78. The upper limit of possible
formed on the basis of past experience
Previously we mentioned the anecdote about how a river was dammed and the frequency of floods decreased, but their magnitude increased, leading to greater overall construction. One factor was a false sense of security created by the dam, which caused building closer to the river. This tends to be a general feature of dams and embankments in that they create a false feeling of safety. The river and dam example is also useful for considering the fact that our imagination of the upper limit of a possible catastrophe is formed on the basis of our past experience. We do not account for once-in-a-hundred-year events, because we haven't lived through them. Few large structures on the Hayward fault in the San Francisco Bay Area are built to cope with a once-in-a-hundred-year earthquake, though one will eventually occur sooner or later. We need to prepare for disasters which are significantly larger than anything we have seen.