A string of earthquakes which rocked Horsham, Crawley and Gatwick earlier this year were not caused by oil extraction activity in nearby Surrey.
That’s the finding of new research carried out by Imperial College in London.
The series of 34 quakes - dubbed the ‘Surrey swarm’ - struck between April 2018 amd May this year. They all happened within 10 kilometres of two active oil extraction sites at Brockham and Horse Hill in Surrey.
Thousands of people in Crawley, Horsham, Horley, Charlwood, Newdigate and Dorking were left shocked as the largest quake reached a magnitude of 3.2 in February.
Houses shook and residents reported hearing a ‘thunderous noise’ as the tremor struck a wide area at 3.42am. It was followed by two smaller tremors.
There were reports of buildings in Crawley and Horsham shaking and furniture moving.
Now, the first in-depth study of the quakes by Imperial, the University of Bristol, and the British Geological Survey, has shown no direct link between oil extraction and earthquakes in the region.
The study’s authors say they believe “natural causes were behind the earthquakes.”
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.”
During the early stages of the swarm researchers installed seismometers – instruments that measure ground vibrations – around the affected areas. The highly sensitive devices tracked the timings, strengths, and distribution of earthquakes.
The researchers also used earthquake data from existing sensors in people’s homes - known as ‘RaspberryShakes’ - that had been ‘listening’ since late 2017 for seismic activity in the area.
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.
A spokesman for Imperial said: “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.
“The researchers found that the Surrey swarm quakes moved ancient faults horizontally, indicating that the quakes would probably have happened regardless of nearby oil extraction.”
The researchers say 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.
Dr Hicks added: “This is not the first time earthquakes have come seemingly from nowhere and without human input. Decades of instrumental recordings and hundreds 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.”
The researchers say they are “continuing to monitor quakes in the area for the foreseeable future.”