The 2018-2019 Surrey Earthquake Swarm: A Comprehensive Analysis of the UK’s Most Significant Seismic Sequence

A Comprehensive Analysis of the UK’s Most Significant Seismic Sequence

The British Geological Survey recorded an unprecedented sequence of seismic events between April 2018 and May 2019 that would become known as the “Surrey earthquake swarm.” This series of tremors, centered near the village of Newdigate in Mole Valley, represented the first recorded earthquake activity in Surrey for over 268 years, catching both residents and scientists by surprise . The largest event, registering a magnitude of 3.2, was felt across a wide area including Dorking, Horley, Charlwood, and even reaching into West Sussex towns like Crawley and Horsham . For a region unaccustomed to such geological disturbances, the swarm naturally raised significant public concern, particularly given its proximity to active oil extraction sites at Brockham and Horse Hill . The scientific community was presented with a compelling question: were these tremors a natural phenomenon, or had nearby industrial activity triggered this unusual seismic sequence? This article provides a comprehensive examination of the Surrey earthquake swarm, exploring its characteristics, the scientific investigation into its origins, and the wider implications for understanding seismic activity in the United Kingdom.

Understanding the “Surrey Swarm”: A Timeline of Tremors

The earthquake sequence began on Easter Sunday, April 1, 2018, with a magnitude 2.7 tremor that struck near Newdigate . This initial event was followed by two smaller quakes within the same day, registering magnitudes of 1.8 and 1.7 . Over the following months, the seismic activity continued in clusters, with residents experiencing tremors on multiple occasions. June 2018 saw two significant events: a 2.6 magnitude quake on June 27 felt in Newdigate, Charlwood, and Crawley, and a 2.4 magnitude tremor just two days later . The pattern of activity escalated in July with a 3.0 magnitude quake on July 5, marking the largest event in the sequence at that time . The most powerful earthquake of the entire swarm occurred on February 27, 2019, registering a magnitude of 3.1 and generating a remarkable 1,251 reports from members of the public who felt the shaking . The British Geological Survey received reports describing the experience as ranging from feeling “as if a large lorry had crashed into the side of the house” to experiencing “a loud bang woke me up” and “windows and doors shook” . In total, the swarm comprised 34 distinct earthquakes detected between April 2018 and May 2019, with the British Geological Survey’s more sensitive instruments detecting a total of 168 low-magnitude events during this period . The shallow depth of these tremors, typically around 2 to 2.5 kilometers below the surface, meant that even relatively modest magnitude events were felt strongly by local residents .

The Scientific Investigation: Distinguishing Natural from Induced Seismicity

One of the primary concerns raised by local communities and experts alike was whether the Surrey earthquake swarm could be linked to nearby oil extraction activities. The Brockham and Horse Hill oil sites, located within 10 kilometers of the epicentral area, had been operating in the region, and some academics had speculated that the timing of the tremors coincided with periods of activity at these sites . To address this critical question, a comprehensive study was undertaken by researchers from Imperial College London, the University of Bristol, and the British Geological Survey, with results published in the journal Seismological Research Letters . The research team installed a network of temporary broadband seismometer stations in the epicentral region to complement existing BGS sensors and RaspberryShake instruments installed in residents’ homes, which provided crucial high-precision data . By examining multiple parameters of the earthquakes, the scientists systematically assessed the likelihood of an induced origin.

The investigation focused on four key areas: the direction of fault movement, the depth of the earthquakes, the timing of events relative to oil extraction activities, and the spatial relationship between the tremors and the drilling sites . The direction of rock movement during an earthquake provides a crucial fingerprint of its origin. Most natural earthquakes in the United Kingdom occur along pre-existing faults where the rocks on either side of the fault line move horizontally, known as strike-slip faulting. In contrast, earthquakes induced by oil extraction typically exhibit vertical movement along faults . The research team found that the Surrey swarm earthquakes predominantly displayed strike-slip mechanisms, indicating that they were consistent with tectonic reactivation of ancient faults rather than human-induced activity . The depth analysis provided further compelling evidence against an induced origin. The research team determined that the majority of the Surrey earthquakes occurred at a depth of approximately 2.3 to 2.5 kilometers, significantly deeper than the geological formations from which oil is extracted, which are located less than 1 kilometer below the surface . This depth discrepancy represents a fundamental inconsistency with the oil extraction hypothesis, as any causal link would require the drilling activity to affect rock formations considerably deeper than the targets being exploited .

The timing of the seismic events was also critically examined. The first cluster of earthquakes occurred in April 2018, which was long after the oil extraction tests conducted in 2016 and well before further extended tests began in July 2018 . This temporal disconnect, together with the fact that the injected volumes of fluid at the oil sites were near zero and production amounts were orders of magnitude smaller than other documented cases of extraction-induced seismicity, led the research team to conclude that the oil extraction was highly unlikely to be the cause . The spatial distribution of the earthquakes also argued strongly against an induced source. Rather than clustering around the extraction sites, the tremors were distributed in a tight area more than 3 kilometers away from the drilling operations . Dr. Stephen Hicks, the lead author of the study from Imperial College London, noted that “it would be unprecedented for this type and scale of oil extraction to affect sites more than a kilometre away” . Based on this comprehensive analysis, the researchers concluded that the Surrey swarm was almost certainly a natural phenomenon, with the proximity to oil extraction sites being a coincidence . Dr. Hicks further emphasized this conclusion by stating, “If oil extraction caused the earthquakes, then it did so by a mechanism that hasn’t yet been reported anywhere else in the world” .

