Senior Seismic Hazard Analysis Committee Research Articles

Mean-to-median spectral acceleration ratios in Senior Seismic Hazard Analysis Committee Level 3 probabilistic seismic hazard analyses: An analysis of reported results

In many countries, seismic characterization of the site selected for a critical structure or industrial facility is required in terms of site-specific seismic ground motion hazard. For this purpose, a probabilistic seismic hazard analysis (PSHA), performed under the Senior Seismic Hazard Analysis Committee (SSHAC) protocol, is an extended practice for nuclear facilities. In the past decade, SSHAC Level 3 studies have been performed for sites in North America, Europe, Japan, Taiwan, and South Africa. When analyzing PSHA results, the mean-to-median spectral acceleration ratios given by the hazard curves can be interpreted as a measure of the degree of epistemic uncertainty associated with the results. In this article, results of 33 SSHAC Level 3 studies have been used to determine mean-to-median spectral acceleration ratios and the statistics of these ratios, as a function of spectral frequency and annual frequency of exceedance (AFE). The purpose was to develop a reference for the range of uncertainty that is typically captured in this kind of studies. It has been found that, for a given AFE, ratios corresponding to different sites are within a relatively small interval, especially for the spectral frequency band between 2.5 and 10 Hz, which is the band normally more relevant for the seismic design of nuclear installations. In this band, for 10−4 yr−1 AFE, a mean/median ratio of 1.40 would envelop practically all investigated sites.

Development of shear-wave velocity profiles and kappa compatible with Ground Motion model for Taiwan

A new velocity model (GMM) compatible with a ground motion prediction model (GMM) for Taiwan is presented in this study. The model is developed using the data set extracted from the flatfile compiled for Taiwan Senior Seismic Hazard Analysis Committee (SSHAC) level 3 project (NCREE, 2015). The Phung et al. (2020) ergodic Sa GMM was used as a backbone model for estimating the site amplification in the frequency domain using inverse Random vibration theory (IRVT). The main motivation for developing a velocity model is that the estimation of the site scaling in terms of the time-averaged shear-wave velocity over the top 30 m (VS30), the method also provides the corresponding depth-dependent VS(z) profiles and the value for the selected VS30 value as a result of the inverse quarter-wave-length method (IQWL). Not all of the empirical amplification can be explained by the 1-D VS profiles andvalues. For soft sites (VS30 < 500 m/s), and intermediate periods (0.5-2.0 sec), there is additional amplification in the empirical data which is attributed to 3-D path and site effects. Using the proposed method, the resulting GMM provides a VS profile, k, and 3-D effect for each VS30 value, which provides a more informative handoff of ground-motion information for use in site-specific site response studies.

Probabilistic Tsunami Hazard Assessment of Offshore Wave Heights for Southern Region of Pingtung County in Southern Taiwan

This study applies the unit tsunami method and probabilistic tsunami hazard analysis to investigate the tsunami hazard for the southern region of Pingtung County in southern Taiwan. The FUNWAVE-TVD model is used to calculate the aleatory and epistemic uncertainties. The aleatory uncertainty is analyzed using a comparison of simulated and observed maximum tsunami wave heights and the epistemic uncertainty is evaluated using a scenario caused by earthquakes in the Manila and Ryukyu subduction zones according to probabilistic seismic hazard analysis in Taiwan using Senior Seismic Hazard Analysis Committee Level 3 methodology. A unit tsunami method database is established for the Manila and Ryukyu subduction zones and used to carry out a probability-based offshore tsunami wave height simulation for the southern region of Pingtung County. A hazard map for tsunamis caused by earthquakes in the Manila and Ryukyu subduction zones is also derived.

Conditional Ground-Motion Models for Horizontal Peak Ground Displacement for Active Crustal Regions

