Seismic hazard assessment for the UK offshore Exclusive Economic Zone
Mosca, I.; Baptie, B.; Haslam, R.; Gafeira, J.; Jenkins, G.. 2024 Seismic hazard assessment for the UK offshore Exclusive Economic Zone. Nottingham, UK, British Geological Survey, 168pp. (OR/24/012) (Unpublished)
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Abstract/Summary
This report presents the new seismic hazard maps for the UK offshore Exclusive Economic Zone (EEZ) developed using a Monte Carlo-based approach. These are intended to update the current maps for UK waters published in 2002. The work done by the BGS team in this project has been informed at key stages by external experts who have reviewed the main components of the seismic hazard model. The analysis is based on a comprehensive catalogue of earthquake activity across the region developed by combining existing earthquake catalogues and data from regional and local monitoring agencies. Anthropogenic events such as those related to blasting, underwater explosions, mining and hydrocarbon exploration have been removed where possible. We also remove foreshocks and aftershocks to ensure that the catalogue only contains time-independent events. We then use published empirical magnitude conversion relationships to convert different magnitudes to moment magnitude (Mw) to ensure that the catalogue has consistent and homogeneous magnitudes across the region of interest. Finally, we assess catalogue completeness for different time intervals using both published information for the earthquake catalogues from the region and by examining cumulative and annual numbers of earthquakes for specific magnitudes. We model earthquake occurrence across the region using a seismic source characterisation (SSC) model that consists of a series of zones, where seismicity is considered to be homogeneous. The shape and extent of each zone are based on knowledge of the tectonics, geology and seismicity of the study area. We use four different seismic source zone models within the SSC to capture the epistemic uncertainty in different rupture scenarios but we do not include specific fault sources in the SSC model because of the difficulty relating earthquakes to particular faults. We estimate the rate of earthquake occurrence for each seismic source zone using the earthquake catalogue derived for the study. A logic tree approach was used to account for the epistemic uncertainty in earthquake activity rates, maximum magnitude, earthquake depth distribution, and faulting style. Ground motions are estimated for different rupture scenarios using a ground motion characterisation (GMC) model that consists of five recently published multiple ground motion prediction equations (GMPEs) considered to be applicable to the region. The GMPEs are included in a logic tree where the weights are informed by the fit between observed and modelled ground motions. The GMC model also includes the host-to-target adjustments (HTTAs) and a single-station sigma model. We calculate the hazard using Monte Carlo-based simulations to generate artificial catalogues by random sampling of the probability distributions in the SSC model. This follows the same methodology used for the latest national seismic hazard maps for the UK. Hazard is calculated at individual points spaced at 0.125° in latitude and 0.25° in longitude for peak ground acceleration (PGA) and spectral acceleration at 0.2 s (SA0.2 s) and 1.0 s (SA1.0 s) for 5% damping and Vs30 = 800 m/s (rock conditions) as a proportion of g and for the return periods of 95, 475, 1100, 2475, and 5000 years. This is the first time that maps of the seismic hazard at short (0.2 s) and long periods (1.0 s), which are particularly relevant for offshore structures, have been produced for UK waters. Uniform hazard spectra are calculated for two offshore carbon capture and storage sites in the North Sea (Endurance storage site, the Acorn area) and the HyNet North West in the southeast Irish Sea and a disaggregation of the hazard for these sites has also been undertaken to identify the earthquakes that control the hazard for key return periods. For a return period of 475 years, the PGA hazard is lower than 0.04 g for much of the UK offshore EEZ, except for the Irish Sea close to North Wales, the northern North Sea and the southern North Sea. The hazard is up to 0.06 g in the region offshore North Wales, 0.09 g in the northern North Sea, and 0.06 g in the southern North Sea. A similar spatial pattern in the hazard is observed at 0.2 s with the highest hazard in the northern North Sea (0.20 g) with more pronounced variations. At 1.0 s, the hazard is less than 0.03 g and there is little variation across the UK offshore EEZ. For 2475 years, the northern North Sea, the offshore area close to western Scotland, the Irish Sea close to Wales, and the southern North Sea are the areas of highest hazard for PGA and SA0.2 s. The highest hazard values (0.24 g for PGA and 0.53 g for SA0.2 s) are observed in the northern North Sea. The Acorn site has the lowest hazard of the three carbon capture and storage license areas with a PGA hazard value of 0.05 g for a 2475-year return period. The PGA hazard values at Endurance and the HyNet are 0.14 g and 0.09 g for the same return periods. For SA0.2 s the hazard values for the 2475-year return period increase to 0.12 g at Acorn, 0.31 g at Endurance, and 0.20 g at HyNet. The uniform hazard spectra for return periods of 475 years and 2475 years show the hazard values for different periods of ground motion with an equal probability of exceedance and demonstrate that the hazard values peak between periods of 0.1 to 0.4 s. The comparison between the maps for the 475-year return period from this study and the previous maps shows a good agreement although the input data and the SSC and GMC models used for the calculations between the two studies are different. It is important to note that these seismic hazard maps are not a substitute for site-specific hazard assessment and high-consequence-of-failure installations (designated CC4-Highest in the new edition of EN1990, 2002) in particular. The user must take responsibility for checking that use of the results contained in this report is appropriate for the case in question. The products of this project will be accessible to the public through a dedicated webpage (http://www.earthquakes.bgs.ac.uk/hazard/UKhazard.html) and an interactive mapping tool (https://www.bgs.ac.uk/map-viewers/geoindex-offshore/). The computer code can be made available on request to ukeqs@bgs.ac.uk.
| Item Type: | Publication - Report |
|---|---|
| Funders/Sponsors: | Industrial Decarbonisation Research and Innovation Centre, British Geological Survey |
| Additional Information. Not used in RCUK Gateway to Research.: | This item has been internally reviewed, but not externally peer-reviewed. |
| Related URLs: | |
| Date made live: | 09 Sep 2024 13:26 +0 (UTC) |
| URI: | https://nora.nerc.ac.uk/id/eprint/537986 |
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