Rare but Conceivable: Determining the Likelihood of Meteors and Other Infrequent Events

In the aftermath of a rare but devastating disaster, there is a tendency to overestimate how frequent such an event may be, hence a tendency to overestimate the risk associated with such an event. Unlike “Black Swans,” which are inconceivable until they occur, events like meteors, hurricanes, and airplane crashes that impact chemical plants are rare, but conceivable. They occur with calculable likelihood and have quantifiable consequences, but most would not fault a PHA or LOPA team for dismissing a meteor strike as “not credible”.

This paper considers several “meteors”, both natural and man-made, that often become a cause of general concern after such an event occurs: meteors, earthquakes, and tsunamis, tornadoes and hurricanes, and airplane crashes, pipeline ruptures, train derailments, and truck crashes. It describes approaches to estimating the frequency of such events at a specific facility, and provides specific methods given certain assumptions. With these frequencies in hand, the paper goes on to develop a framework for giving meaning to the term “credible”.