ABSTRACTGround-motion models (GMMs) are developed for peak ground displacement (PGD) and for bandlimited PGD based on strong-motion data that has been filtered as part of standard processing and the total PGD that includes the tectonic deformation as well as the vibratory ground motion. For the bandlimited PGD, we develop conditional ground-motion models (CGMMs) using subsets of the Pacific Earthquake Engineering Research Center Next Generation Attenuation-West2 Project (NGA-W2) database and the National Center for Research on Earthquake Engineering Taiwan Senior Seismic Hazard Analysis Committee level 3 project database. The CGMM approach includes the observed pseudospectral acceleration (PSA(T)) as an input parameter in addition to magnitude and distance. The period of the PSA(T) is used as an input parameter; it is magnitude dependent and is based on the period for which there is the highest correlation between the ln(PGD) and ln(PSA(T)). Two CGMMs are developed: a global model based on the NGA-W2 data and a region-specific model for Taiwan. The conditional PGD models are combined with traditional GMMs for PSA(T) values to develop GMMs for both the median and standard deviation of PGD without the dependence on PSA. A second set of PGD GMMs are developed to correct for two factors: the effect of the high-pass filtering from standard record processing and the stronger large magnitude (M>6.5) scaling due to tectonic deformation. For magnitudes greater than 7, the PGD values from the total PGD GMMs are 2–5 times larger than the bandlimited PGD values based on the strong-motion data sets, but the increase is at very long periods. The appropriate PGD model to use, bandlimited PGD or total PGD, depends on the period range of interest for the specific engineering application.

Ground motion prediction equation for crustal earthquakes in Taiwan

We develop a ground motion prediction equation (GMPE) for estimating horizontal ground motion amplitudes caused by crustal earthquakes, based on an integrated data set that includes strong motion recordings mainly from Taiwan earthquakes and only from large magnitude earthquakes in the NGA-West2 database. This GMPE is developed for probabilistic seismic hazard analysis study, which is introduced as a part of Taiwan Senior Seismic Hazard Analysis Committee Level 3 projects. The functional form developed by Chiou and Youngs was carefully studied to determine the key modeling parameters needed to regress against ground motion in the target region. Using this functional form, the GMPE achieves considerable improvement over previously developed Taiwan GMPEs. In particular, the use of a high-order function in magnitude scaling enables representation of the saturation effects of large earthquakes. Moreover, consideration of focal mechanisms, depth effects, and dip effects are used to correct the magnitude scaling; consideration of nonlinear site amplification is conditioned on VS30and reference ground motion on rock; and consideration of basin depth effect is a function of Z1.0in correlation with VS30. In addition, ground motion data used in this study are not only expanded by more than three times as many earthquakes and records compared with a previous Taiwan model but also provide the metadata of these records that were not available or were previously incomplete. In this study, we compare the proposed model with the NGA-West2 models and discuss the regional difference in ground motion in terms of spectral shape, magnitude scaling, distance scaling, depth scaling, style of faulting, and site effects. We provide median and single standard deviations of peak ground acceleration and 5% damped pseudospectral acceleration response ordinates of the orientation-independent average horizontal component of ground motion (RotD50) for the spectral period of 0.01–10 s.

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.

Assessing the need for an update of a probabilistic seismic hazard analysis using a SSHAC Level 1 study and the Seismic Hazard Periodic Reevaluation Methodology

A key decision for nuclear facilities is evaluating the need for an update of an existing seismic hazard analysis in light of new data and information that has become available since the time that analysis was completed. We introduce the newly developed risk-informed Seismic Hazard Periodic Reevaluation Methodology (referred to as the SHPRM) and present how a Senior Seismic Hazard Analysis Committee (SSHAC) Level 1 probabilistic seismic hazard analysis (PSHA) was performed in an implementation of this new methodology. The SHPRM offers a defensible and documented approach that considers both the changes in seismic hazard and engineering-based risk information of an existing nuclear facility to assess the need for an update of an existing PSHA. The SHPRM has seven evaluation criteria that are employed at specific analysis, decision, and comparison points in a graded approach, based on the nuclear facility's seismic design category. The SHPRM is implemented using a SSHAC Level 1 study performed for the Idaho National Laboratory, USA. The implementation focuses on the SHPRM evaluation criteria that are based directly on the seismic hazard, and the inputs to these criteria are derived from the SSHAC Level 1 PHSA. To illustrate outcomes of the SHPRM that do not lead to the need for an update and those that do, the example implementations of the SHPRM are performed for nuclear facilities that have target performance goals expressed as the mean annual frequency of unacceptable performance at 1×10−4, 4×10−5 and 1×10−5.

Use of the SSHAC methodology within regulated environments: Cost-effective application for seismic characterization at multiple sites

Abstract Essential elements of license applications and safety reviews for nuclear facilities are quantifications of earthquake and other natural hazards. A Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 or 4 process provides regulatory assurance that the hazard assessment considers all data and models proposed by members of the technical community and the associated uncertainties have been properly quantified. The SSHAC process has been endorsed as an acceptable hazard assessment methodology in US NRC regulatory guidance. Where hazard studies are required for multiple sites, regional SSHAC Level 3 or 4 studies followed by site-specific Level 2 refinements can provide major benefits in cost and duration.

