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Verification of hazard calculations made during the Thyspunt

SSHAC Level 3 project: Phase 3

J. Douglas

Council of Geoscience

Report Number 2013-0004 Rev. 0

Confidential

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CGS REPORT

2013-0004

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Verification of hazard calculations made during the Thyspunt SSHAC Level 3 project: Phase 3

DATE OF RELEASE:

15 January 2013

CONFIDENTIAL

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N. Keyser

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G. Graham

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Verification of hazard calculations made during the Thyspunt

SSHAC Level 3 project: Phase 3 Final report

BRGM/RC-61704-FR December 2012

J. Douglas

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This report was checked by: ULRICH Thomas date: 07/12/2012

Approved by:

Name: E. FOERSTER Date : 17 December 2012 Signature :

BRGM’s quality management system is certified ISO 9001:2000 by AFAQ.

Keywords: probabilistic seismic hazard analysis (PSHA), hazard calculations, annual exceedance frequencies (AFEs), verification, nuclear power plants, Thyspunt, South Africa In a bibliography, this report should be cited as follows: Douglas, J. (2012) – Verification of hazard calculations made during the Thyspunt SSHAC Level 3 project: Phase 3. Final Report BRGM/RC-61704-FR, 15 pp., 4 illustrations. © BRGM, 2012, No part of this document may be reproduced without the prior permission of BRGM.

Verification of hazard calculations for Thyspunt SSHAC Level 3 Project

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Synopsis

The Thyspunt Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 project is seeking to provide estimates of the expected earthquake ground motions for the proposed site of a nuclear power plant at Thyspunt (South Africa) derived using state-of-the-art probabilistic seismic hazard analysis. The hazard calculations will be conducted by the Council for Geoscience (CGS) using the FRISK software from Risk Engineering Inc. As part of the quality-assurance measures the Project Execution Plan calls for independent checks of these hazard calculations. To do this, independent calculations have been conducted for a selection of source-backbone-sigma model-response period combinations using nhlib (a New Hazard Library, part of OpenQuake) by employees of the Global Earthquake Model (GEM). This report concerns the independent comparison of these calculations by BRGM and the iterations required to achieve convergence. A separate report (BRGM/RC-61697-FR) covers an additional task that consisted of checking the implementation of the ground-motion prediction equations in FRISK and nhlib used in these hazard calculations. A final task undertaken by BRGM is the checking of a Matlab routine written by CGS to post-processes the hazard results computed using FRISK. The results of this task were reported by completion of a form (template) provided by CGS (no separate BRGM report was produced).

Estimates of the annual exceedance frequencies (AFEs) for 18 ground-motion levels for 15 source-backbone-sigma model-response period combinations computed by FRISK (by CGS) and nhlib (by GEM) were independently compared by BRGM. The relative differences in the AFEs from the two sets of calculations were calculated. After the discovery and correction of various errors in the CGS and GEM calculations, all of these differences were agreed with the Project Technical Integrator and other project participants to be sufficiently minor for verification purposes. In general, for the annual exceedance frequencies of interest to the Thyspunt project the relative differences between the expected ground motions were much less than 25% for all AFEs of interest to the project. Therefore, it is concluded that the hazard calculations undertaken by CGS and GEM for this project are being correctly performed.


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Contents

1. Introduction ...................................................................................................................... 1

2. Verification of the hazard calculations ........................................................................... 2

2.1. FIRST ITERATION FOR FAULT SOURCES 3

2.2. SECOND ITERATION FOR FAULT SOURCES 4

2.3. THIRD ITERATION FOR FAULT SOURCES 5

2.4. FOURTH ITERATION FOR FAULT SOURCES 5

2.5. FIFTH ITERATION FOR KNG 6

2.6. ITERATIONS FOR AREA SOURCES 7

3. Conclusions ..................................................................................................................... 9

4. Bibliography ................................................................................................................... 10

List of illustrations

Illustration 1 : The 15 source-backbone-sigma model-response period combinations for which spot checks of the hazard calculations were made (fault sources are in italics). ................ 2

Illustration 2 : The 18 ground-motion levels for which AFEs were computed for all 15 scenarios.The same ground-motion levels are used for all calculations. ............................ 3

Illustration 3 : Comparison of the hazard curves from the five consider fault-source scenarios after final iterations (figure courtesy of Prof. Bommer, PTI). Solid lines are from CGS calculations and dashed are from GEM calculations. ......................................................... 7

