A series of more than 100 small earthquakes in Surrey in 2018 and 2019 might have been triggered by oil extraction from a nearby well, suggests a new study by UCL researchers.
The earthquakes, which occurred in Newdigate and surrounding areas from April 2018 until early 2019, were recorded as being between 1.34 and 3.18 magnitude, and were linked to cracks in walls and ceilings and other damage to people's homes, with reports of houses and beds shaking.
Geologists have been divided over whether these earthquakes could have been triggered by extraction at the Horse Hill well in Horley about 5km to 10km away.
Previously, some researchers had argued that the pattern of earthquakes did not correspond to the oil extraction, with periods of increased extraction not followed directly by an increase in seismic activity.
The new study, published in Geological Magazine, ran more than a million simulations estimating the frequency of earthquakes based on the timing and volume of oil extraction and found the model predictions roughly matched what occurred, suggesting a link between the extraction of the oil and the earthquakes.
Lead author Dr Matthew Fox (UCL Earth Sciences) said: "Our study suggests there is a link between the ground earthquakes and oil extraction at Horse Hill but we cannot rule out that this link is a coincidence. More work needs to be done to understand if this is cause and effect. However, our findings indicate it is plausible that oil extraction triggered the earthquakes."
Oil extraction at Horse Hill was stopped in October after planning permission was quashed in a landmark legal case. Supreme Court judges ruled that the full climate impact of the project should have been considered before Surrey County Council approved it.
However, another project has received council approval to conduct exploratory oil drilling in Balcombe, West Sussex. This is subject to a legal challenge by a residents' group, with the case expected to be heard by the Court of Appeal this month [January 2025].*
Extracting oil changes the fluid pressure in the rock the oil is extracted from. This change in pressure can potentially diffuse through tens of kilometres underground to intersect a fault (a shear fracture between two rock blocks). The speed that this change in pressure moves depends on the rock's permeability (how quickly fluids can pass through it). As faults are often under considerable strain, even a small change in stress can cause the rapid movement of rock that leads to an earthquake.
For the new study, the researchers used a mathematical model to predict the number of earthquakes that might occur based on how much oil had been extracted, accounting for the two different rock types that oil was extracted from.
As much key information is unknown - for instance, what the time lag would be from extracting oil to an earthquake occurring - the team used a Bayesian machine learning tool that randomly varied unknown parameters such as time lag.
Building on the work of the late University of Glasgow geologist Dr Rob Westerway, the researchers were able to account for oil extraction switching between two different rock types, with one, Portland rock, being a million times more permeable than the other, Kimmeridge, meaning the lag time between extraction and a potential induced earthquake would be a matter of days rather than weeks.
Although the earthquakes began just before oil extraction started, the researchers said these earthquakes could have been triggered by preparatory works - for instance, checking the pressure in the well - which could have led to a similar pressure change travelling underground to the fault line.
The researchers said the picture was complicated by the fact that seismometers (instruments detecting ground vibrations) were installed in the area only after the first larger earthquakes had occurred. Any smaller earthquakes that might have occurred prior to these instruments being installed were not recorded. This highlights the need for detailed seismic monitoring before oil extraction begins at proposed sites.
Co-author Professor Philip Meredith (UCL Earth Sciences) said: "Our study highlights the importance of monitoring the seismicity of areas where oil extraction might occur before any works start. Caution should be the byword. It is no good saying you don't have a problem when you potentially do.
"There has been no significant seismic activity in Surrey for decades, so these earthquakes were unusual events. However, unusual events do happen in nature, so we can't rule out the possibility that the timing associated with the oil extraction was a coincidence."
*Lead author Dr Matthew Fox has provided an expert witness statement in the case.
