Oil exploitation blamed for causing 100 earthquakes in UK

© Ivan/Pexels

In the peaceful county of Surrey, residents of Newdigate saw their homes shake repeatedly in 2018 and 2019. The BBC had also reported it in this article, describing one of them, dated February 27, 2019, as ” one of the strongest earthquakes to have shaken the region since the start of this series last year was felt as far as Surrey and parts of Sussex “.

These earthquakes, which have fueled concerns among the local population, were only the beginning of a scientific controversy that would extend over several years. Eyes quickly turned to nearby oil fields, suspected of causing these unusual earthquakes.

Newdigate is a typical English village of 19.18 km², located south of London. © Screenshot/Google Maps

What lies beneath the English fields ?

The series of earthquakes that shook southern England has some particularly intriguing features. More than a hundred tremors, with magnitudes ranging from 1.34 to 3.18 on the Richter scale, were recorded in a relatively small area. Relatively small values, ranging from micro-earthquakes to so-called minor earthquakes, which can be felt by a few people, especially if they are at rest or on a high floor of a building.

The epicenter of these seismic events was located between 5 and 10 km from the Horse Hill drilling site, operated by the UK Oil & Gas company. This geographical proximity immediately raised questions within the scientific community.

The British Geological Survey (BGS), a leading body in geological monitoring, had initially concluded that the phenomena were natural. This initial analysis was initially based on the historical rarity of earthquakes in these locations. Indeed, the Newdigate region is located in a relatively stable geological area, far from the large subduction zones or major active faults (see our article on tectonic forces) which are often the cause of larger earthquakes.

Secondly, the apparent absence of direct temporal correlation between mining activities and tremorshad not aroused suspicion. However, this interpretation of the geological data was soon to be challenged by a new methodological approach.

A new look at the Surrey earthquakes

It was researchers Matthew Fox and Philip Meredith of University College London who proposed a new reading of these apparently unrelated events. Using a million computer simulations, they were able to dissect what was going on in the bowels of Surrey, layer by layer, to understand how the subsoil was responding to oil extraction.

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The subsoil of Surrey is like a kind of natural millefeuille, alternating two types of rock with radically different behaviors. On one side, a limestone called Portland stone, a real mineral sponge riddled with tiny cavities and its porosity that allows fluids and pressures to circulate quickly. On the other, Kimmeridge clay, dense and impermeable, which acts as a natural barrier, considerably slowing down these same movements.

This particular geological configuration explains why previous studies had not detected the link between oil extraction and earthquakes. In areas made up of Portland stone, the effect is almost instantaneous: remove oil, and the underground pressure changes immediately, like when you squeeze a sponge. In the Kimmeridge clay areas, however, it's a different story: pressure changes propagate slowly, creating a time lag between extraction and earthquakes.

Fox explains: “I think that increases the likelihood that oil extraction led to the earthquakes. The correlation between these two phenomena – between oil extraction and seismicity – is quite strong, and this suggests that there is a link ».

Natural faults or boreholes ?

Professor Stuart Haszeldine of the University of Edinburgh, whose independent research corroborates Fox and Meredith's findings, points to the existence of a  clear correlation between mining activities and earthquakes ». This convergence of analyses strengthens the probability of a causal link, even if some aspects still require in-depth statistical studies.

Faced with these elements, UK Oil & Gas obviously kicked the issue into touch. A company spokesperson told New Scientist: ” This incident was dismissed years ago when BGS seismologists decided it was a natural phenomenon, attributed to the movement of a deep, distant fault unrelated to the site .”

A fragile and insufficient defence, since the combined pressure of scientific discoveries and citizen mobilisation has brought about concrete changes. Following legal action brought by environmental activists, supported by Friends of the Earth; an American NGO; Surrey County Council withdrew the operating licence for the Horse Hill site last October. Solid scientific evidence and equally solid citizen engagement were therefore enough to make UK Oil & Gas leave the Surrey region. A company already affected by several controversies, notably horizontal drilling and hydraulic fracturing, techniques highly contested because of their impact on ecosystems and human health.

• A series of unusual earthquakes in Surrey has raised suspicions of a link to nearby oil drilling by UK Oil & Gas.
• New research has shown a probable link between drilling and tremors, caused by the complex geology of the region.
• Under scientific and citizen pressure, the Horse Hill drilling site has had its operating permit withdrawn.

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