Earthquakes that rocked Surrey and Sussex sparking mass protests were not caused by oil extraction, a new report suggests.
Scientists found no evidence that oil extraction was responsible for the string of recent tremors near Gatwick Airport, known as the ‘Surrey Swarm.’
The series of 34 quakes, reaching magnitudes of up to 3.2 on the Richter scale, shook homes and panicked residents within a few miles of two active oil extraction sites at Brockham and Horse Hill in Surrey.
Green campaigners called for a ban on oil extraction during the swarm, which occurred between April 2018 and May this year.
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Earthquakes that rocked Surrey and Sussex sparking mass protests were not caused by oil extraction, a new report suggests. Pictured: Dr Stephen Hicks and Dr David Hawthorn installing a seismometer close to one of the affected areas
The British Isles do not lie along a tectonic plate boundary and earthquakes are rare, raising concerns that the shocks were triggered by nearby drilling and extraction.
But scientists at Imperial College London believe natural causes were behind the quakes and said their closeness to oil extraction sites was ‘probably a coincidence.’
The first in-depth study of the quakes found no direct link between oil extraction and earthquakes in the region.
Study Lead author Dr Stephen Hicks, of Imperial’s department of earth science and engineering, said: ‘The quakes seem to have occurred naturally and our findings suggest their closeness to oil extraction sites is probably a coincidence.
‘This is not the first time earthquakes have come seemingly from nowhere and without human input.
‘The swarm, like most natural earthquakes in the UK, could have been caused by ongoing collision of the African and Eurasian tectonic plates in the Mediterranean Sea – the UK’s nearest plate boundary – which stresses the crust and causes earthquakes across Europe.’
Scientists found no evidence that oil extraction was responsible for the string of recent tremors near Gatwick Airport, known as the ‘Surrey Swarm.’ Pictured: A map of earthquake data in England and Wales, showing the study area south of London
Pictured: A computer-generated 2D image of the subsurface beneath the earthquake area showing the locations of earthquakes and ancient geological faults. Green vertical lines show fault lines
The Imperial team tracked the timing, strength and distribution of the earthquakes by using seismometers to measure vibrations in the ground.
They also mapped the distances between the quakes and the extraction sites and examined the depth at which they occurred.
Their results showed the quakes were distributed in a tight cluster more than 1.8 miles (3km) away from the extraction sites which researchers said was too far away to link them with oil extraction.
Dr Hicks said: ‘It would be unprecedented for this type and scale of oil extraction to affect sites more than a kilometre away.’
Researchers found that the Surrey Swarm quakes moved ancient faults horizontally, indicating that the quakes would probably have happened regardless of nearby oil extraction.
Most natural earthquakes in the UK cause rocks on either side of weaknesses in the ground, known as faults, to move horizontally.
In contrast, earthquakes caused by oil extraction cause rocks either side of faults to move vertically.
Dr Hicks said: ‘The ground vibrations recorded from earthquakes provide clues that hint at their cause.
‘There are increasing examples worldwide of human activity causing earthquakes, but it can be difficult to work out which newer cases are natural, and which are human-caused.’
Researchers also used earthquake data from existing sensors in citizens’ homes – known as ‘Raspberry Shakes’ – that had been listening for seismic activity in the area since late 2017.
Based on data from the seismometers, the study team examined a variety of properties of the Surrey quakes and compared them to previous ones that were caused by both human activities and by natural causes in the UK and elsewhere.
Dr Hicks added: ‘Decades of instrumental recordings and hundreds of years of historical accounts of earthquakes show that similar seismic swarms have happened in the UK before due to long-term tectonic stresses and without any clear link to human activities.’
To measure the depth at which the quakes occurred, scientists compared the location of the earthquakes with images of rock layers beneath.
The images were created by measuring the reflection of sound waves off each layer.
They found that although the Surrey earthquakes were shallow (around 2.5 km deep), they occurred deeper than rock formations from which oil is extracted (less than 1 km deep).
The full findings of the study were published in the journal Seismological Research Letters.