Elevated freeways, highway overpasses, and garages built under homes are vulnerable to crashing down when the earth shudders, said seismologist Jack Boatwright of the US Geological Survey.

"The automobile culture is really a knife in the heart of earthquake preparedness," Boatwright told AFP. "We are only as strong as our weakest overpass."

Another key weakness is structural, including building code oversights exposed by the deadly Northridge earthquake that struck in the Los Angeles area in 1994, scientists said.

San Francisco is preparing to commemorate the major earthquake that laid ruin to the city 100 years ago on April 18.

Despite that history, significant building code reforms weren't instituted in the state until the 1970s.

While brick buildings and other risky structures have been bolstered, emergency officials concede much of the city would likely crumble in a temblor on par with the 7.8-magnitude earthquake of 1906.

Scores of small earthquakes are logged daily in California. Earthquakes have been embraced as part of the state's identity, with sports teams named after them and jokes made in films and commercials.

A "great strike" earthquake such as the one in 1906 relieves stress built up by jammed tectonic plates, reducing the odds of another temblor at the same spot.

"I don't think we quite have it nailed down, but it would appear the recurrence time for the 1906 event is 200 years," Boatwright said. "So, it looks like we are safe for a while from a recurrence. I'm pretty happy about that fact."

While the chances of another 1906-type quake along the San Andreas Fault in the next 30 years is one in twenty, there are a plethora of other faultlines overdue for trouble.

Rampant development has resulted in cities and communities built on top of faultlines, meaning they would be hit with the full force of shock waves released by earthquakes, according to scientists.

"We are better off than the people were in the Bay Area at the turn of the (20th) century," Boatwright said. "But, there is plenty of risk spread around for smaller events in inhabited areas."

The 1906 earthquake was centered off the coast of San Francisco. Another deadly quake that struck in 1989 was centered in the Santa Cruz Mountains south of San Francisco.

The Hayward Fault that runs under cities a short distance inland from San Francisco is a "poster child" for seismic hazard because it is due for a major earthquake.

"It will be an enormous mess where the earthquake occurs," Boatwright said. "It's horrifying to think that along faultlines where there used to be farmland you now have Silicon Valley filled with towns."

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