Seismic Zone UK — BS EN 1998-1 UK NA · agR and kh by region
BS EN 1998-1:2004 (Eurocode 8 Part 1) together with its UK National Annex sets the earthquake actions framework for the United Kingdom. The reference peak ground acceleration on type A ground, agR, is very low across the country compared to Mediterranean or Pacific Rim nations: typically below 0.02 g for most of England and under 0.04 g even in the more active zones of North Wales, central Scotland and the English Midlands. The UK NA confirms that the product agR·S below 0.05 g allows the designer to skip seismic design for ordinary structures. This tool returns agR values by region and the pseudo-static horizontal coefficient kh where seismic checks are warranted, for example on tall retaining walls, nuclear and COMAH-regulated sites.
What it is and when it applies
BS EN 1998-1 applies to structures in seismic regions within the Eurocode framework. The UK National Annex recognises that British seismicity is low and that Category I buildings (ordinary structures) on firm ground typically do not need explicit seismic design. Nevertheless, earthquake actions must be considered for: nuclear installations (ONR SAPs, BSL), oil and gas terminals under COMAH, major dams (ICE Reservoirs Act 1975 and BDS Engineering Guide to Seismicity 2014), offshore platforms (ISO 19901-2), and certain tall structures of national importance. For geotechnical design where a check is required, agR is used directly to build the pseudo-static horizontal coefficient: kh = 0.5·agR/g for retaining walls and slopes that tolerate limited Newmark displacement (10-25 cm per Hynes-Griffin & Franklin 1984), and kh = agR/g for rigid structures.
Governing equations
Design ground acceleration (EN 1998-1 Clause 3.2.1):
ag = γI · agR, where γI is the importance factor (0.8 to 1.4) and agR is from the UK NA map
Pseudo-static horizontal coefficient:
kh = 0.5·(agR/g) → retaining walls and slopes with acceptable deformation
kh = 1.0·(agR/g) → rigid structures with no tolerated deformation
kv = 0 (typical) or kv = ±0.5·kh (near-fault projects, rarely required in UK)
Surface acceleration with soil factor (EN 1998-1 Table 3.2):
amax = S · agR, with S by ground type: A=1.0, B=1.2, C=1.15, D=1.35, E=1.4
No-seismic-design threshold (UK NA):
If agR · S < 0.05 g, seismic design actions may be omitted for ordinary structures
Worked example
| Parameter | Value |
|---|---|
| Location | Snowdonia area, North Wales — higher UK hazard |
| agR | 0.03 g |
| Ground type D (loose-to-medium soils) | S = 1.35 |
| Use | Retaining wall with acceptable deformation |
| Pseudo-static kh | 0.5 × 0.03 = 0.015 |
| Surface amax | 1.35 × 0.03 = 0.041 g |
| UK NA check (agR·S < 0.05 g) | 0.041 < 0.05 → seismic design may be omitted |
In this example the product agR·S equals 0.041 g, below the UK National Annex 0.05 g threshold. Seismic actions may therefore be omitted for an ordinary Category II retaining wall following Eurocode 8 logic, and design proceeds under BS EN 1997-1 (Eurocode 7) using only static earth pressure (Rankine, Coulomb or numerical methods). If the same site hosted a COMAH-regulated tank farm or a nuclear safety-related wall, seismic checks would still apply via the relevant sector standard regardless of the UK NA threshold. Even where seismic design is omitted, a qualitative sensitivity study is good practice for walls above 6 m supporting critical infrastructure.
