Scientists and engineers with the United States Geological Survey say there is a 63 percent chance of a magnitude 6.7 quake happening here within the next 30 years. How big is a 6.7? That was the strength of the Northridge quake. Loma Prieta was 6.9. If it happens on the San Andreas fault it could be as strong as the 7.8 quake of 1906.
"There would be tens of thousands of buildings badly damaged, there would be 250,000 San Franciscans, perhaps on that order, who would not be able to return to their homes," says geotechnical engineer Tom Tobin who just finished a study commissioned by the city of San Francisco, which he says is particularly vulnerable. "In San Francisco, the characteristic weakness is that the ground floor where there is parking or where there are businesses is weaker than the floors above."
Tobin's study warned the city that 90 percent of its buildings are residential and that is where most of the risk exists.
The Bay Area's other major fault is the Hayward one which lies directly beneath the most densely populated area of the East Bay.
"It would be a very near-source earthquake with very strong shaking -- cities from the south of Fremont up through Pinole," says Tobin.
Seismologists say the Hayward fault is capable of a 7.3 and is overdue. On average, the fault has a major shift every 140 years. It has been 143 years since the last one.
Dana Buntrock is an associate professor of architecture at UC Berkeley specializing in construction practices in Japan and the United States. She says the city of Berkeley has done a relatively good job of giving homeowners incentives to get prepared.
"To bolt their foundation down, to put in sheer walls, to stiffen up that chimney," says Buntrock who adds that many other communities have put off measures. "And the result is that if we had a major seismic event we have less confidence that our buildings would be able to shelter us."
Buntrock sums up the situation this way -- there is no top end for earthquakes, but there is a limit on what we're willing to spend, and in this recession she sees those spending decisions being pushed off into the future.