The Role and Meaning of Uncertainty and Probability in Natural Hazard Assessment (Invited)

About one decade ago, Donald Rumsfeld provided a comprehensive view of the uncertainties in hazard assessment: "There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know". This apparently tongue-in-cheek definition describes well the variety of different types of uncertainty that scientists face in describing the Nature. The ubiquitous and deep uncertainties in the formal representation of natural systems imply that the forecasting of emergent phenomena such as natural hazards must be based on probabilities. Notwithstanding the very wide use of the terms 'uncertainty' and 'probability' in natural hazards, the way in which they are linked, how they are estimated and their scientific meaning are far to be clear, as testified by the last Intergovernmental Panel on Climate Change (IPCC) report and by its subsequent review. The lack of a formal framework to interpret uncertainty and probability in a coherent way paved the way to some of the strongest critics to hazard analysis; in fact, it has been argued that most of natural hazard analyses are intrinsically 'unscientific'. The purpose of this talk is to confront and clarify the conceptual issues associated with the role of uncertainty and probability in hazard analysis. Of particular concern is the taxonomy of uncertainty and the logical framework in which a probability can be tested and then considered 'scientific'. Specifically, we discuss the different types of uncertainties (aleatory variability, epistemic uncertainty, and ontological error), their differences, the link with the probability, and their estimation using data, models, and subjective expert opinion, in the framework of two particular problems, the Probabilistic Seismic Hazard Analysis (PSHA), and the Probabilistic Volcanic Hazard Analysis (PVHA).