Illustration 4 : Comparison of the hazard curves for the ten consider area-source scenarios obtained after a series of iterations (figures courtesy of Prof. Bommer, PTI). Solid lines are from CGS calculations and dashed are from GEM calculations. .......................... 8


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1. Introduction

The Thyspunt Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 project is seeking to provide estimates of the expected earthquake ground motions for the proposed site of a nuclear power plant at Thyspunt (South Africa) derived using state-of-the-art probabilistic seismic hazard analysis (PSHA). The hazard calculations will be conducted by the Council for Geoscience (CGS) using the FRISK software from Risk Engineering. As part of the quality assurance measures the Project Execution Plan (Bommer and Coppersmith, 2010) calls for independent spot checks of the hazard calculations for sample runs selected by the Project Technical Integrator (PTI). The comparison by John Douglas (BRGM) of two independent implementations of these spot checks is the subject of this report. The hazard calculations for the sample runs were undertaken by employees of the Global Earthquake Model (GEM) using nhlib (a New Hazard Library), which is being developed as part of OpenQuake. Therefore, the role of BRGM was to compare the two independent sets of hazard calculations and coordinate investigations into any significant differences found. Both groups (CGS and GEM) sent their calculated annual frequencies of exceedance (AFEs) independently to Dr Douglas and these calculations were kept from the groups until convergence between the results was obtained. Other communications between the actors in this task were copied to all groups for faster resolution of any problems.

An associated report (Douglas, 2012) concerns a separate verification task conducted during Phase 3: checking of the coding of the ground-motion prediction equations (GMPEs) for the Thyspunt project within FRISK and nhlib. Douglas (2012) concluded that the GMPEs for this project were correctly implemented within FRISK and nhlib because the absolute differences in evaluated response spectral accelerations were less than 0.01%. This was a prerequisite before checking of the hazard calculations could begin. A final task undertaken by BRGM is the checking of a Matlab routine written by CGS to post-processes the hazard results computed using FRISK. The results of this task were reported by completion of a form (template) provided by CGS (no separate BRGM report was produced). These other tasks are not discussed further in this report.

The requirements for the checking of the hazard calculations were received from the PTI, Prof. Julian Bommer, in an email on Tuesday 13th November 2012. The document provided in this email (entitled Specifications for OpenQuake Checks on Final Hazard Calculations) included the list of the 15 source-backbone-sigma model-response period combinations that would be used for the spot checks of the hazard calculations. The current report is associated with a CD ROM containing Excel spreadsheets with the results from the iterations required to achieve convergence in the hazard calculations between the two groups (CGS and GEM).



2. Verification of the hazard calculations

To make the hazard-calculation checks as quick and straightforward as possible, it was decided to undertake the verification in the following manner. The two groups (Fleur Strasser, CGS; Marco Pagani and Damiano Monelli, GEM) were provided on Tuesday 13th November with an Excel spreadsheet with 15 worksheets: one for each of the source-backbone-sigma model-response period combinations used for the spot checks of the hazard calculations. This spreadsheet was produced by John Douglas in accordance with the email sent by the PTI on Tuesday 13th November 2012. The two groups then independently evaluated the considered scenarios and included their estimated AFEs for the 18 ground-motion levels of interest in the Excel file as an additional column.

The first scenarios considered were those for the fault sources since the necessary input parameters for these scenarios were the first that were finalised (these calculations were revisited later in the procedure) and then the calculations for the area sources were compared. The completed Excel file was provided to John Douglas (without copying in the other group to maintain independence) and the AFEs were cross-checked against those provided by the other group. The relative percentage differences in the AFEs between the two evaluations were computed and any significant differences indicated to both groups (without sending them the completed Excel file). In accordance with project quality assurance, all communication for this task was copied to the PTI and the Project Manager, Erna Hattingh (CGS).

Due to the large number of results obtained, no tables of results are presented in this report but Excel spreadsheets listing all the results are provided on the CD ROM associated with this report. The Excel spreadsheets hazardchecks_phase3_area_iteration1.xlsx and hazardchecks_phase3_fault_iteration5.xlsx contain the AFEs computed by CGS and GEM after convergence was achieved. Also contained on the CD ROM are spreadsheets listing the results for various previous iterations and some specific tests (see below).

The 15 source-backbone-sigma model-response period combinations for which spot checks were made are listed in Illustration 1. The 18 ground-motion levels for which AFEs were computed are listed in Illustration 2.