Result: North Wales · agR = 0.03 g · kh = 0.015 · amax = 0.041 g · below UK NA 0.05 g threshold → static design only
Regional hazard table (BS EN 1998-1 UK NA)
| agR | Region | Notes |
|---|---|---|
| < 0.02 g | Most of England, Northern Ireland, lowland Scotland | London, Birmingham, Manchester, Belfast, Edinburgh — seismic design typically omitted |
| 0.02-0.04 g | North Wales, central Scotland, English Midlands | Snowdonia, Highlands border, Shrewsbury area — historical M4-5 events |
| 0.04-0.08 g | Colchester area, Dover Straits, parts of Cornwall | Colchester 1884 M4.6 caused damage to 1,200 buildings; most hazardous UK earthquake of recent history |
Interpretation of results
The United Kingdom sits in a stable continental interior within the Eurasian Plate, far from any active plate boundary. Seismicity is driven by reactivation of pre-existing Caledonian and Variscan fault systems under current stress conditions, producing mainly M3-5 events with rare M5+ earthquakes. Notable historical events include Dogger Bank 1931 (M6.1, offshore), Colchester 1884 (M4.6 onshore, highest recent damage), Carlisle 1979 (M4.7), and Folkestone 2007 (M4.3). For most ordinary buildings, retaining walls and foundations, EN 1998-1 seismic design is not required under the UK NA 0.05 g rule. Specific sectors override this: nuclear (ONR SAPs require deterministic 10,000-year hazard assessments with agR sometimes reaching 0.25 g at Hinkley Point and Sizewell through site-specific PSHA), oil and gas (COMAH), and large dams (BDS 2014 requires seismic checks for Category A dams regardless of regional agR). For offshore wind foundations in the southern North Sea, ISO 19901-2 earthquake actions apply.
Reference standards
- BS EN 1998-1:2004+A1:2013 — Eurocode 8: Design of structures for earthquake resistance, Part 1
- NA to BS EN 1998-1:2004 — UK National Annex to Eurocode 8 Part 1
- BS EN 1997-1:2004 — Eurocode 7: Geotechnical design, Part 1
- ONR SAPs 2020 — Office for Nuclear Regulation, Safety Assessment Principles
- BDS 2014 — British Dam Society Engineering Guide to Seismicity for Dams
- BGS 2020 — British Geological Survey, UK earthquake catalogue and hazard map
- Hynes-Griffin & Franklin (1984) — Rationalizing the seismic coefficient method
Frequently asked questions
Does the UK really have earthquakes?
Yes, around 200-300 tectonic earthquakes are recorded by the British Geological Survey every year, most below magnitude 3 and not felt. Historically, Colchester 1884 (M4.6) damaged about 1,200 buildings, Dogger Bank 1931 (M6.1, offshore) was felt across northern Europe, and Market Rasen 2008 (M5.2) was the largest onshore event in 25 years. Seismicity is low but real, particularly along Caledonian and Variscan structural zones.
What does the UK NA 0.05 g threshold mean?
The UK National Annex to BS EN 1998-1 states that when the product agR·S at the site is less than 0.05 g, seismic actions may be omitted for ordinary Category II structures. This applies to most of the UK land area when combined with rock or stiff ground. However, the threshold does not exempt nuclear installations, COMAH sites, major dams, or any structure where the owner or regulator requires seismic resilience.
How is nuclear seismic design different?
The Office for Nuclear Regulation (ONR) SAPs require Design Basis Earthquake (DBE) assessment to a 10,000-year return period, evaluated through site-specific PSHA combined with DSHA for relevant scenarios. Typical DBE PGA values at UK nuclear sites range from 0.10 g (Hunterston) to 0.25 g (Hinkley Point C). This is far above the UK NA threshold and requires full dynamic analysis, liquefaction screening and soil-structure interaction modelling regardless of the regional EN 1998-1 map.
Is BS EN 1998-1 enough for geotechnical design in the UK?
For ordinary projects on stiff ground where agR·S < 0.05 g, EN 1998-1 is effectively superseded by the UK NA allowance and static Eurocode 7 design is sufficient. For higher-hazard projects, EN 1998-1 gives agR and the ground type framework. Geotechnical design complements it with EN 1998-5 (foundations, retaining structures, geotechnical aspects), project-specific Mononobe-Okabe analysis, and for critical infrastructure full nonlinear dynamic FEM.