The United States is becoming more vulnerable to natural hazards mostly because of changes in population and national wealth density—more people and more societal infrastructure have become concentrated in disaster-prone areas. For most of the 20th century, the United States has been largely spared the expense of a catastrophic natural disaster. A great earthquake (magnitude 8 or larger) has not struck a major metropolitan area since the 1906 San Francisco earthquake. An extreme or catastrophic hurricane (Class 4 or 5) has not struck directly a major urban area since the one that hit Miami, Florida, in 1926. Yet even without such disasters, which might create losses well over $100 billion, the overall costs of natural hazards, such as extreme weather, drought, and wildfires, are estimated at $54 billion per year for the past 5 years, or approximately $1 billion per week [National Science and Technology Council, 1997].