Information in the public domain on the maximum difference in AFEs to expect between hazard analyses made using different software for PSHA is sparse. One of the few publicly-available reports presenting hazard curves derived using the same source and ground-motion models for various codes is that by Thomas et al. (2010). For many of the hazard curves presented in this report the match is exact, sometimes because of the simplicity of the source and ground-motion model (e.g. their Figure 3.7) but even for more complex models (e.g. their figure 3.68), but others show differences of up to a factor two, particularly for high amplitudes (low AFEs) (e.g. their Figure 3.20). Unfortunately Thomas et al. (2010) only present ‘Early results’ hazard curves for more realistic source and ground-motion models (their Test Set 2) and not curves following iterations once differences in implementations had been resolved. Therefore, it was not possible to know a priori how large the differences between correctly-implemented hazard calculations from two codes would be for the 15 cases considered here but a factor of two for low AFEs was thought possible.

Illustration 1 : The 15 source-backbone-sigma model-response period combinations for which spot checks of the hazard calculations were made (fault sources are in italics).

Number Source Backbone Sigma Period T(s) JCALC



1 ECC AS08 HOMmd T01 0.01 102393 2 ECC AS08 HOMmd T06 0.10 102393 3 ECC CY08 HETmd T06 0.10 402396 4 ECC CY08 HETmd T09 1.00 402396 5 ECC AC10 HOMmd T01 0.01 502393 6 ECC AC10 HETmd T09 1.00 502396 7 SYN AS08 HETmd T06 0.10 102396 8 KAR AC10 HOMmd T06 0.10 502393 9 CK CY08 HOMmd T06 0.10 402393 10 NAM CY08 HOMmd T06 0.10 402393 11 KNG AS08 HETmd T09 1.00 102396 12 AFZ AC10 HETmd T09 1.00 502396 13 GAM AS08 HETmd T09 1.00 102396 14 PLE AC10 HOMmd T09 1.00 502393 15 WOR CY08 HOMmd T09 1.00 402393

where: ECC, SYN, KAR, CK, NAM, KNG, AFZ, GAM, PLE and WOR are various area and fault sources of the seismic source zonation for the Thyspunt project; AC10 is the GMPE by Akkar and Cagnan (2010), AS08 is the GMPE by Abrahamson and Silva (2008) and CY08 is the GMPE by Chiou and Youngs (2008) (all three GMPEs are modified by the central Vs-kappa adjustment); and HOMmd and HETmd are the median homogenous and the median heterogeneous sigma models, respectively.

Illustration 2 : The 18 ground-motion levels for which AFEs were computed for all 15 scenarios.The same ground-motion levels are used for all calculations.

GM# 1 2 3 4 5 6 7 8 9 SA(g) 0.0001 0.0002 0.0005 0.001 0.002 0.005 0.01 0.02 0.05 GM# 10 11 12 13 14 15 16 17 18 SA(g) 0.1 0.2 0.3 0.4 0.5 0.75 1 2 5

2.1. FIRST ITERATION FOR FAULT SOURCES

The Excel spreadsheet hazardchecks_phase3_fault_iteration1.xlsx that is associated with this report contains the hazard results from CGS and GEM for the five fault sources from the first iteration. Dr Strasser sent the results from CGS on Friday 16th November and Dr Pagani the results from GEM on Thursday 22nd November. These results were compared by John Douglas (BRGM) on Monday 26th November. The first comparison showed very large differences in the calculated AFEs. The results from CGS for AFZ, GAM, PLE and WOR were more than 30 times smaller than those computed by GEM for the same scenarios and all considered accelerations and those for KNG were more than 5 times smaller. However, the hazard curves were roughly parallel in log-log space, which suggested that there was a difference one or more of the underlying basic assumptions used by CGS and GEM when computing the hazard for all cases. Dr Douglas confirmed with both groups that they were reporting AFEs (not probabilities for longer intervals, e.g. 50 years) and they were computing AFEs using g and not m/s2, which could have led to a constant offset.

Later on the 26th November Dr Pagani informed the group that the probability of the sources AFZ, GAM, PLE and WOR were active was actually being mistakenly taken as 1.0 rather than 0.2 as in the scenarios considered for these sources in the test cases considered here. Correcting for this difference (by multiplying the AFEs from GEM by 0.2) led to a reduction from a factor of more than 30 to a factor of around 6 for these four scenarios. This meant that for all five fault scenarios there was a roughly constant scale factor between the CGS and GEM AFEs. Since the GMPE implementations of CGS and GEM had previously been checked (Douglas, 2012) the most probable reason for the differences in AFEs was in the modelling of the fault source activity.