Comment on J.U. Klügel's: Problems in the application of the SSHAC probability method for assessing earthquake hazards at Swiss nuclear power plants, in Engineering Geology, vol. 78, pp. 285–307

The Senior Seismic Hazard Analysis Committee’s ( SSHAC, 1997) guidance for performing probabilistic seismic hazard analysis (PSHA) has been used to evaluate earthquake hazards at Swiss nuclear power plants (Klugel, 2005). Klugel (2005) found many inconsistencies and deficiencies in the SSHAC guidance and concluded b the results can not be explained by the available information on seismic activity and ground motion recordings in Switzerland. Q In other words, the results (hazard estimates) can not be physically compared to the geological and seismological observations. Also, Klugel also found that b the better the experts tried to discover and to quantify uncertainties following the SSHAC guidance, the more diffuse the resulting distribution will be. All these observations raise questions about the SSHAC guidance. The SSHAC guidance was developed by a seven member committee to address the large differences in PSHA results by different practitioners. The most important conclusion reached by the committee is b that differences in PSHA results are due to procedural rather than technical differences (SSHAC, 1997). In other words, b many of the major potential pitfalls in executing a PSHA are procedural rather than technical in character (SSHAC, 1997). The SSHAC guidance only deals with procedural problems in executing a successful PSHA, but not technical problems with PSHA itself. The technical problems may be one of the main reasons for the large differences, however. Thus, it would be beneficial to review the basics of PSHA and its drawbacks.

Problems in the application of the SSHAC probability method for assessing earthquake hazards at Swiss nuclear power plants

From 2000 to 2004 a large scale probabilistic seismic hazard analysis (PEGASOS) was created and performed as a research project, sponsored by the Swiss NPP utilities to improve the assessment methodology for seismic risk in support of the plant-specific seismic PRAs. The project followed the methodology of the SSHAC [ Senior Seismic Hazard Analysis Committee (SSHAC), 1997. Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts. NU-REG/CR-6372] procedures at its most elaborate way—level 4. Before practical implementation was to occur, a detailed review was performed including validation tests and analysis of uncertainty propagation. This paper presents the main results of the review. The review revealed that current PSHA (Probabilistic Seismic Hazard Analysis) methodology as based on logic trees, in conjunction with the SSHAC procedures, potentially leads to a significant overestimation of the seismic hazard in areas with low seismic activity due to the inherent possibilities of unconstrained accumulation of uncertainties. The preliminary results of the project did not pass any of our logical geological–scientific benchmark tests used in our attempts to perform a validation process of the PEGASOS analysis method. Some of the problems encountered are of generic nature and shall be studied carefully before making the decision of whether or not the Swiss nuclear power industry will adopt the recommended use of SSHAC-procedures as a basis for the evaluation of the seismic hazard for individual nuclear power plant seismic PRA without correction.

Two methods for the structured assessment of model uncertainty by experts in performance assessments of radioactive waste repositories

The assessment of the performance of high-level radioactive waste repositories is based on the use of models for predicting system behaviour. The complexity of the system together with the large spatial and temporal scales imposed by the regulations introduce large uncertainties in the analysis. The difficulty of validating the relevant models creates the need of assessing their validity by means of expert judgments. This paper addresses the problem of model uncertainty both from a theoretical and a practical point of view and presents two mathematical approaches to the treatment of model uncertainty that can assist the experts in the formulation of their judgments. The formal elicitation of expert judgments is investigated within the Technical Facilitator/Integrator (TFI) framework that has been proposed by the Senior Seismic Hazard Analysis Committee. Within this framework, the mathematical formulations for the treatment of model uncertainty are regarded as tools for sensitivity analyses that give insights into the model characteristics and are helpful in structuring the expert opinion elicitation process itself. The first approach, referred to as the alternate-hypotheses formulation, amounts to constructing a suitable set of plausible hypotheses and evaluating their validity. The second approach to model uncertainty is referred to as the adjustment-factor formulation and it requires that a reference model be identified and its predictions be directly modified through an adjustment factor that accounts for the uncertainty in the models. Furthermore, both approaches require a clear understanding of the distinction between aleatory and epistemic uncertainties. The implications that these two formulations have on, and the issues that they raise in, the elicitation of expert opinions are explored. A case study of model uncertainty regarding alternative models for the description of groundwater flow and contaminant transport in unsaturated, fractured tuff is presented.