Between 2018 and 2019, the quiet county of Surrey experienced over 100 small earthquakes, puzzling residents and geologists alike. These tremors, ranging in magnitude from 1.34 to 3.18, caused widespread concern and some structural damage to homes in areas like Newdigate. Reports from locals detailed houses shaking, cracks forming in walls, and even furniture moving during the earthquakes. Now, new research conducted by University College London (UCL) suggests that these tremors might have been triggered by oil extraction activities at the Horse Hill well in nearby Horley.
The question of whether oil extraction was behind these earthquakes has long divided experts. Some argued that the pattern of seismic activity did not align directly with the timeline of oil extraction at Horse Hill. For instance, periods of intensified oil drilling were not immediately followed by increases in seismic activity. Others, however, pointed out that even small changes in underground pressure caused by drilling could potentially trigger dormant faults under stress. This new study, published in Geological Magazine, provides evidence that strengthens the latter argument.
Using advanced modeling techniques, the UCL researchers ran over a million simulations to estimate the likelihood of earthquakes occurring in relation to the timing and amount of oil extraction at Horse Hill. Their findings suggest a strong correlation: the model predictions closely aligned with the real-world data of earthquake occurrences. While the study does not definitively prove that oil drilling caused the earthquakes, it establishes a plausible link, suggesting that the seismic activity could be induced rather than entirely natural.
Dr. Matthew Fox, the lead author of the study, explains, “Our research indicates a possible connection between oil extraction and the earthquakes. However, there is still uncertainty, and further investigations are needed to determine if this is a cause-and-effect relationship or merely coincidental. What we do know is that the link is scientifically plausible and warrants deeper scrutiny.”
The cessation of oil extraction at Horse Hill in October followed a major legal development in which the UK Supreme Court quashed the planning permission for the project. The ruling emphasized the failure to account for the project’s full climate impact during the planning approval process by Surrey County Council. This decision was hailed as a landmark for environmental accountability in oil and gas projects.
Despite the suspension at Horse Hill, debates about oil exploration in the UK continue. In West Sussex, Surrey’s neighboring county, the council recently approved a proposal for exploratory oil drilling in Balcombe. However, this project faces a significant legal challenge from local residents. The case is expected to reach the Court of Appeal later this month, reflecting the broader tension between environmental concerns and energy development in the region.
Oil extraction involves altering the underground pressure within rock formations as oil is pumped out. These changes in fluid pressure can travel vast distances—potentially tens of kilometers—depending on the permeability of the surrounding rock. Once this pressure reaches a fault line, even a small change can destabilize the fault, releasing strain in the form of an earthquake. Faults, which are natural fractures between blocks of rock, are often under immense stress. As the UCL researchers emphasize, the relationship between pressure changes and seismic activity is highly complex but increasingly well-understood.
For their analysis, the team employed sophisticated mathematical models that incorporated the geology of the Horse Hill site. Two primary rock types—Portland and Kimmeridge—were considered. Portland rock, which is highly permeable, allows fluid pressure to move through it within days. By contrast, Kimmeridge rock is much less permeable, leading to pressure changes that take weeks or longer to propagate. This distinction was vital in determining the time lag between oil extraction and the observed earthquakes.
The researchers also examined the possibility that initial earthquakes, which predated the start of active drilling, could have been linked to preparatory activities at the site. Actions such as well pressure tests might have caused minor disturbances that traveled underground, potentially triggering seismic activity along fault lines already under stress. These considerations add complexity to the question of causality.
One significant challenge the study faced was the absence of early seismic data. Seismometers capable of detecting ground vibrations were only installed in the region after the larger earthquakes had occurred. As a result, any smaller seismic events before the installation of these instruments were not recorded. This gap in data makes it more challenging to establish a comprehensive timeline of seismic activity and its possible connection to drilling.
Professor Philip Meredith, a co-author of the study, stressed the importance of proactive monitoring. “Our findings underscore the critical need for baseline seismic monitoring before oil extraction begins. Having reliable data from the outset can help distinguish natural seismicity from potential induced events,” he said. “It’s concerning to realize that seismic activity in Surrey had been dormant for decades, yet significant earthquakes suddenly emerged in conjunction with oil drilling. While coincidences are possible in nature, ignoring potential links could be shortsighted.”
The implications of this research extend beyond Surrey. As countries like the UK explore domestic energy resources amidst global energy challenges, understanding the environmental and seismic risks associated with oil extraction becomes increasingly urgent. Other regions considering drilling projects, whether in the UK or elsewhere, could learn from Surrey’s experience. Detailed geological assessments, continuous monitoring, and strict regulatory oversight will be vital to ensure the safety and sustainability of future projects.
For residents of Surrey and similar regions, the study is a reminder of the intricate connections between human activities and natural systems. Whether or not the earthquakes were definitively induced by oil extraction, the phenomenon has raised awareness about the potential risks associated with energy development. It also highlights the need for a precautionary approach when it comes to projects that could alter the subsurface environment.
The Surrey earthquakes offer a valuable case study for scientists, policymakers, and local communities. As the debate continues and further research is conducted, one message is clear: understanding and managing the geological impact of human activity is essential for balancing energy needs with environmental safety and public well-being.
Reference: Geological Magazine (2025). DOI: 10.1017/S0016756824000505