To track down the reason for the differences in the computed AFEs the KNG fault source: using the Youngs and Coppersmith (1985) characteristic-earthquake model, a single crustal thickness (15km), a fault dip of 45°, a fault length of 101km, a crustal rigidity of µ=3.0×1010



Pa, only using the slip rate (of 0.1mm/yr), a characteristic magnitude of 6.75, a b value of 0.85, was selected as a typical example. The first iteration for this scenario (26th November) led to similar differences between the results from CGS and GEM (a factor of roughly five, with AFEs from CGS being lower).

After receiving feedback from John Douglas on these differences Dr Monelli (GEM) discovered two errors in their implementation of the characteristic model. First of all the calculation of the a value of the exponential distribution was made assuming the cumulative rate at Mchar-1.25 to be equal to the characteristic rate, while the correct value should be computed assuming the incremental rate at Mchar-1.25 to be equal to the incremental rate at the characteristic magnitude. Secondly, the characteristic rate was not distributed uniformly over Mchar±0.25, but instead the same characteristic rate value was returned for all magnitudes in Mchar±0.25. Once these errors were corrected the match between the CGS and GEM AFEs for this case were much closer (less than 10% up to 0.05g and less than 50% for higher amplitudes). The results before and after finding these errors are provided in hazardchecks_phase3_KNGsource.xlsx. Therefore, it was requested on Tuesday 27th November by Prof. Bommer that GEM recomputed their AFEs for the five fault sources taking account of the corrections made to their implementation of the characteristic model.

2.2. SECOND ITERATION FOR FAULT SOURCES

The second set of hazard results for the fault sources were received from GEM on Tuesday 26th November. Comparison of these to the original ones from CGS revealed a much closer match, particularly for low-amplitude motions (high AFEs) (see hazardchecks_phase3_fault_iteration2.xlsx). AFEs computed by GEM are generally less than 20% higher than those by CGS up to about 0.02g but the match becomes poorer for higher amplitudes (sometimes reaching more than 200% for some sources). The AFEs from GEM were always slightly higher than those from CGS for all considered amplitudes. The poor match for very low AFEs (high-amplitude motions) is not unexpected since it is related to choices made on what default value is used when ground motions go beyond the six-epsilon limit (numerical equivalent for ‘unbounded’) in FRISK. The results for the sources KNG, AFZ and WOR (at considerable distances from the site) showed slightly larger differences than for the fault sources close to the Thyspunt site (PLE and GAM).

Both groups were asked to provide the minimum and maximum distances used in their calculations to check whether differences in these values were the source of the larger differences for the distant sources. From the Excel spreadsheet summarising these distances (fault_distancescomparisons.xlsx) it can be seen that the size of the differences in computed distances is small (invariably less than 5%) and consequently it was concluded that these slight differences could not be the source of the discrepancies in the calculated AFEs. Other choices for the input parameters for the hazard calculations (e.g. fault surface discretization and number of epsilons used for truncation) were also checked but no reasons for the obtained differences were found.

In an attempt to track down the differences in the AFEs, the occurrence rates for the simplified KNG fault were computed by both groups and compared. In addition, following the discovery of a difference in the way large earthquakes rupturing beyond the seismogenic depth were modelled by CGS and GEM, length-conversion (see Section 2.6) was implemented by both groups. Following communication between Drs Strasser and Monelli on their respective implementations of the characteristic model, Dr Strasser noticed an error in the way that nhlib currently models characteristic events, which meant that nhlib was not implementing the equations of Youngs and Coppersmith (1985) exactly.



2.3. THIRD ITERATION FOR FAULT SOURCES

After correction of these errors in nhlib, on Monday 3rd December Drs Strasser and Monelli once again compared their occurrence rates for the KNG simplified scenario using the Youngs and Coppersmith (1985) approach and again noticed some slight differences (hazardchecks_phase3_KNGsource_sliprate.xlsx). These were corrected and GEM recomputed the AFEs for all fault scenarios (hazardchecks_phase3_fault_iteration3.xlsx) but some small differences in the AFEs for very low motions remained. On Tuesday 4th December Dr Monelli circulated his code for the evaluation of the Youngs and Coppersmith (1985) model in which it was discovered by Dr Douglas that he was using 16.1 as the constant in the conversion between moment and magnitude rather than 16.05, the value used by CGS. This resolved the remaining minor difference in the occurrence rates.