The Tectonic Context: Why Did This Occur in the UK?

The United Kingdom is not typically associated with significant earthquake activity in the public consciousness, as it does not lie along the active plate boundaries that generate the world’s most devastating seismic events. However, the British Isles are subject to ongoing tectonic stresses that result in a consistent background level of seismicity . The British Geological Survey typically records approximately 20 to 30 earthquakes each year that are felt by people, with the vast majority being magnitude 3 or below and causing no damage . The largest recorded earthquake in British history was a magnitude 6.1 event at Dogger Bank in the North Sea in 1931, which was powerful enough to damage properties on the east coast despite being 60 miles offshore . The tectonic stresses affecting the UK primarily originate from two distant sources: the ongoing collision between the African and Eurasian tectonic plates in the Mediterranean Sea, which acts as the UK’s nearest plate boundary, and the Mid-Atlantic Ridge, where the Eurasian and North American plates are slowly diverging . This complex interplay of forces, driven by plate tectonics hundreds of miles away, generates stress throughout the European crust, which is occasionally released along ancient fault lines that crisscross the British Isles. These faults, often formed during past geological epochs, can remain dormant for thousands of years before being reactivated by accumulated stress. The Surrey swarm, according to the researchers, was most likely the result of such a natural reactivation of a pre-existing fault system, with the ongoing tectonic pressure causing a series of small slips along the fault .

Conclusion: Understanding Seismic Risk and the Need for Vigilance

The Surrey earthquake swarm provided a rare and valuable opportunity for scientists to study seismic activity in a region with low background seismicity and to explore the complex challenge of discriminating between natural and induced earthquakes. The comprehensive scientific investigation ultimately established that the sequence was natural, a conclusion supported by multiple lines of evidence including the horizontal movement along faults, the depth of the tremors, and the temporal and spatial disconnect from nearby oil extraction activities. This resolution, however, does not diminish the importance of continued research and monitoring. As Dr. Hicks noted, the researchers are continuing to monitor quakes in the area for the foreseeable future, recognizing that “the more data we have, the more we’ll know about the causes and effects of these earthquakes” . The Surrey swarm highlights the importance of maintaining and expanding robust seismic monitoring networks across the United Kingdom, especially in areas where industrial activities coincide with geological structures capable of producing felt earthquakes. Furthermore, the study underscores the need for transparent and publicly accessible data on industrial operations, as self-reporting by operators and the lack of easily available oilfield operational data continued to be a point of concern for local residents, even though the study concluded that the earthquakes were natural . While the Surrey swarm remains an intriguing geological episode rather than a cause for alarm, it serves as a reminder that even in regions far from major plate boundaries, the Earth’s crust is dynamic and subject to change. For residents of Surrey and surrounding areas, the tremors of 2018-2019 have left a lasting impression, transforming the understanding of their region’s geological character and reinforcing the importance of rigorous scientific investigation in addressing public concerns about industrial activity and seismic risk.

Frequently Asked Questions

1. Was the Surrey earthquake swarm caused by oil extraction?

No, a comprehensive scientific study by researchers from Imperial College London, the University of Bristol, and the British Geological Survey concluded that the Surrey swarm occurred naturally . The study found no direct link between the earthquakes and nearby oil extraction activities at Horse Hill and Brockham. The quakes occurred more than 3 kilometers from the extraction sites, deeper than the targeted oil-bearing formations, and moved faults horizontally rather than vertically, which is characteristic of natural UK earthquakes . Dr. Stephen Hicks, lead author of the study, stated that any causal link would be “unprecedented” and would require a mechanism not yet reported elsewhere .

2. How many earthquakes were recorded in the Surrey swarm?

The British Geological Survey detected 34 distinct earthquakes during the swarm period from April 2018 to May 2019 . However, using more sensitive temporary seismometers installed in the affected area, researchers detected a total of 168 small-magnitude earthquakes over this period . Many of these smaller events were not felt by residents but provided crucial data for the scientific investigation into the swarm’s origin.

3. What was the largest earthquake recorded in the Surrey swarm?

The largest earthquake in the Surrey swarm was a magnitude 3.2 event . Some reports also refer to a magnitude 3.1 earthquake on February 27, 2019, as the largest felt event . This quake generated significant public interest, with the British Geological Survey receiving 1,251 reports from people who experienced shaking, described as strong enough to make “entire sections of all buildings tremble” .

4. Why was the Surrey earthquake swarm unusual?

The Surrey swarm was unusual because it represented the first recorded seismic activity in the county for more than 268 years, as far back as British Geological Survey records go . Additionally, southeast England is one of the least seismically active regions in the UK, making the occurrence of such a dense sequence of shallow tremors highly notable . The shallow depth of the earthquakes, around 2 to 2.5 kilometers, meant that even moderate magnitude events were felt by a wide population .

5. What causes earthquakes in the UK?

Earthquakes in the UK are caused by ongoing tectonic stresses acting on pre-existing faults in the Earth’s crust . The primary sources of this stress are the collision between the African and Eurasian tectonic plates in the Mediterranean Sea and the divergence of the Eurasian and North American plates at the Mid-Atlantic Ridge . These distant plate boundaries generate stresses throughout the European crust, which are occasionally released along ancient faults within the British Isles. The Surrey swarm was most likely caused by such a natural reactivation of a fault system under these long-term tectonic stresses .