Site-specific seismic hazard assessment for nuclear facilities in low seismicity regions

Transcription

1 NPSAG Seismic PSA Workshop 13/14 March 2013 Radisson Blu Arlandia Hotel, Sweden Site-specific seismic hazard assessment for nuclear facilities in low seismicity regions Prof Willy Aspinall (Aspinall & Associates / University of Bristol) willy@aspinall.demon.co.uk Disclaimer The views expressed in this talk are my own and should not be construed as being shared by any organization with whom I may be or have been associated.

2 UK Seismic Hazard Working Party methodology Brown books: PSHA methodology documents (13 no.) Yellow books: CEGB/ Nuclear Electric new NPP site-specific seismic ground motion hazard assessments (8 no.) Green books: Site-specific seismic hazard assessments for other UK nuclear facilities (N no.) White books: Partial site-specific seismic hazard assessments for existing sites (PSRs) (N no.) Blue books: Uniform Hazard Spectra (UHS) assessments for other organizations ( N no.) Case history: Armenia NPP Metsamor PSHA4PSA

3 The notorious 1988 Spitak earthquake, magnitude 7 distance ~ 80km from ANPP Basic approach: The study implements a conventional probabilistic seismic hazard assessment (PSHA) methodology, meeting IAEA Guidelines as far as possible.. acquisition of different (existing) datasets from Armenian specialists conflation of datasets into project-specific catalogues seismotectonic synthesis of available information construction and parameterization of seismic source model definition of logic tree framework to incorporate parameter uncertainty distributions computation of probability of exceedance hazard curves on basis of model and parameters presentation of main results (pga, and URS) multiple sensitivity tests (mainly for parameters with ill-defined epistemic uncertainty) The study is not a full PSHA, such as would be undertaken for a new build NPP.

4 Ancient Armenian chronicles Map of historical epicentres in ANPP area of interest - symbol sizes scaled with magnitude Complete data for relevant threshold periods (Set 1) are identified by crosshatched insert symbols

5 Map of instrumental epicentres for ANPP area of interest Colour coded by catalogue source (n.b. not magnitude scaled) Map of identified active faults in Armenian specialists reports, overlaying instrumental epicentres

6 Comparison of model fault zones (red) with Georisk mapped faults (black) and earthquakes of magnitude 5Mw or greater and depths less than 35km Grey areas are catchment bands around mapped faults for associating individual earthquakes with particular fault. K-means cluster analysis of remaining diffuse seismicity not associated with faults. Symbols are not scaled to magnitude. 12 separate spatial clusters are identified in 3-dimensions (Lat., Long., depth)

7 Side view of cluster analysis results, viewed from south-west, showing depth partitions Application to durations of dome-building eruptions The seven remaining clusters are combined to form shallow area source zones GS3 GS10. Note GS3 & GS4, and GS5 & GS8 belong to subdivided K- means clusters, to reflect different area activity rates within each cluster

8 Estimating activity rate and b-value distributions for logic tree models from data, cont Let Mmax be the zone maximum magnitude, M 0 the engineering threshold magnitude, and a the zone activity rate (no. events / year / unit area exceeding M 0 ). Statistical uncertainty in estimate of a - given data available - has the form of the standard Gamma distribution:..for x > 0. α 1 x 1 f ( x) = x exp α β β Γ α ( ) Calculating this can be easily implemented in a spreadsheet: Mthres Mo Mmax b 1st nd rd th th th F1= No. yrs eq. M4 sample Ni (+1) reduced tim F2= No. yrs eq. M4 sample reduced tim F3= No. yrs eq. M4 sample reduced tim F4= 1 No. yrs eq. M4 sample reduced tim 50 F5= No. yrs eq. M4 sample 0 0 reduced tim wt ave F6= No. yrs eq. M4 sample 0 0 Prob invgamma reduced tim M4 cmp F = (10^bMt - 10^bMmax)/(10^bMo - 10^bMmax) pt weighted Gamma distribution for activity rate for 4 events above relevant thresholds in 400 years

9 Updating alternative b-value weights for logic tree input. BAYESIAN UPDATING OF B-VALUE WEIGHTS, GIVEN SPECIFIC DATASET first b-val 1.19 alt b-val 1.28 prior wt 0.5 prior wt 0.5 likelihood likelihood Mo beta1: obs. Data Mmax beta2: denom for col E denom for col H Posterior wt for b2= Posterior wt for b1= Typical logic-tree combinations of different active rates and b-values, representing uncertainty in the true Gutenberg- Richter relationship.. and comparison with actual experience in a low seismicity area!!

