Review of Recommendations for Probabilistic Seismic Hazard Analysis Guidance on Uncertainty and Use of Experts (1997) / Chapter Skim
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TREATMENT OF UNCERTAINTY
TREATMENT OF UNCERTAINTY
Pages 31-40
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From page 31... ...
Throughout its report, SSHAC emphasizes the need to distinguish between these two types of uncertainty, the quantifications of their contributing sources, and the propagation and full display of the epistemic component to users (see, e.g., Sections 1.S and i.9~. SSHAC deals win techniques to assess, elicit, combine, propagate, document, and display epistemic uncertainty, and it is clear that much if not most of the effort in any probabilistic seismic hazard analysis (PSHA)
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From page 32... ...
Consequently, the fractile hazard curves that represent epistemic uncertainty would also differ greatly. A reasonable interpretation of the probabilistic models used in seismic hazard analysis is that they represent not intrinsic randomness but uncertainty on the part of the analyst about the actual states and laws of nature for example, about the number of earthquakes of magnitude 6 to 7 that will occur In He next 50 years in a given crust volume.
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From page 33... ...
Note that as interest in PSHA is typically in the occurrence of rare events in the near future and because the occurrence of such events depends to a large extent on He current physical conditions of the earth's crust near the site, ignorance or epistemic interpretation of the occurrence probability is more appropriate than the long-term relative frequency or aleatory interpretation. In certain parts of its report, SSHAC concedes that in reality there may be just one type of uncertainty.
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From page 34... ...
where it is stated that: "because epistemic and aleatory uncertainties are treated differently in making design and retrofit decisions, and because the median hazard is sometimes the preferred central measure of hazard due to its stability, it is also important to allocate uncertainties in the proper category." While it is true that the median curve is often preferred to the mean curve, a clear rationale for this practice or, more generally, a procedure for dealing with epistemic uncertainty in decision making is not presented in the SSHAC report. Finally, in Section 7.6 reference is made to the need for multiple hazard curves in the context of probabilistic risk assessment studies.
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From page 35... ...
Different needs for uncertainty representation characterize these phases. In the elicitat~on/combinat~on phase, experts need to be aware of all pertinent sources of uncertainty, including parameter and model uncertainties and their correlations, and the limitations and errors of the available data, so that they can make an informed assessment of the validity of alternative hypotheses, the accuracy of alternative models, and the value of data and can convey such uncertainties to the TI/TFI.
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From page 36... ...
What changes with the model is not the mean hazard but the amount of epistem~c uncertainty and, therefore, all the Tactile hazard curves including the median. Therefore, any decision that is based on the fractile curves rather than the mean curve depends on the essentially arbitrary choice of how much epistem~c uncertainty is included in the seismicity and ground motion models.
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From page 37... ...
now by different members of the scientific community reflect possible future fluctuations in the overall community mean hazard, this should be useful input to the decision maker. For example, this information would allow the decision maker to see how the decision he/she must make would vary if different experts in the informed scientific community had to make that same decision.
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From page 38... ...
However, this is true only in theory, as it is clear that different amounts of information are needed to estimate with confidence the mean value of a random variable, as opposed to its complete distribution. For example, the use of best estimates for recurrence and ground motion models often leads to hazard values that are close to the mean hazards obtained by considering a large number of alternative models.
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From page 39... ...
Treatment of Uncertainty 39 it this way implies that, during the time interval of interest, new evidence and knowledge may end up "proving right" one member of the present group of experts. While this may not be a valid assumption, documentation of the expert-to-expert variability in the mean hazard may be preferable to the full display of epistemic uncertainty proposed by SSHAC.
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