| Seismic safety
Mapping seismic risk
Low risk countries such as the UK still need to take potential seismic activity into consideration when designing infrastructure such as dams. That’s why recent efforts focused on updating national seismic hazard maps
ALTHOUGH THE UK MAY be a country with low levels of seismic risk, national seismic hazard maps are still important for providing guidance when designing earthquake-resilient structures. And this is why existing national seismic maps published 18 years ago were recently updated. Ilaria Mosca is a Seismic Hazard Researcher at the British Geological Survey and has been working for British Dam Society (BDS) since 2013. As she explained at a recent BDS meeting, work was carried out between 2018 and 2020 to update existing UK hazard maps that have been published by Musson and Sargeant in 2007. Funding for the project was provided by the Institution of Civil Engineers and the British Geological Survey. It was considered to be the appropriate time to carry out the work as significant advances have been made in probabilistic seismic hazard assessment since publication of the previous maps. In addition, updated maps were also required to provide guidance for the revised edition of Eurcode 8. Eurcode 8 is a European building code for seismic
requirements when designing engineering structures, which is to be published in 2026. And although the UK is a low seismicity region and the design seismic action is not required for standard residential and commercial buildings, it is recommended for buildings with high economic, social, and environmental consequences such as dams.
Hazard Maps A seismic hazard map shows the distribution of
earthquake shaking levels that have a certain probability of occurring in a region or country. National hazard maps are important, even for countries with low seismic level such as the UK, because they provide a baseline model to discriminate between tectonic and any induced seismicity due to operations if it occurs. For example, in Lancashire, 58 earthquakes were linked to fluid injection during hydraulic fracturing in 2011. The maps also indicate the first order approximation level of seismic hazard for engineering structures, including dams, and help to identify regions of high seismic hazard to aid site-specific hazard assessment. The updated 2020 national seismic hazard model
uses an up-to-date earthquake catalogue for the British Isles, for which the completeness periods have been reassessed. It also uses a modified version of the 2007 source model and incorporates some advances in ground motion modelling since then, including host- to-target adjustments for the ground motion models selected in the logic tree
For the first time, the national maps for the UK are
provided for not only peak ground acceleration but also spectral acceleration at 0.2 s and 1.0 s for 5% damping on rock and the four return periods (i.e. 95, 475, 1100, and 2475 years).
Low risk As the UK is far from any plate boundaries, it is
classified as an intraplate region with low levels of seismicity. The largest historical earthquake was back in June 1931 – a 5.0Mw instrumentally recorded earthquake that occurred in the North Sea. There’s no evidence of surface ruptures produced by known earthquakes across the country and although earthquakes of over 6.0Mw are expected to be rare in the UK, there is concern that moderate earthquakes of over 4.0Mw could generate potentially damaging ground motion. Indeed, the updated maps confirm that seismic
hazard is generally low in the UK and is slightly higher in North Wales, the England-Wales border region, and western Scotland.
Although national seismic hazard maps provide general indication of the relative level of seismic hazard across the UK, it should also be noted that they are not a substitute for a site-specific hazard analysis. To increase the visibility of the updated national seismic hazard maps they were made accessible and available via a dedicated webpage, downloadable data, models and outputs, interactive mapping tools, linkages with professional bodies and industry, as well as public seminars, webcasts, and attendance in scientific conferences.
More information
For more details visit
www.bgs.ac.uk/map-viewers/ geoindex-onshore/
Above: Seismic risks may be low in the UK but national seismic hazard maps are still important for helping to design earthquake-resilient structures such as dams
References
Advancing UK seismic safety: exploring the 2020 hazard maps, piezometer challenges, and soil dispersion insights. Event organised by The British Dam Society. 28 April 2025.
www.ice.org.uk/events/ recorded-lectures/bds-agm- and-presentations-advancing- uk-seismic-safety
Above: View over Snowdonia National Park in North Wales. According to the updated 2020 national seismic hazard maps, for a return period of 2475 years, the highest hazard values (0.25g for PGA and 0.47g for SA0.2s) are observed around Snowdonia
www.waterpowermagazine.com | December 2025 | 35
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