10 PSHA Results for ANPP site are calculated for: Conf. Levels (model): Expected, for annual probabilities of exceedance: 10e -1, 10e -2, 10e -3, 10e -4, 10e -5 and for horizontal and vertical motions at pga plus 18 spectral frequencies 40Hz 20Hz 16.7Hz 12.5Hz 10.0Hz 8.3Hz 7.1Hz 5.0Hz 3.3Hz 2.8Hz 2.5Hz 2.0Hz 1.7Hz 1.4Hz 1.0Hz 0.8Hz 0.5Hz 0.4Hz PSHA curves for ANPP site 10-1 ANPP seismic hazard conf. Expected 0.1 conf Annual probability of exceedance exp Peak ground acceleration [g]

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12 PSHA Sanity check Comparison of Intensity hazard using PSI4PSA seismogenic model, with Intensity experience at ANPP site according to macroseismic dataset Intensity: annual exceedance probability Annualised Intensity exceedance rate from experience, (counts from ) Expected annual rate from hazard assessment D:\Risktec\IntRisk\ANPP Intensity Experience vs Model.draw II III IV V VI VII VIII IX. Intensity

13 Adding active eastward extension Yerevan fault: pga sensitivity Sensitivity tests Yerevan Fault extension sensitivity test result : 0.285g 0.32 g expected 10-4 annual probability FS1 & FS2 active status = 0.5 test result : 0.285g g expected 10-4 annual probability GS3 & GS5 activity increased 25% test result : 0.285g g expected 10-4 annual probability GS3 & GS5 b-value = 0.8 test result : 0.285g g expected 10-4 annual probability GS3 & GS5 max. magnitudes increased by +0.5 test result : 0.285g g expected 10-4 annual probability GS3 & GS5 depths all fixed to 5 km test result : 0.285g g expected 10-4 annual probability

14 Sensitivity tests GS3 & GS5 b-value and Mmax test result : 0.285g g expected 10-4 annual probability GS3 & GS5 b-value, Mmax and depth test result : 0.285g g expected 10-4 annual probability Attenuation sigma increased to 0.7 test result : 0.285g g expected 10-4 annual probability Attenuation sigma decreased to 0.6 test result : 0.285g g expected 10-4 annual probability Attenuation sigma increased to 0.7 with zonal parameters modified conservatively, as above, gives a result : 0.285g g expected 10-4 annual probability Attenuation sigma decreased to 0.6 with zonal parameters modified conservatively, as above, gives a result : 0.285g g expected 10-4 annual probability IAEA Review The IAEA Review asked for a sensitivity test for pga hazard in which the CDP fault (inferred from geophysical survey) was included in the model, and to report back to the meeting.

15 IAEA Review Sensitivity test for pga hazard in which the CDP fault (inferred from geophysical survey) was included in the model, and to report back to the meeting. The following parameters were agreed for this exercise: The fault position was taken from the AESP map (App 5.6.jpg see Appendix 7), extended at both ends to give a total fault length of 20 km. M MAX was taken to be 5.5 Ms (advice of L. Serva) Activity rate was assumed to be 1/20 th of that of GS5 (i.e. area source surrounding CDP fault source), with activity rate of GS5 reduced accordingly (consultation with L. Serva). Depth and b-value distributions as per GS5. With this additional source included, the pga hazard changed from: 0.285g 0.30 g expected 10-4 annual probability IAEA Review Postscript: IAEA Issue (Ground movement hazard assessment) R3. This query concerns the definition of the horizontal component of ground motion. In the present study, the peak acceleration in the larger of the two horizontal record components is taken to characterize the ground motion hazard. Different definitions are sometimes used. For cases where it is required to determine the hazard associated with the maximum resolved peak horizontal acceleration, this information needs to be obtained from the original strong motion records. In the absence of that information, however, it can be stated that the largest resolved peak horizontal motion cannot exceed x the peak acceleration in the larger of the two components (i.e. the square-root of the sum-of-squares of two equal values). In this case the peak horizontal acceleration hazard result could therefore be no higher than: 0.40 g expected 10-4 annual probability and must be significantly less, given the stochastic geometry of instrument orientations relative to causative earthquakes in the real world.

