 'Earthquakes around the British Isles in the last 100 days' which shows that there are several of these kinds of events every month around the UK, usually at low levels.

It shows 18 in the UK in the last month, six of them are recorded in Blackpool, but there have also been cases in Nottingham, Surrey and Workington, and several more off the UK coast.

The BGS states: "Since hydraulic fracturing operations started at Preston New Road, near Blackpool, we have detected some small earthquakes close to the area of operations.

"This is not unexpected since hydraulic fracturing is generally accompanied by microseismicity (very small earthquakes that are too small to be felt)."

An earthquake itself is defined as "the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic waves.

"Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities."

Seismic activity, or seismicity, is term used to describe the frequency, type and size of earthquakes experienced over a period of time.

The word tremor is often used to describe small earthquakes, such as the ones that Lancashire has experienced, which are too low on the Richter Scale to be felt, but there is no scientific difference between the two and an earthquake can be of any strength, from very minor to extremely severe.