An earthquake is the vibrations and shaking of the Earth’s crust which are caused by movement in the Earth’s plates. The plates do not always move smoothly against one another and can sometimes get stuck which causes pressure. Earthquakes occur when the built up pressure is eventually released from inside the earth’s crust.
The point inside the crust where the pressure is freed is called the focus, the point right above the focus, on the Earth’s surface, is called the epicentre. The energy released from the earthquake is seismic waves and these waves spread out from the focus. The waves are stronger at the epicentre.
The strength or magnitude of an earthquake is measured by the Richter scale. The Richter scale is numbered from 1-10.
Anything lower than 2 on the Richter scale and the earthquake usually isn’t felt. At 3, the vibrations can feel like a passing truck. 4 and there is noticeable shaking. 5 can cause damage to poorly constructed buildings. 6-9 can cause serious damage and loss of life. The Japanese earthquake and tsunami on 11th March 2011 was measured at 9, and the most serious on the warning scale, causing irreparable damage and immense loss of life.
Approximately every two years Britain will experience a magnitude 4 earthquake and a 5 every 10-20 years. Research predicts that the largest possible earthquake in the UK is around a 6.51.
In 2011 Blackpool experienced two small earthquakes. On the 1st April an earthquake measuring 2.3 on the Richter scale struck and later on the 27th May an earthquake of 1.5. It was found that both of these happened close to the Preese Hall drilling site where hydraulic fracturing was taking place. The epicentre of the second quake was within 500 metres of the drilling site at a depth of 2km.
Cuadrilla Resources issued a press release on the 2nd November 2011 to announce that they triggered a number of minor seismic events; but that the combination of geological factors that created the higher than normal seismic event was extremely rare and that the events would be limited to around magnitude 3 on the Richter scale as a ‘worst case scenario’2. During fracking the micro seismic events generally register below 0 on the Richter scale.
How did the fracking cause the earthquakes?
Hydraulic fracturing, fracking, works by injecting huge volumes of a water and sand mixture deep below the earth’s surface, past the water table and freshwater aquifiers into the dense shale rock producing an immeasurable amount of long and narrow fractures in the rock formation through which gas can escape to the surface.
The UK quakes were not caused by the violent rupturing of the rocks, but by the presence of water. The water lubricates the rocks and pushes them apart, allowing them to slip past each other.
So should we expect lots more earthquakes from fracking?
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.
Dr Brian Baptie of the British Geological Survey, however, 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.3
In America fracking has caused a number of earthquakes across the states, there are many articles to read if you type ‘fracking’ and ‘earthquakes’ into a search engine. Trinidad in Colorado experienced an earthquake at 5.3. Texas had 11 earthquakes in 24 hours recently and in Poland Township, Ohio; in less than one month a total of 77 earthquakes happened, and although they were small, when the fracking was halted on a nearby well, the rate suddenly dropped.
The mere fact that earthquakes are happening more and more regularly even though they aren’t high on the Richter scale is worrying. There are a lot of unknowns in the long term dangers and effects of hydraulic fracturing so who is to know what will happen in the future when fracking is a prominent feature in the UK.
References
1 http://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/UK.html
2 http://www.cuadrillaresources.com/news/cuadrilla-news/article/press-release-geomechanical-study/