16 IAEA Review Postscript: IAEA Issue (Ground Movement Hazard Assessment. Hazards calculation) R2: This recommendation requests sensitivity studies on the URS results, to show the results are not different than those for pga. If the depth of focus of all seismicity in the area sources closest to the site (GS3 & GS5) is changed to be fixed at 5 km (i.e. instead of 5 km, 15 km & 25 km), the 5Hz URS hazard changes from: 24.3 cm/s 26.2 cm/s expected 10-4 annual probability This is much less dramatic than the corresponding change in pga due to fixing focal depth (from 0.285g to 0.321g) and serves to demonstrate that spectral response does not track peak acceleration in any simple linear manner. IAEA Review Postscript: IAEA Issue (Ground Movement Hazard Assessment. Hazards calculation) R2: This recommendation requests sensitivity studies on the URS results, to show the results are not different than those for pga. To understand the sensitivity of the whole URS frequency spectrum to changes in model parameters would require a complete set of tests, frequency by frequency. Remark: This IAEA Review suggestion evinces some lack of understanding of probabilistic seismic hazard analysis. There are a number of reasons why URS ordinates of different natural frequencies will not track pga trends as the source model is varied: URS attenuation relations differ significantly in terms of distance dependence, and different spectral amplitudes are sensitive to the source event magnitude, and depth of focus, in different ways. The following two tests, using an alternative attenuation relation, are provided for illustration. They replicate the sensitivity tests that produced the largest increases in pga in the earlier tests, reported above.

17 IAEA Review Postscript: IAEA Issue (Ground Movement Hazard Assessment. Attenuation Models) R6. This recommendation suggests counterpart hazard calculations using the relationship due to Smit et al. (2000). If the Smit et al. attenuation relation is used, the pga hazard changes from : 0.285g 0.38 g expected 10-4 annual probability Note: this attenuation relation is valid only up to magnitude 7Ms, and peak accelerations calculated for higher magnitudes and short distance ranges are unconstrained (see functional form of equation) c.f. other relationships, such as Ambraseys et al. (2005). This has the inevitable effect of enhancing hazards level when magnitudes in excess of 7Ms are included in the hazard model. In a similar vein, the Smit et al. URS relations are similarly unconstrained at very high magnitudes, and elevation of the hazard level is again found. For illustration, the 5Hz URS ordinate changes as follows: 24.3 cm/s 58.3 cm/s expected 10-4 annual probability Comparison of Ambraseys et al [2005] attenuation with Smit et al [2000]

18 Decision reporting Decisions in the present hazard model that are likely to be shown by more detailed scrutiny as being pessimistic, optimistic, orthodox (neutral), or unclear - in terms of the pga hazard at 10-4 p.a. probability of exceedance. Table 5. 2 Decision appraisal table for basic factors in primary hazard model Decision Source type Factor Pessimistic Orthodox Optimistic Unclear Remarks General Completeness thresholds YES Testing desirable Smaller events could give Minimum magnitude - YES Possibly - damage concerns for plant in poor condition Include ISC data Possibly Further analysis desirable Decision appraisal Table 5. 3 Decision appraisal table for zonal source factors in primary hazard model Source type Zonal sources Decision Factor Pessimistic Orthodox Optimistic Unclear Remarks Geometries - Assignment of events Activity rates b-values - Maximum magnitudes Focal depths Attenuation relations Attenuation sigma Based on Cluster analysis - - Possibly yes Methodological treatment Methodological treatment YES YES YES - YES but see remarks - - Alternative zonations seem unnecessary No event-specific allowance made for uncertainties in location Depends upon completeness thresholds Depends on Bayesian updating Selected tests desirable Conditioned by data limitations and attenuation relation testing desirable constraints Testing alternatives desirable Single value limits dispersion of confidence bands testing desirable

