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The earthquake engineering community and regulatory agencies are moving, at varying rates, toward risk-informed engineering decisions and design. Risk-informed decision making, in turn, requires that probabilistic seismic hazard analyses explicitly and transparently incorporate uncertainty in hazard-significant seismic source and ground motion parameters. This uncertainty arises from limited data and from the existence of multiple alternative models that purport to explain these data. When uncertainty in the various inputs to the probabilistic analysis is high but is not properly captured, one may obtain misleading results; one also deprives the decision maker of useful context. Proper quantification of uncertainty also helps guide future research efforts.
The Earth science community, following traditional practice (e.g., the scientific method), often publishes a “proponent” interpretation or model with little or no expression of uncertainty beyond the limits of the immediate data that were considered in the research. This practice leaves it incumbent on the PSHA analyst (often a consultant) to capture the proper range of uncertainty for a parameter based on the body of published literature or, at times, based on the analyst’s own interpretations of available data. This is an important, often critical, interface issue between the Earth science community and the engineering community. Over time, some published interpretations or models become incorporated into “common belief” and become accepted paradigms whose uncertainties are rarely challenged even when more recent data or studies no longer support (if not outright reject) the original interpretation or model.
Emerging best practice, originating in the nuclear industry, is to use a formal, structured process to capture the “center, body and range” of uncertainty for inputs to a hazard model. This process engages the Earth science community as resource experts (e.g., providers of data) and proponent experts (e.g., providers of interpretations or model), and requires the PSHA analysts to consider whether full parameter uncertainties are captured within the available data or whether uncertainties ought to extend beyond the available data, expert interpretations, and current paradigms. An overall goal of current PSHA practice ought to be the focus on capturing the full range of uncertainty, so that the next generation of PSHA, which will be constructed with more and better information, will have results that fall within today’s uncertainty limits. This presentation will address some of the issues and questions that have evolved in the assessment of uncertainty and suggestions for a path forward in improved communication of uncertainty between the Earth scientist and the PSHA practitioner.
The following materials are available for download:
William R. Lettis was selected as the Joyner Lecturer for 2017. Dr. Lettis founded William Lettis & Associates, Inc in 1990 and Lettis Consultants International, Inc. in 2011 to provide consultancy at the interface of earthquake science and earthquake engineering. He will deliver his lecture in April 2017 at the SSA Annual Meeting in Denver, CO and in March 2017 at the EERI Annual Meeting in Portland, OR. Dr. Lettis has characterized seismic sources for probabilistic seismic hazard analyses for high-risk facilities at over 100 locations within the United States and throughout the world, in a range of seismic environments. His paleo seismic research on active faults has resulted in over 100 publications and in guidance documents for the Nuclear Regulatory Commission and the International Atomic Energy Agency. Dr. Lettis is a current member of the National Seismic Hazard and Risk Assessment Steering Committee and the California Geological Survey Mapping Committee. From 1996-2008, Dr. Lettis also served as co-chair for the American Nuclear Society Working Group Committee 2.27 to develop evaluation criteria for assessing seismic hazards for nuclear materials facilities. He has performed post-earthquake reconnaissance of numerous earthquakes worldwide, including as co-leader of the EERI 2001 Bhuj Earthquake Reconnaissance Team. Dr. Lettis’ current interest is in developing models to capture uncertainty in earthquake behaviour and ground motion parameters for seismic hazard assessment, including implementation of procedures from the Senior Seismic Hazard Analysis Committee (SSHAC).