Probabilistic Seismic Hazard Analysis Project

1. Probabilistic Seismic Hazard Analysis Project Origins and Objectives 12-05-2012

2. BC has all four types of tectonic regions • Active Crustal: WNA • Stable Continental: CENA • Subduction Zones: Cascadia • Volcanic: Volcanic Arc/Cascades >80 dams 18 Extreme Consequence 6 > 10,000 PAR

3. Background • BC Hydro has been involved in probabilistic seismic hazard analysis (PSHA) since the 1980s. • Evolution in Approach: • 1984 – first PSHA for Lower Mainland & Vancouver Island, using GSC seismic source model • 1991-92 – Provincial Seismic Hazard Review; developed BCH seismic source model • 1997 – Introduced “multiple model” approach, with BCH and GSC “H” & “R” source models as alternates, & alternate ground motion models • Post-1997 – Ongoing introduction of new ground motion models • Results have been used for seismic performance evaluations and designs for dams and the electric system as they have become available

4. Concerns About BCH PSHAs in 2000s • Seismic source and ground motion models becoming outdated • Cascadia subduction zone not incorporated into probabilistic model • Large computed ground motions • Large uncertainties • Sensitivity of computed ground motions to some model inputs (especially subduction zone ground motion models) • Stability and consistency of hazard estimates, same site and site-to-site • Dr. C. A. Cornell retained in 2007. • Alternative approaches to update BCH’s PSHA were considered.

5. Project Objectives and Startup • Develop an up-to-date and technically sound PSHA model for the BC Hydro service area. • Achieve technical stability of the PSHA model and its results for 10-15 years. • Model to be used by BC Hydro for seismic hazard assessment and characterization at all sites. • Initial work undertaken mainly with internal resources • Review by Dr. Cornell indicated that more rigour required to meet Project Objectives • Dr. Cornell recommended a SSHAC III approach

6. Probabilistic Seismic Hazard Analysis Project Results and Policies for Use +$10M and 4 years later… .

7. PSHA Model – ‘Shallow’ Seismicity

8. PSHA Model – ‘Deep’ Seismicity Subduction slab represented by 4 source zones which become increasingly deeper to the northeast. Source zone boundary uncertainty is included in the model The Nootka Fault is represented as a separate source

9. PSHA Model – Evaluated & Selected GMPEs Volcanic Arcs: Evaluated McVerry et al (2011) but did not use it.

10. PSHA Model: BC Hydro Subduction Model

11. Uncertainty in PGA Hazard Now what??

12. Median Mean 84th percentile Now that we understand the uncertainty… • what to use? • Different philosophies at different sites Typical Results (actual data : Site C) Relative confidence Level (%)

13. PSHA Policy • For High, very High and extreme Consequence dams, ground motion hazards at a mean 10-4 AEF to be a starting point • Best representation of the uncertainties • Use full spectrum of results for risk analyses • Consider the potential for more extreme events, and the risks associated with those events • Policy decision to embrace risk informed decision making (finally…) • Consider combinations of events. For example, if a dam is damaged due to an earthquake, would it be possible to subsequently pass the mean annual flood (or larger flood) safely? • Incorporate system thinking and reliability principles in design

14. Effects of PSHA project and policy

15. Effects of PSHA project and policy LaJoie

16. Effect of PSHA Policy and Results • Different effects at different sites • Site C : increase in ground motion • (median to mean) • HLK: decrease in ground motion • (84th to mean) • RUS : decrease in seismic hazard • JHT : decrease in PGA, but • increase in hazard • JOR : increase in ground motion

17. Probabilistic Seismic Hazard Analysis Project Sharing the Work Giving away the hard-earned IP…. 12-05-2012

18. Requests for Model/Model Components

19. Sharing the Model • Project documents ALMOST signed off : • - GIS based Seismic Source Model • logic trees that characterize the various source areas • - Ground Motion model (attenuation formulae) • logic trees • IP will be shared by Publication of scientific papers • Access to hardcopy reports • Actual software code held by AMEC • With access to IP, others can develop alternative codes

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21. What Has Changed Since 1997? • Seismic Data Base • Another 15 years of data : increased data base of western Canada recordings • no large, “surprise” earthquakes • Several important worldwide earthquakes, some with strong motion records • Improved understanding of BC seismotectonics, e.g. • greatly advanced knowledge of 3D structure & geological evolution of Canada’s continental landmass and its margins. • Improved understanding of the configuration & behaviour of the Cascadia subduction zone, its seismic history and seismic potential • Ground Motion Models • Ongoing development & publication of new attenuation relationships, e.g.

22. Elements of PSHA – Cornell Methodology • Identify and model sources of aleatory (random) and epistemic (model and parameter) uncertainty • Available information often supports multiple, credible (scientifically sound) interpretations • SSHAC : The goal is to develop inputs that represent the composite distribution of the informed scientific community Uncertainty in Attenuation Fault Confidence Limits Frequency of Exceedance Acceleration Site Cumulative Rate (EQ > M/yr) Magnitude M1 Area Sources Mean M2 Mx Magnitude (M) Acceleration Distance STEP 3 Ground Motion Attenuation STEP 2 Recurrence Model STEP 4 Ground Motion Hazard STEP 1 Seismogenic Zone Model Idriss Extreme Upper Upper BCH Upper Sadigh et al. Best Estimate GSC-H Best Estimate Campbell Lower GSC-R Lower Extreme Lower BJF Alternate Mx values for each recurrence curve Alternate seismogenic zone models Alternate attenuation relationships Alternate recurrence curves for each seismogenic zone

23. Elements of PSHA – Cornell Methodology Uncertainty in Attenuation Fault Confidence Limits Frequency of Exceedance Acceleration Site Cumulative Rate (EQ > M/yr) Magnitude M1 Area Sources Mean M2 Mx Magnitude (M) Acceleration Distance STEP 3 Ground Motion Attenuation STEP 2 Recurrence Model STEP 4 Ground Motion Hazard STEP 1 Seismogenic Zone Model Ground Motion Model Seismic Source Characterization Model Ivan Wong led the SSC group!

24. PSHA Policy Review • Peer Reviewed as part of Project • April 28, 2011 at Stanford U Marty McCann • Ivan Wong • Kevin Coppersmith • All agreed on use of mean : entire scientific process of hazard analyses revolves around the understanding of uncertainty • Use of mean already considered best practice in the US • Confirmed that policy is defendable • Use of Risk Informed approach most welcome : • “Dam community is two decades behind the Nuclear industry” • USBR and USACE have now moved to risk-informed decision making

25. PSHA Policy : Use of the Mean • Broadly representative of the distribution • Means add to means mathematically for combined risk analyses • Accounts for uncertainty Uncertainty not accounted for if Median is adopted Preliminary Results for Site C Relative confidence Level (%) Median Mean 84th percentile

26. PSHA Policy : Use of the Mean • Broadly representative of the distribution • Means add to means mathematically for combined risk analyses • Accounts for uncertainty Do we really need this little uncertainty? Preliminary Results for Site C Relative confidence Level (%) Median Mean 84th percentile

27. Use of mean supported by ICOLD draft bulletin Median Mean 84th percentile PSHA Policy : Use of the Mean • Broadly representative of the distribution • Means add to means mathematically for combined risk analyses Relative confidence Level (%)