19 Table 5. 4 Decision appraisal table for fault source factors in primary hazard model Decision appraisal Decision Source Factor Pessimistic Orthodox Optimistic Unclear Remarks type Ignores Yerevan Fault Overall - YES Possibly - & CDP faults : sources model needs testing Based on conservative Geometries probably interpretation of information supplied All faults taken to Active have active status possibly status of 1 along full length Within assumed control width, Assignment ignoring YES of events locational uncertainties testing desirable Depends upon Activity methodologic completeness rates treatment thresholds Depends on Methodologic b-values Bayesian treatment updating Maximum Selected tests YES magnitudes desirable Conditioned by data limitations Focal and attenuation YES depths relation constraints testing desirable Testing Attenuation YES alternatives relations desirable Single value limits Attenuation YES but see dispersion of sigma remarks confidence bands testing desirable Hazard sensitivity tests and their role Sensitivity tests provide an important input to seismic hazard assessments on two levels: (a) without an adequate general understanding of how the decisions that are being made are likely to impact on the hazard results, it is hard to see how expert judgement can properly be exercised, and (b) it is necessary to use sensitivity tests to explore and, eventually, to demonstrate (to regulatory authorities, etc.) the effects in detail of those decisions in the particular circumstances under consideration.

20 A near-meaningless table Comparison of PSI4PSA results with other recent ANPP site-specific pga hazard assessment results: 50%ile Expected [mean] 84%ile Armenenergoseismicproject Co. (1995) [deterministic] 0.21g g Georisk (2004) g - This study (2006) 0.28g 0.29g 0.32g [for 10-4 p.a. pga exceedance probability] Logic-tree estimates: effect of attenuation scatter on pga hazard Cobweb plot of range of PRISK logic-tree pga results for a site with average British seismicity (for clarity, M MIN and M MAX are fixed in this example) From The Mallard Partnership (2003) Uncertainty, Conservatism and the Use of Expert Judgement in Probabilistic Sitespecific Earthquake Ground Motion Hazard Assessments for the UK Nuclear Industry. Report for HSE Nuclear Safety Directorate.

21 Sensitivity to attenuation σ value Attenuation σ value for pga 10-4 p.a. pga [%g] 10-4 p.a. 1Hz PSV HARD [cm/sec] 10-4 p.a. 1Hz PSV SOFT [cm/sec] (weight) (50%) EXPECTED (90%) (50%) EXPECTED (90%) (50%) EXPECTED (90%) BASE MODEL (0.2) (0.5) (0.3) (i) 0.6 (1) (ii) 0.65 (1) (iii) 0.5 (1) (iv) (0.2) (0.5) (0.3) Truncated at 3 σ (v) (0.2) (0.5) (0.3) Truncated at 2 σ Sensitivity to attenuation σ value

22 Sensitivity to b-value Sensitivity to maximum magnitude

23 Sensitivity to focal depth Sensitivity to activity rate

24 Sensitivity to minimum magnitude Generic sensitivity test findings summarized The following trends which can be expected to be encountered in any typical lo-seismicity area hazard assessment: Source parameters: increasing activity rate increases hazard increasing b-value reduces hazard increasing minimum magnitude reduces hazard increasing maximum magnitude produces very minor increase in pga hazard, but somewhat greater increase in low frequency motion increasing focal depth reduces hazard Attenuation relations : increasing attenuation reduces hazard increasing attenuation scatter increases hazard

25 Magnitude-dependent hazard result sensitivities - revealed by disaggregation Comparison of site-specific URS hazard results with UK piecewise linear DBE spectrum (hard and soft sites)

26 Hard site Figs 6.5 & 6.6 Comparison of piecewise linear spectral ordinates with 10-4 p.a. URS values, for two sites Soft site ZONELESS METHODS FOR SEISMIC AREA SOURCE MODELLING 54

27 Adaptive Seismic Hazard Source Modelling The methodology for modelling seismic sources should be adaptive to the region. In the early days of seismic hazard modelling, the same Cornell-McGuire approach, and associated programs (EQRISK, FRISK, SEISRISK, etc..), could be used globally. but one-size does NOT fit all. The seismic source modelling method should be tailored to each region. Computational complexity was a limitation in 1968, but is not a factor in 2013! Regions Favourable for Seismicity Smoothing Areas where there is an extensive well-researched historical earthquake catalogue. Areas where the delineation of active faults is unclear. Areas where the association of earthquakes with faults is very ambiguous.

