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Much has been written about the Newdigate swarm… Whilst it cannot be proven that there is a defined causal link between the earthquakes and oil exploration activities, this link cannot be disproven either.  And the fact that there was no observed seismic activity before drilling for hydrocarbons began at Horse Hill is, in itself, probably the strongest indicator that there is indeed a causal link.  This blog sets out the evidence.  We invite you to come to your own conclusions.

Note that many of the arguments put forward in support of the claim that the cause of the swarm is entirely natural are potentially flawed.  It is plain for all to see that a conclusion that the cause of the swarm may not be entirely natural, could add support to calls for a moratorium on all hydrocarbon exploration and extraction activity in the area.  And this could seriously harm the financial prospects of several oil and gas companies.  Then it is not surprising that the general consensus of the geological science community, most of whom derive financial support for their research from the industry (see Channel 4 report on this issue), is in support of the claim that the cause is entirely natural.  The questions are: are they truly independent and can you believe them?


As of November 2020, over 150 tremors have been registered by theBritish Geological Survey (BGS) in Newdigate, and they are continuing. The swarm started on 1 of April 2018. When a 3.0ML tremor hit a few months later, in early July 2018, the level of public interest and suspicions over links with the oil & gas activities at nearby Horse Hill and Brockham sites increased dramatically.  After our initial skepticism, the brief blog we published on 6 July 2018 on this topic remains one of our most read posts ever.  The strongest tremor measuring 3.1ML was recorded on 27 February 2019. The earthquakes caused damage to property and there remains a concern over the possibility of similarly strong tremors.

The Times Open Letter & OGA Workshop

Following the summer 2018 sequence the Times published an open letter from four leading academic geologists on both sides of the “fracking” debate, including Professor Richard Selley – a Brockham resident. The letter called for a moratorium on all drilling, re-injection and flow-testing in the affected area.

This demand was ignored, but the Oil & Gas Authority (OGA) held a workshop on 3 October 2018, including the letter signatories, other academics, regulators, and industry people, and chaired by the BGS.  Participants concluded that, based on the evidence presented, there was no causal link between the seismic events and oil and gas activity. One person disagreed. More on this below.

BGS / OGA Workshop Conclusions

The BGS used the Davis and Frohlich (1993) test to determine whether the tremors were induced. It limited its analysis to fluid injections and production activity and concluded the causes were natural because 1) injection at Brockham was only of low volumes and too far away, and 2) there had been no injection or production at Horse Hill prior to the onset of earthquakes on 1 April 2018. Here are the key sections from its report:

The case that the events were induced is primarily supported by the temporal correlation between the restarting of production at Brockham and the start of the earthquake sequence on 1 April. However, the very limited period and volume of production seems unlikely to cause a significant stress perturbation at a distance of 8 km. Oil production has been ongoing for many years in the region, without any previous seismicity. Finally, although this is a low seismicity region, even by UK standards, earthquakes are not unprecedented here.

Regarding Horse Hill, the BGS said:

Information provided to OGA by the UK Oil and Gas PLC [UKOG] and the Horse Hill operator, Horse Hill Developments Ltd [HHDL], states that pumping to increase wellbore pressure to try to get the well to flow was carried out in HH-1 on 9 July 2018. The earthquake sequence was already underway at this time. UK Oil and Gas PLC states that no fluids have been injected into the HH-1 well in 2018. This appears to rule out operations at HH-1 as a possible causative factor for the earthquake sequence.

The BGS only applied the test to fluid injection and hydrocarbon extraction activities. It said:

Strictly speaking, these [Davis and Frohlich criteria] are only applicable for seismicity induced by fluid injection. However, they can also be used to assess the evidence that the Newdigate earthquake sequence may have been triggered by fluid injection or extraction.

But could other oil & gas activities be responsible for inducing the earthquakes through a mechanism that’s not yet fully understood? And would this change the timeline to correlate with the original onset of earthquakes on 1 April 2018?

The University of Edinburgh Theory

One such mechanism is proposed by Stuart Haszeldine and Andrew Cavanagh of the University of Edinburgh, who using the same Davis Frohlich criteria, suggested to the Oct 2018 OGA workshop participants that the earthquakes were likely induced by oil and gas operations at the Horse Hill site. Building on the work of David Smythe, Emeritus Professor of Geophysics in the University of Glasgow, who demonstrated that the HH-1 well has been drilled through a fault zone, the Edinburgh group proposed that tremors are induced by a rapid de-pressurisation of the critically stressed fault zone after a period of gradual pressurisation. The issue is periodically addressed by draining off (bleeding off) the annulus after a long period of shut-in. This alters the distribution of pressure and fluid in a connected fault zone and causes a shallow earthquake. See also a follow up report to Surrey County Council in February 2019 here.

Hicks et al.

In June 2019, a study lead by Dr Stephen Hicks of Imperial College London found no clear links between the swarm of earth tremors in Surrey and local oil drilling activity. Our main issue with this paper is that it relied on operational data provided by UKOG/HHDL, which has not been made publicly available in full and, as far as we are aware, had not been verified by any of the regulators to be complete or true. Apparently, it is common practice to rely on operator data, but we believe that in a study of such high public interest this fundamental limitation should have been clearly explained, especially if the data remains off limits to anyone other than the paper’s authors. Otherwise, scientific credibility is given, without wider scrutiny, to unverified data provided by the very oil company that is suspected of causing the tremors.

Geosierra Theory

Later in 2019, another qualified voice surfaced linking the tremors with activity at Horse Hill, similarly to Stuart Haszeldine. The theory by Grant Hocking – president and founder of GeoSierra, US based energy and environmental company – is illustrated by a striking image of the coincidences between the larger tremors and activities that reduce the downhole pressure in the Portland layers in Horse Hill -1 well (HH-1).


Our Input: Documents Obtained Under The Freedom of Information Act (FOIA) 

We filed a number of Freedom of Information Act requests with the various regulators trying to uncover relevant records. The reporting and monitoring regime is very opaque, but we managed to find out some interesting details. We put a timeline together combining the FOIA documents, public records and seismic events. See it here

Further reading:

British Geological Survey: Earthquakes near Newdigate, Surrey


(First published 6 May 2019. Updated November 2020).