Can shale gas extraction cause earthquakes?
In 2011, the UK Government halted shale gas extraction activities and commissioned an independent review into the association between hydraulic fracturing (or fracking) of shale gas and seismic activity, given the detection of seismic tremors at a shale gas extraction site in Lancashire.
A review by the Royal Society and the Royal Academy of Engineering found, for the Preese Hall site in the Bowland Shale area, seismic activity could be attributed to the fracking activity underway, noting the most likely cause of events was injection of fracking fluid under pressure to a nearby (but previously unidentified) pre-stressed fault in the shale.
Their investigation found no association between small quantities of water injected and seismic activity, but did find that large quantities of water pumped under the surface could cause seismic tremors under certain conditions. Put simply, in the Lancashire case, tremors occurred as fracturing fluid flowed along a fault already under stress, causing it to move.
The independent assessment concluded that shale gas extraction could in principle cause small earthquakes; but that these were unlikely to lead to structural damage. Blowouts could potentially occur but were deemed a rare event. The assessment highlighted that mini seismic events could occur from extraction activities using hydraulic fracturing, and the effects were not only confined to shale extraction. The report noted also that the risk of seismic activity was greater where pre-stressed faults existed, and so recommended on-going mapping of faults to identify potentially high-risk sites to avoid.
To help prevent future seismic tremors linked to shale gas extraction, a number of recommendations were put to the UK Government, including minimising pressure changes at depth, avoiding injecting fracking fluid into active faults, and commissioning national surveys to identify existing faults in UK shales.
Ongoing monitoring and recording of seismic activity was also recommended, alongside comprehensive risk assessments and site surveys to identify potential faults and mitigate against potential seismic activity.