28 Seismic Zones as Seismotectonic Provinces There is often a lack of scientific consensus over the boundaries of a seismotectonic province. The concept of a seismotectonic province is questionable in a region of low to moderate seismicity. The relevance of seismotectonic province to a site-specific seismic hazard assessment is dubious when its size is so large as to cover the area contributing to the site hazard. The contrasts between alternative zonations, proposed by different seismic hazard experts, for the same engineering project and for different projects in the same region - reflects the fundamental ambiguity in the zonation process. 57 Political Sensitivity of Zonation Zone boundaries are susceptible to bias at international frontiers. Seismicity catalogues may not be harmonized across frontiers. Seismotectonic mapping may be discontinuous across frontiers. Seismic hazard analysts in neighbouring countries may have different opinions on zonation. 58

29 Application of Kernel modelling to seismicity In non-parametric statistical data analysis, a major advance is the technique of kernel smoothing, and it is natural to extend the application of this more sophisticated technique to the statistical analysis of spatial seismicity data. The kernel method is the most straightforward and widely used of the standard methods, and it is hard to see substantial reasons for going beyond it. David Vere-Jones 1992 Vere-Jones D. (1992) Statistical methods for the description and display of earthquake catalogues. Statistics in the Environmental & Earth Sciences, Halsted Press. Kagan Y., Johnson (1999) Testable Earthquake Forecasts for 1999, Seism. Res. Lett., Vol.70. No KERFRACT implementation The basic principle underlying the program KERFRACT is that the epicentre of each past shallow event is smoothed geographically to generate a spatial probability distribution for event recurrence. Uncertainties in location and magnitude condition Kernel properties. This is consistent with the seismological observation that, in areas with a long historical catalogue, earthquakes tend not to occur in areas they have not occurred before historically. Woo G. (1996) Kernel estimation methods for seismic hazard area source modeling. BSSA, Vol.86, No.2. 60

30 Logic-tree Branches for Seismic Sources Kernel smoothing model Background seismic source model Area zonation models At the very least, kernel smoothing merits consideration as a branch of a background seismic source logic-tree alongside a variety of zonation models. Testing zoneless models against conventional source models

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32 Zoneless Seismic Hazard modelling European applications 65 Synthetic catalogues and Monte Carlo PSHA Mismatch of model activity rate with historical record [Musson & Winter, 2008 Fig 4a]

33 Testing alternative spatial source models Activity rate test for (a) BGS 2007 UK source zone model for EC8; (b) kernel method; (c) epicentral cell method [Goda et al 2013 SRL Fig 6] Kernel seismicity modelling - summary Source modelling methods should be adaptive according to the region of seismic hazard analysis. Kernel seismicity smoothing techniques are especially useful where there is an extensive historical earthquake catalogue, such as exists in Europe, the Middle East and Asia. These techniques merit inclusion within a logic-tree PSHA framework. 68

34 Summing up For seismic inputs to PSA: Construct a parsimonious best science seismotectonic seismic source PSHA model Use coherent data, relationships (e.g. same magnitude scale; correct distance parameter..) Calculate hazard confidence bounds, using logic tree formulation Report basis for modelling decisions Conduct sensitivity tests Summing up Seismic inputs can dominate NPP PSA (core damage) risks, compared to internal events Characterization of earthquake hazards in low probability events in low seismicity areas is very challenging In the UK, where probabilistic seismic hazard assessment methodology was pioneered, progress stalled in 1990 s: review / updating needed in the light of Tohoku-oki, Christchurch and other earthquakes, especially for use with PSAs

35 Christchurch, New Zealand, earthquake 22 February 2011 Peak acceleration 2.2 g - applying attenuation scatter truncation, this pga would not appear in a PSHA at ANY probability of occurrence 71 Tack så mycket!