"Several hundred thousand, perhaps even half a million people could be made homeless," said geophysicist Mary Lou Zoback of the U.S. Geological Survey (USGS) in Menlo Park, California.
As in 1906, the first sign of a serious quake will be a wave of very strong shaking that will ripple out at lightning speed from the earthquake epicenter.
"Even if you are sitting watching the news, you will barely have time to crawl under the table," said Zoback, who coordinates earthquake hazard programs for USGS.
Last December Zoback led journalists on a bus tour of quake landmarks during a meeting of the American Geophysical Union.
Standing atop Twin Peaks, San Francisco's highest point, she swept her arm across the panoramic vista, pointing out the places most likely to crumble when the next big one hits.
"All the areas built on fill [soft mud and sand], such as the Bayfront, Marina district, financial district, and SoMa [south of Market], will be very vulnerable," she said.
Stopping in the Marina district Zoback described how earthquakes essentially turn these soft sediments to quicksand.
"Liquefaction makes soils flow downslope, fracturing pipelines and tilting buildings that don't have deep foundations," she said.
In addition to being built on very wobbly land, many of the Bay City's homes are not designed for serious shaking.
"San Francisco's typical architectural style of having a garage on the first story and then the residence on topwhich we call a soft first story [building]is very vulnerable," Zoback said.
More than 80 percent of the city's housing was built prior to the introduction of earthquake-proof building codes in 1970.
Zoback and her colleagues estimate that nearly 40 percent of the city's buildings will be destroyed in the next big quake. The hardest hit will be the soft-first-story structures in the west of the city, built on the reclaimed land.
In such buildings just a small amount of shaking is enough to collapse open and unsupported space at ground level, bringing down the rest of the building above.
"A simple solution is to strengthen the lower layer by inserting a steel frame around the garage door," Zoback said.
Even such basic techniques have often gone unused.
San Francisco has a higher percentage of rented residences than any other United States city other than New York. Strict rent control laws here hold down rents but give landlords little incentive to shell out for earthquake proofing.
"We are sitting on a tectonic time bomb, and it is a fight against time to get all these structures retrofitted," said David Schwartz, a USGS geologist on the tour.
San Francisco's water supply is particularly susceptible to earthquake damage.
Eighty-five percent of the city's water arrives via a single pipeline from the Sierra Nevada mountains, more than 150 miles (240 kilometers) away. On its journey to San Francisco the pipeline crosses three major earthquake faults.
"Until we complete our ten-year capital-improvement program, which is just underway, the Bay Area's Sierra supply is highly vulnerable to disruption for up to 60 days following a major earthquake on the San Andreas or Hayward Faults," said Tony Winnicker, a San Francisco Public Utilities Commission spokesperson.
That could mean a San Francisco without running water and limited fresh water for two months. It's a dangerous prospect, as seen in New Orleans after Hurricane Katrina, when many residents came to depend on emergency shipments of bottled water.
To avoid such a crisis, the utilities commission is now laying a new pipeline in short, easy-to-replace sections. Sophisticated microwave sensors along the route will be able to signal the precise location of ruptures.
When the work is done the system should be able to provide a basic water supply for 24 hours after a major break. Full water delivery would be available again after 30 days, Winnicker says.
But if a major quake occurs before 2016, when the repairs are expected to be complete, residents could be in for a long and possibly perilous wait.
No Way Out?
Faced with no home and no water, many San Franciscans would likely flee the city, as their compatriots in New Orleans did after Hurricane Katrina last September.
But "many roads, including much of the north-south freeway, will be destroyed, and it is also likely that the Bay Bridge will fail again," USGS hazard coordinator Zoback said.
When the magnitude 6.7 Loma Prieta earthquake struck in 1989, a section of the San Francisco-Oakland Bay Bridge failed, and a double-decker portion of the Nimitz Freeway collapsed, killing 42 people.
Today a multibillion-dollar retrofit is underway on the Bay Bridge. Two side-by-side roadways are to replace the double-decker eastern span.
Innovative new four-legged columns, which are being driven deep into solid rock below the bay, will support the new sections. The legs on each column will be connected by easily replaceable beams, which in the event of a quake, crack and deform to absorb the shock.
Megaprojects like these give the sense that in some ways San Francisco has awoken to the danger of the next big one. But will the city be prepared in time?
If the ground doesn't hold relatively firm for another decade or so, Zoback believes the U.S. will have another major catastrophe on its hands.
"The picture's going to look very similar to New Orleans after Katrina," she said.
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