PUTTING PEOPLE AND RISK IN THE SAME PICTURE VIA HAZARD ENSEMBLE DIAGRAMS

Probabilities and average recurrence intervals (ARI) based on magnitude-frequency relationships characterize the prevalent mode of teaching about the risk of geologic hazards, and are embodied in national policies such as FEMA’s Flood Insurance Rate Maps. While the statistics of recurrent events are well-established, the use of ARI for communicating risk is problematic. For example, the focus on a characteristic recurrence interval leads to the misconception that hazardous events occur with regularity (e.g., the 100-year flood). Also, the large standard deviation of an ARI (e.g., 100 years ± 100 years) is not widely known or appreciated by the public.

Lutz (J. Geosci. Educ. 59, 5-12 (2011)) introduced the hazard ensemble diagram as an alternative way to communicate risk that is arguably more effective, particularly for non-science audiences. A hazard ensemble diagram is a graph of a hazard variable (such as flow for floods) against time, with lines that represent 100 or more forward simulations of a cumulative maximum hazardous event. A hazard ensemble diagram demonstrates recurrence without introducing a characteristic time that can lead to erroneous expectations of regularity or scarcity of events. This talk extends the application of statistical ensembles to other hazards, such as earthquakes, volcanoes, tornadoes, or even nuclear power accidents, and expands the value of hazard ensemble diagrams in risk communication and teaching.

For example, the distribution of event magnitude is necessarily conditional on the length of the record, so that the duration of exposure to a hazard becomes an explicit predictor of risk. Hazard ensemble diagrams show this counter-intuitive behavior: that a constant annual probability results in rare events becoming more likely over time. This leads to a significant advantage of the hazard ensemble diagram over the traditional ARI: That the natural hazard problem is framed in terms of human behavior rather than the statistics of the hazard. Because natural hazards arise where humans and hazardous natural processes coincide, hazard ensemble diagrams offer ways to engage students and others in more concrete analysis and reflection. Examples of implementing ensemble diagrams for floods using an Excel workbook will be presented.