The Horse Hill oil site was discovered in 2015Invicta Kent Media/ShutterstockOver 100 earthquakes that shook the south of England are likely to have been triggered by oil extraction, say experts. Previous research had ruled out a link, but fresh information about the geology of the area has established a probable connection.The earthquakes were all fairly minor, measuring between 1.34 and 3.18 on the Richter scale, and took place near Newdigate in Surrey in 2018 and 2019. Residents reported their houses shaking for several seconds. AdvertisementThere was speculation at the time that the earthquakes were linked to oil extraction by UK Oil & Gas, although the epicentre of the earthquakes was between 5 kilometres and 10 kilometres from the companys Horse Hill drilling site. But previous research by the British Geological Survey (BGS) and others determined that the earthquakes, while rare and relatively near to the drilling, were natural and coincidental.Now, Matthew Fox and Philip Meredith at University College London have taken another look, using the latest understanding of the rock makeup of the area, and suggest that the earthquakes were indeed induced by oil extraction.The pair ran more than a million computer simulations based on the location and magnitude of oil extraction, along with details of the geology of the surrounding area, and found that the predictions of earthquakes matched real events more closely than previously thought. Unmissable news about our planet delivered straight to your inbox every month.Sign up to newsletterWhen oil is extracted from deep underground it changes the pressure beneath the surface, which can cause rock movements that lead to earthquakes. Fox says there were previously discrepancies between dates on which earthquakes occurred and when oil was extracted, which can now be accounted for by factoring in the geology of areas being drilled.The pair found that in regions composed of porous Portland stone, pressure changes and rock movement could be almost simultaneous, while in areas of dense Kimmeridge Clay, there could be a delay as pressure changes took time to propagate.For me, this increases the probability [that oil extraction led to earthquakes], says Fox. The correlation between these two between oil extraction and seismicity is quite strong, and that does suggest that there is a link.Fox says there is still a chance the link is coincidental, but that this is now less likely than previous research suggested. He expects further statistical analysis to precisely quantify the probability of a coincidence, but stopped short of making an estimate with current data.UK Oil & Gas controlled oil extraction at Horse Hilluntil October last year, when work was stopped after the withdrawal of planning permission from Surrey County Council following a court case brought by campaigners supported by Friends of the Earth.A spokesperson for the company told New Scientist: This is an incident that was answered and dealt with many years ago when the BGS seismologists were satisfied it was a natural event associated with movement on a deeper unassociated fault many kilometres deeper and distant from the site.ButStuart Haszeldine at the University of Edinburgh, who wasnt involved in this particular study but has done his own work with colleagues, says that a connection between oil extraction and the earthquakes is now extremely likely.We have made a detailed study of these small earthquakes in Surrey and the possibility that tremors were linked to activities at the Horse Hill oilfield, says Haszeldine. In my professional judgement, there was a clear linkage of earthquake timing and operational activities to produce oil and gas from the Horse Hill site.Journal reference:Geological Magazine DOI: 10.1017/S0016756824000505Topics: