Uncertainty in Seismic Hazard Assessments (Truth in Advertising/Full Disclosure) M MAX Workshop USGS-Golden September 8/9, 2008 Jon Ake, U.S. NRC Figure A3–1

Uncertainty in Seismic Hazard Assessments Regulatory Requirements/Guidance: 10 CFR recognizes the nature of uncertainty in seismological and geological evaluations and the need to account for these uncertainties. Further developed in Regulatory Guides and Similar considerations are imbedded in DOE Standards, recent ANS/ANSI Standards, ICODS Guidelines Good Practice Figure A3–2

Uncertainty and Variability 1. Aleatory Variability -The natural randomness in a process -Not predictable -Can’t be reduced (theoretically) 2. Epistemic Uncertainty -Knowledge-based -Unknown models/parameters -Competing models -Potentially reducible -Generally incorporated via logic trees Figure A3–3

Examples Epistemic: Alternative models to predict magnitude based on Intensity -Maximum Intensity (MMI MAX ) vs Area within a MMI contour (A I-VI ) -Each of these models has an uncertainty associated with it. Epistemic: Alternative models to predict magnitude based on fault characteristics -Fault length vs displacement -Each of these models has an estimate of variability associated with it Figure A3–4

Uncertainty in Seismic Hazard Assessments Need to take a pragmatic approach A snapshot in time Problems especially acute in SCR Space for time substitution is a given (both for development of models and estimates of variability) Figure A3–5

Summary Necessary to formally include uncertainty/variability in our assessments We need to develop a well-defined process that will include and track the aleatory and epistemic uncertainties, but avoid “double-counting” of uncertainty Need to define credible alternative models Figure A3–6