An exploratory study of parameter sensitivity, representation of results and extensions of PSHA: case study - United Arab Emirates

Despite the wide use of probabilistic seismic hazard analysis (PSHA) for the evaluation of seismic hazard, some degree of confusion and misunderstanding exists regarding how the hazard calculations should be performed as well as how the hazard results should be interpreted. In this thesis, different aspects of PSHA that are commonly misunderstood, as well as some new developments, are investigated. To this end, a comprehensive case study PSHA for three cities in the United Arab Emirates is carried out. Previous publications present contradictory interpretations of the earthquake threat in this country, creating confusion regarding appropriate seismic design levels. The results of this PSHA confirm low hazard levels in most of the country (UBC97, Zone 0) that increase as one moves northwards (UBC97, Zone 1). Using the case study as a point of reference, the mechanics and implications of performing hazard disaggregation when using multiple ground-motion prediction equations (GMPEs) within a logic-tree framework are investigated. Logic-tree approaches receive significant attention as different ways of representing hazard results from logic trees are discussed as well as issues associated with the identification of hazard-dominating scenarios and how these may influence the definition of scenario spectra for the selection of ground-motion records for seismic design. The sensitivity of the hazard results to key parameters in PSHA such as: the minimum magnitude deemed to be of engineering significance; the activity parameters of seismic sources; the use of alternative GMPEs and the standard deviations associated with these models; and the allocation of weights to logic-tree branches is investigated. Furthermore, recently proposed alternatives to the specification of a minimum magnitude as the criteria for identifying non-damaging earthquakes are studied. Finally, correlations between the hazard results obtained in terms of spectral accelerations and hazard results in terms of peak ground velocity and spectral intensity are explored

Probabilistic seismic hazard assessment for a new-build nuclear power plant site in the UK

A probabilistic seismic hazard analysis (PSHA) has been conducted as part of the Safety Case justification for a new-build nuclear power plant in the UK. The study followed a cost-efficient methodology developed by CH2M and associates for safety-significant infrastructure where high-level regulatory assurance is required. Historical seismicity was re-evaluated from original sources. The seismicity model considered fourteen seismic sources which, when combined, formed six alternative seismic source models. Separate models for the median ground-motion and aleatory variability were considered. The median ground-motion model comprised a suite of ground-motion equations adjusted to the site-specific conditions using VS-kappa factors. A partially non-ergodic sigma model was adopted with separate components for the inter-event variability, and single-station intra-event variability, adjusted by a partially ergodic site-to-site variability term. Site response analysis was performed using equivalent-linear random vibration theory with explicit incorporation of the variability in the ground properties using Monte Carlo simulations. The final PSHA results were obtained by convolution of the hazard at the reference rock horizon with the site amplification factors. The overall epistemic uncertainty captured by the logic tree was assessed and compared against results from earlier PSHA studies for the same site

A streamlined approach for the seismic hazard assessment of a new nuclear power plant in the UK

This article presents a streamlined approach to seismic hazard assessment aimed at providing regulatory assurance, whilst acknowledging commercial and program constraints associated with the development of safety–critical facilities. The approach was developed based on international best practice and followed the spirit of the Senior Seismic Hazard Analysis Committee (SSHAC) Level 2 requirements, while incorporating the key features of the SSHAC Level 3 process aimed at achieving regulatory assurance, but with a more flexible implementation. It has also benefited from experience gained by others regarding the implementation of the SSHAC process in projects in the USA, Switzerland and South Africa. The approach has been successfully applied as part of the Safety Case for the new-build nuclear power plant at Hinkley Point, UK. The proposed approach can be considered as a cost-effective solution for the seismic hazard evaluation of safety-significant facilities where a high level of regulatory assurance is required

An exploratory study of parameter sensitivity, representation of results and extensions of PSHA : case study - United Arab Emirates

Despite the wide use of probabilistic seismic hazard analysis (PSHA) for the evaluation of seismic hazard, some degree of confusion and misunderstanding exists regarding how the hazard calculations should be performed as well as how the hazard results should be interpreted. In this thesis, different aspects of PSHA that are commonly misunderstood, as well as some new developments, are investigated. To this end, a comprehensive case study PSHA for three cities in the United Arab Emirates is carried out. Previous publications present contradictory interpretations of the earthquake threat in this country, creating confusion regarding appropriate seismic design levels. The results of this PSHA confirm low hazard levels in most of the country (UBC97, Zone 0) that increase as one moves northwards (UBC97, Zone 1). Using the case study as a point of reference, the mechanics and implications of performing hazard disaggregation when using multiple ground-motion prediction equations (GMPEs) within a logic-tree framework are investigated. Logic-tree approaches receive significant attention as different ways of representing hazard results from logic trees are discussed as well as issues associated with the identification of hazard-dominating scenarios and how these may influence the definition of scenario spectra for the selection of ground-motion records for seismic design. The sensitivity of the hazard results to key parameters in PSHA such as: the minimum magnitude deemed to be of engineering significance; the activity parameters of seismic sources; the use of alternative GMPEs and the standard deviations associated with these models; and the allocation of weights to logic-tree branches is investigated. Furthermore, recently proposed alternatives to the specification of a minimum magnitude as the criteria for identifying non-damaging earthquakes are studied. Finally, correlations between the hazard results obtained in terms of spectral accelerations and hazard results in terms of peak ground velocity and spectral intensity are explored.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Methods for assessing the epistemic uncertainty captured in ground-motion models

