A new study from UCL researchers suggests that over 100 minor earthquakes that took place in Surrey between 2018 and 2019 may have been caused by oil extraction activities at a nearby well.
The earthquakes struck Newdigate and nearby areas from April 2018 to early 2019, registering magnitudes between 1.34 and 3.18. They were associated with cracks in walls and ceilings, and reports of shaking in homes and beds.
Geologists have debated whether the earthquakes were connected to the oil extraction at the Horse Hill well located roughly 5km to 10km away.
Some researchers had previously asserted that the timing of the earthquakes did not align with the oil extraction activities, noting that increases in extraction did not directly correspond with spikes in seismic activity.
The latest research, published in Geological Magazine, involved running over a million simulations to estimate earthquake frequency based on the timing and quantity of oil extracted, finding that the model’s predictions closely matched actual earthquake occurrences. This suggests a possible connection between oil extraction and the seismic events.
Dr. Matthew Fox, the lead author from UCL Earth Sciences, commented, “Our findings indicate a potential correlation between the earthquakes and oil extraction at Horse Hill, but we cannot dismiss the possibility that this connection is coincidental. Further investigation is required to determine if a cause-and-effect relationship exists. Nonetheless, our results support the idea that oil extraction may have triggered the earthquakes.”
Oil extraction at Horse Hill was halted in October after a court decision revoked planning permission, ruling that the full climate implications of the project should have been evaluated before it was approved by Surrey County Council.
Conversely, another project has been granted council approval for exploratory oil drilling in Balcombe, West Sussex. This is currently being challenged legally by a group of residents, with the case set to be reviewed by the Court of Appeal this month [January 2025].*
The process of extracting oil alters fluid pressure within the surrounding rock, which can affect pressure levels throughout extensive underground regions and potentially intersect faults (shear fractures between two blocks of rock). The rate at which this pressure change propagates depends on the rock’s permeability (how quickly fluids can flow through it). Because faults often experience significant strain, even minor changes in stress can cause the rapid rock movement that leads to earthquakes.
In their study, the researchers developed a mathematical model to forecast the number of earthquakes that might arise based on the volume of oil extracted, taking into account the two different types of rock involved.
Due to uncertainties such as the time delay between oil extraction and earthquake occurrence, the research team utilized a Bayesian machine learning tool to introduce variability in unknown factors like time lags.
Building on the foundational work of the late Dr. Rob Westerway from the University of Glasgow, the researchers incorporated transitions between two types of rock—Portland rock, which is significantly more permeable than Kimmeridge rock. This difference means that the delay between extraction and a potential earthquake could range from days to weeks.
While the earthquakes began just prior to the start of oil extraction, the researchers suggested they may have been instigated by preparatory activities, such as measuring well pressure, which could similarly affect underground pressure levels reaching the fault line.
The researchers noted that the situation was complicated by the fact that seismometers to detect ground vibrations were only installed after the initial significant earthquakes, which means that smaller tremors beforehand went unrecorded. This underscores the importance of thorough seismic monitoring at proposed oil extraction sites before any operations commence.
Co-author Professor Philip Meredith from UCL Earth Sciences stated, “Our research emphasizes the necessity of monitoring seismic activity in areas where oil extraction is planned before any operations begin. A cautious approach is essential; claiming there’s no problem without investigation can be misleading.”
“Surrey has not experienced significant seismic activity for decades, making these recent earthquakes notable. However, unusual natural events can occur, so we must consider the possibility that the timing related to oil extraction is coincidental.”
*Lead author Dr. Matthew Fox has served as an expert witness in this case.