2.4. FOURTH ITERATION FOR FAULT SOURCES

Dr Monelli sent GEM’s updated results for the simplified KNG scenario on 4th December, which were compared on 5th December by BRGM to those originally provided by CGS (hazardchecks_phase3_fault_iteration4.xlsx). The match now at very low ground-motions for AFZ, GAM, PLE and WOR is almost exact (GEM’s values are 1 or 2% higher). For these scenarios the match between the two sets of calculations is very good (less than 10% difference) down to AFEs of at least 10-5 for GAM, PLE and WOR and less than 30% difference down to AFZ. Even for smaller AFEs the match between the two sets of calculations is excellent, except for WOR and AFZ where larger differences are seen (but these are thought due to the default value used for very low motions). These differences for very low AFEs are of little consequence for the project. Therefore, for these scenarios it was agreed that the hazard calculations were sufficiently close to conclude that the computations were correctly implemented by CGS.

For KNG on the other hand, the match even for very low ground motions is still not exact (CGS’s AFEs are about 40% higher than those from GEM). The hazard curves have similar shapes and the differences between the curves remains quite small (less than 50% difference) even for very low AFEs. As the differences in calculations of the occurrence rates seem now to have been resolved (seen by the excellent match for the other faults), it was requested that CGS and GEM check carefully their implementations of this scenario.

Both groups confirmed that their input parameters were correct for this scenario. Dr Strasser noted that this is the only scenario for which earthquake recurrence intervals were used as an alternative to fault slip rates and, therefore, she suggested that there could be an error in the GEM implementation of this option. Dr Monelli checked the calculation for this option in nhlib and did indeed find an error. To check that this was the cause of the differences between the results from the two groups for the KNG scenario the simplified KNG scenario (see above) was repeated but using an earthquake recurrence of 1.25× 10-4 rather than slip rate.

Both groups sent Dr Douglas their results on 5th December (hazardchecks_phase3_KNGsource_earthquakerecurrence_iteration1.xlsx). The match is good but the hazard curves still show an offset at low ground motions (CGS's values are about 15% higher than those from GEM). The difference increases with increasing ground motions. It was again requested that each group checked their calculations for this simplified scenario. Dr Monelli found an error in the normalization for GEM’s calculations and, hence, sent an updated set of AFEs. When compared with those from CGS a perfect match was found for very low ground motions and an excellent match (less than 20% difference) down to at least 10-5 (hazardchecks_phase3_KNGsource_earthquakerecurrence_iteration2.xlsx).. Therefore, it was concluded that all the errors in nhlib when using earthquake recurrence had been successfully corrected.



2.5. FIFTH ITERATION FOR KNG

Consequently GEM re-ran their analysis for the full KNG scenario. This was checked against the AFEs originally provided by CGS and an excellent match was found for all AFEs of interest to the Thyspunt project (hazardchecks_phase3_fault_iteration5.xlsx). Consequently it was agreed with Prof. Bommer that both sets of calculations for the five fault scenarios were consistent. Illustration 3 shows the comparison between the hazard curves computed by the two groups. As can be seen the difference in spectral accelerations between the two implementations in less than 10% for all fault sources even at very low AFEs.



Illustration 3 : Comparison of the hazard curves from the five consider fault-source scenarios after final iterations (figure courtesy of Prof. Bommer, PTI). Solid lines are from CGS calculations and dashed

are from GEM calculations.

2.6. ITERATIONS FOR AREA SOURCES

Dr Strasser sent the CGS results for the ten area source scenarios on 26th November; the results for the SYN scenario were updated on Wednesday 28th November after the discovery, by GEM, of an error in the SYN geometry. Following the discovery and correction of this error and a problem with the weights for the strike of ruptures for the KAR source (again discovered by GEM), Dr Monelli (GEM) provided the results of their calculations for the SYN scenario on Thursday 29th November. When compared the hazard curves showed a close match for low amplitudes but very large differences for low AFEs (high amplitudes) (hazardchecks_phase3_area_SYN_iteration1.xlsx). The reason for this difference was identified as being due to the way that the virtual faults for the area source were simulated when the rupture width became too large for the seismogenic thickness assumed. CGS modelled ruptures so that the magnitude-rupture-length correlation was retained whereas GEM modelled ruptures so that the magnitude-rupture-area correlation was obeyed.

