10Assessing Hazards

“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.”

Donald Rumsfeld, US Secretary of Defense

10.1    Five Tough Questions

As the previous chapters illustrate, experience shows that assessing a natural hazard well is challenging. It involves five basic questions:

  • How is the hazard defined?
  • Where will major hazardous events occur?
  • When will they occur?
  • How big will they be?
  • What will happen when they occur?

Because our estimates of all the quantities involved have large uncertainties, scientists' goal should be to do the best they can while recognizing ...

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