A key task when developing a ground-motion model (GMM) is to demonstrate that it captures an appropriate level of epistemic uncertainty. This is true whether multiple ground motion prediction equations (GMPEs) are used or a backbone approach is followed. The GMM developed for a seismic hazard assessment for the site of a UK new-build nuclear power plant is used as an example to discuss complementary approaches to assess epistemic uncertainty. Firstly, trellis plots showing the various percentiles of the GMM are examined for relevant magnitudes, distances and structural periods to search for evidence of “pinching”, where the percentiles narrow excessively. Secondly, Sammon’s maps, including GMPEs that were excluded from the logic tree, are examined to check the spread of the GMPEs for relevant magnitudes and distances in a single plot. Thirdly, contour plots of the standard deviation of the logarithms of predicted ground motions from each branch of the logic tree (sigma_mu) are compared with plots drawn for other relevant hazard studies. Fourthly, uncertainties implied by a backbone GMM derived using the Campbell (2003)’s hybrid stochastic empirical method are compared to those of the proposed multi-GMPE GMM. Finally, the spread of the percentile of hazard curves resulting from implementing the GMM are examined for different return periods to check whether any bands of lower uncertainty in ground-motion space resulted in bands of lower uncertainty in hazard space. These five approaches enabled a systematic assessment of the level of uncertainty captured by the proposed GMM

Ground-motion models for earthquakes occurring in the United Kingdom

This article presents models to predict median horizontal elastic response spectral accelerations for 5% damping from earthquakes with moment magnitudes 3.5 to 7.25 occurring in the United Kingdom. This model was derived using the hybrid stochastic-empirical method based on an existing ground-motion model for California and a stochastic model for the UK, which was developed specifically for this purpose. The model is presented in two consistent formats, both for two distance metrics, with different target end-users. Firstly, we provide a complete logic tree with 162 branches, and associated weights, capturing epistemic uncertainties in the depth to the top of rupture, geometric spreading, anelastic path attenuation, site attenuation and stress drop, which is more likely to be used for research. The weights for these branches were derived using Bayesian updating of a priori weights from expert judgment. Secondly, we provide a backbone model with three and five branches corresponding to different percentiles, with corresponding weights, capturing the overall epistemic uncertainty, which is tailored for engineering applications. The derived models are compared with ground-motion observations, both instrumental and macroseismic, from the UK and surrounding region (northern France, Belgium, the Netherlands, western Germany and western Scandinavia). These comparisons show that the model is well-centred (low overall bias and with no obvious trends with magnitude or distance) and the branches capture the body and range of the technically defensible interpretations. In addition, comparisons with ground-motion models that have been previously used within seismic hazard assessments for the UK show that ground-motion predictions from the proposed model match those from previous models quite closely for most magnitudes and distances. The models are available as subroutines in various computer languages for ease of use

Correction to “Ground-motion models for earthquakes occurring in the United Kingdom”

Correction to: Douglas, J., Aldama-Bustos, G., Tallett-Williams, S. et al. Ground-motion models for earthquakes occurring in the United Kingdom. Bull Earthquake Eng 22, 4265–4302 (2024). https://doi.org/10.1007/s10518-024-01943-8 (first released online 9 Jun 2024

Development of a suite of stochastic ground-motion models for the United Kingdom

Since 1995, various estimates of stochastic ground-motion parameters have been computed for UK earthquakes and a few UK stochastic models proposed. These models have been developed by inverting the available weak-motion data to estimate ranges for the key parameters and using expert judgement and evidence from other regions when data are insufficient. The resulting ground-motion models have been used within site-specific seismic hazard assessments for critical infrastructure and for the 2020 UK National Seismic Hazard Model developed by the British Geological Survey. Often stochastic models have been given a lower weight within these assessments than empirical models from other regions, particularly due to doubts over how the stochastic models scale to larger magnitudes. As part of a broader project to develop a backbone ground-motion model using a hybrid stochastic-empirical method, here we present a summary of analysis conducted using an expanded ground-motion database from the UK and surrounding region to determine stochastic parameters. The ground-motion data have been adjusted to a single rock condition using an approximate technique. We used an approach to determine the stochastic models that is appropriate for their final use, namely within a scaled backbone approach that provides a suite of consistent models with appropriate weights. Due to the trade-off amongst the key parameters (e.g., stress (drop) parameter, geometrical spreading and site attenuation), constraints from the literature and expert judgement are applied. The resulting suite of models captures the uncertainties inherent in the inversion owing to the limited magnitude, distance and structural period range of the ground-motion data. These models will be the basis of a UK ground-motion model due for completion in 2023