CGS repeated their calculations for the SYN making the same assumption as GEM for large events. A comparison of the hazard curves for this case showed a close match even for high ground motions (low AFEs) (hazardchecks_phase3_area_SYN_iteration1.xlsx). Therefore, it was concluded that this difference in virtual fault ruptures for large events was the cause of the discrepancy for this scenario. GEM modified their modelling to better match the procedure used by CGS when rupture width becomes too large for the seismogenic layer. A test for a single branch of the SYN scenario was conducted by both CGS and GEM and the match was also perfect, even for very low AFEs (high ground motions) (hazardchecks_phase3_area_SYN_simplified_lengthconservation.xlsx).

Before conducting the calculations for all area scenarios it was decided to conduct a test for scenario 3 (for the ECC source, which is probably the most important for the site) using a single branch. The aim was to check that the modifications made to how the virtual faults were simulated by GEM and other minor changes to the input parameters used for the calculations, following discussions between Drs Strasser and Monelli, led to closer agreement. The comparison between these calculations by CGS and GEM revealed an excellent match (less than 10% difference in the AFEs) down to AFEs of 10-5 and an adequate fit (less than 70% difference) down to AFEs of 10-9. Therefore, it was agreed that GEM should conduct their complete area source calculations for all ten scenarios.



On Thursday 29th November the results were received from GEM for all ten area sources. These were compared by John Douglas on Friday 30th November and an excellent match (differences less than 20%) was found for all ten area scenarios and AFEs down to at least 10-5 (hazardchecks_phase3_area_iteration1.xlsx). For lower AFEs higher differences in AFEs (up to 80%) are observed, which can be related to differences in how the hazard codes handle very small ground motions and the implicit truncation at six epsilon used by FRISK. The obtained hazard curves are shown in Illustration 4.

Prof. Bommer consulted with members of the site-response group (Prof. Ellen Rathje and Dr Peter Stafford) to ascertain whether these larger differences in the hazard curves are likely to be important for the subsequent steps in the analysis. It was agreed by the site-response group that the, up to 25% differences in ground-motion amplitudes for a AFE of 10-6, are unlikely to cause significant differences in the results of their analyses given that the Thyspunt site is sufficient stiff that nonlinear soil effects are unlikely. Consequently it was concluded that further iterations were not required for the area-source calculations.

Illustration 4 : Comparison of the hazard curves for the ten consider area-source scenarios obtained after a series of iterations (figures courtesy of Prof. Bommer, PTI). Solid lines are from CGS

calculations and dashed are from GEM calculations.



3. Conclusions

In conclusion, after various iterations the AFEs computed by CGS and GEM for all 15 considered source-backbone-sigma model-response period combinations agreed to a sufficient degree (generally there was less than a 20% difference in ground-motion amplitudes for the AFEs of interest to the Thyspunt project). Therefore, it is concluded that the hazard calculations being conducted using FRISK by CGS have been correctly implemented and are providing the correct results. Hence, it was decided that the final hazard calculations for the complete logic tree could be launched by CGS.



4. Bibliography

Abrahamson N. and Silva W. (2008) – Summary of the Abrahamson & Silva NGA ground-motion relations. Earthquake Spectra, 24(1), 67–97.

Akkar S. and Çağnan Z. (2010) – A local ground-motion predictive model for Turkey and its comparison with other regional and global ground-motion models, Bulletin of the Seismological Society of America, 100(6), 2978–2995.

Bommer J. J. and Coppersmith K. J. (2010) – Project Execution Plan: SSHAC Level 3 Probabilistic Seismic Hazard Analysis for Vibratory Ground Motion for the Thyspunt Nuclear Siting Project, South Africa, Report Number 2010-0174, Rev. 0. Council for Geoscience.

Chiou B. S.-J. and Youngs R. R. (2008) – An NGA model for the average horizontal component of peak ground motion and response spectra, Earthquake Spectra, 24(1), 173-215, DOI 10.1193/1.2894832.

Douglas J. (2012) – Verification of GMPE implementation made during the Thyspunt SSHAC Level 3 project: Phase 3. Final Report BRGM/RC-61697-FR.

Thomas P., Wong I. and Abrahamson N. (2010) – Verification of probablistic seismic hazard analysis computer programs. PEER Report 2010/106, Pacific Earthquake Engineering Research Center, College of Engineering, University of California, Berkeley, CA, USA, 176 p., 118 fig., 3 tabl.

Youngs, R. R. and Coppersmith, K. J. (1985) – Implications of fault slip rates and earthquake recurrence models to probabilistic seismic hazard estimates, Bulletin of the Seismological Society of America, 75(4), 939-964.

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