When budgeting for quakes, dig deep.

If the earthquakes that have struck the United States since 1900 are any guide, the nation can now expect to suffer, on average, billions of dollars of seismic damage each year.

Analysts have often adjusted damage estimates from long-past quakes simply by taking into account monetary inflation. But by factoring in population and property-value changes since the old quakes occurred, planners could make damage and death-toll predictions that more accurately reflect today's conditions, says Kevin Vranes, a policy analyst at the University of Colorado at Boulder.

Only three of the United States' post-1900 quakes cost more than $1 billion when they happened. In today's dollar value, three more would have broken the billion-dollar barrier. Factor in current population and property values as well as inflation, and 13 of those temblors would have inflicted $1 billion or more in damages.

If the magnitude-7.9 quake that hit San Francisco on April 18, 1906, occurred in the same place today, it would rank as the United States' costliest natural disaster--with the possible exception of the Dust Bowl of the 1930s, says Vranes. Adjusted for inflation, the $524 million damage estimate from the 1906 event, which killed about 3,000 people, adds up to nearly $9 billion (SN: 4/15/06, p. 234). But by considering today's demographics, the researchers calculated that such a quake would now cost $328 billion and probably kill 24,000 people.

Altogether, quakes that struck the United States since 1900 would cost about $430 billion if they happened today. Quakes that occurred between 1970 and 2005, the period in which damage estimates are considered to be most complete and accurate, today would trigger average annual damages exceeding $2.5 billion, Vranes estimates.--S.P.