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A shallow earthquake swarm close to hydrocarbon activities: Discriminating between natural and induced causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence

A shallow earthquake swarm close to hydrocarbon activities: Discriminating between natural and induced causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence
A shallow earthquake swarm close to hydrocarbon activities: Discriminating between natural and induced causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence
Earthquakes induced by subsurface industrial activities are a globally emotive issue, with a growing catalog of induced earthquake sequences. However, attempts at discriminating between natural and induced causes, particularly for anomalously shallow seismicity, can be challenging. An earthquake swarm during 2018–2019 in southeast England with a maximum magnitude of ML 3.2 received great public and media attention because of its proximity to operating oilfields. It is therefore vital and timely to provide a detailed characterization of the earthquake sequence at present, and to decide based on current evidence, whether the earthquakes were likely natural or induced. We detected 168 low‐magnitude earthquakes and computed detailed source parameters of these events. Most earthquakes occurred at a shallow depth of 2.3 km, >1  km deeper than the geological formations targeted by the oilfields, and laterally >3  km away from the drill sites. We combine the east–west‐trending cluster of the seismicity with 2D seismic reflection profiles to find the causative fault system for the earthquakes. A b‐value close to unity and strike‐slip faulting mechanisms are consistent with tectonic reactivation along a pre‐existing fault. Overall, we find no indicators in the earthquake parameters that would strongly suggest an induced source. Nor do we find any clear trends between seismicity and drilling activities based on operational logs provided by the operators. Injected volumes are near zero and monthly production amounts are many orders of magnitude smaller than other reported cases of extraction‐induced seismicity. On balance, and based on the available evidence, we find it currently unlikely that nearby industrial activities induced the seismic swarm. Most likely, the Surrey earthquakes offer a uniquely detailed insight into shallow seismicity within sedimentary basins. Nevertheless, self‐reporting of injection and production times and volumes by operators, and the lack of easily and publicly available oilfield operational data continues to be a point of concern for local residents.
0895-0695
2095-2110
Hicks, Stephen P.
036d1b3b-bb7a-4a22-b2ce-71618a1723a3
Verdon, James
a0bcb7a3-01cb-4375-8aad-7e687f0a872b
Baptie, Brian
01e042e3-d550-4762-87f0-ebe39f56339a
Luckett, Richard
2e8f57e4-8f73-4526-84de-dd072a3b80fb
Mildon, Zoë K.
e8229941-7034-4ad3-8785-df7dd159dc6a
Gernon, Thomas
658041a0-fdd1-4516-85f4-98895a39235e
Hicks, Stephen P.
036d1b3b-bb7a-4a22-b2ce-71618a1723a3
Verdon, James
a0bcb7a3-01cb-4375-8aad-7e687f0a872b
Baptie, Brian
01e042e3-d550-4762-87f0-ebe39f56339a
Luckett, Richard
2e8f57e4-8f73-4526-84de-dd072a3b80fb
Mildon, Zoë K.
e8229941-7034-4ad3-8785-df7dd159dc6a
Gernon, Thomas
658041a0-fdd1-4516-85f4-98895a39235e

Hicks, Stephen P., Verdon, James, Baptie, Brian, Luckett, Richard, Mildon, Zoë K. and Gernon, Thomas (2019) A shallow earthquake swarm close to hydrocarbon activities: Discriminating between natural and induced causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence. Seismological Research Letters, 90 (6), 2095-2110. (doi:10.1785/0220190125).

Record type: Article

Abstract

Earthquakes induced by subsurface industrial activities are a globally emotive issue, with a growing catalog of induced earthquake sequences. However, attempts at discriminating between natural and induced causes, particularly for anomalously shallow seismicity, can be challenging. An earthquake swarm during 2018–2019 in southeast England with a maximum magnitude of ML 3.2 received great public and media attention because of its proximity to operating oilfields. It is therefore vital and timely to provide a detailed characterization of the earthquake sequence at present, and to decide based on current evidence, whether the earthquakes were likely natural or induced. We detected 168 low‐magnitude earthquakes and computed detailed source parameters of these events. Most earthquakes occurred at a shallow depth of 2.3 km, >1  km deeper than the geological formations targeted by the oilfields, and laterally >3  km away from the drill sites. We combine the east–west‐trending cluster of the seismicity with 2D seismic reflection profiles to find the causative fault system for the earthquakes. A b‐value close to unity and strike‐slip faulting mechanisms are consistent with tectonic reactivation along a pre‐existing fault. Overall, we find no indicators in the earthquake parameters that would strongly suggest an induced source. Nor do we find any clear trends between seismicity and drilling activities based on operational logs provided by the operators. Injected volumes are near zero and monthly production amounts are many orders of magnitude smaller than other reported cases of extraction‐induced seismicity. On balance, and based on the available evidence, we find it currently unlikely that nearby industrial activities induced the seismic swarm. Most likely, the Surrey earthquakes offer a uniquely detailed insight into shallow seismicity within sedimentary basins. Nevertheless, self‐reporting of injection and production times and volumes by operators, and the lack of easily and publicly available oilfield operational data continues to be a point of concern for local residents.

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2018SurreyEarthquakes_SHicks_acceptedpreprint_090719 - Accepted Manuscript
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e-pub ahead of print date: 28 August 2019
Published date: 1 November 2019

Identifiers

Local EPrints ID: 436057
URI: http://eprints.soton.ac.uk/id/eprint/436057
ISSN: 0895-0695
PURE UUID: 368d10dd-21eb-4fcb-9d91-ca0b8683d2b1
ORCID for Stephen P. Hicks: ORCID iD orcid.org/0000-0002-7476-3284

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Date deposited: 27 Nov 2019 17:30
Last modified: 28 Aug 2020 04:01

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