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What will happen when the Hayward Fault--or the San Andreas--goes off? Scientists who study ancient quakes cannot answer that question because it depends on details that sediments do not preserve. But using a new 3-D model of the earth's crust in the Bay Area, USGS geophysicist Brad Aagaard and his colleagues can run simulations that tweak different parameters for earthquakes that have already occurred and for those still to come. The results range from the expected to the quite surprising.
At his computer, Aagaard first conjures up the 1989 Loma Prieta earthquake, which started, many scientists think, along a spur of the San Andreas some 60 miles south of San Francisco. Across a Landsat image of the Bay Area, Aagaard's simulation takes the form of a spreading blob of mixed colors that indicate shaking intensities, from low-intensity blue to medium-intensity yellow and high-intensity red. Then Aagaard calls up 1906. The difference is immediately apparent. This time red flows across the landscape like a river of lava, and among the places that glow the brightest is the area around Santa Rosa, just as the ShakeMap says it should.
Aagaard and his colleagues have started using their earthquake simulator to try to answer the most tantalizing questions of all: What if the rupture of the fault had not started directly off the San Francisco coastline? What if it had started farther south, so that instead of breaking away from the city it had aimed right toward it? What if it had started farther north and broken south? In the first instance, the tentative answer is that San Francisco gets shaken even harder; in the second, it's Silicon Valley and the Livermore Valley that find themselves clamped in the lion's jaws. "1906 is the most powerful earthquake we can imagine hitting Northern California," says Mary Lou Zoback, head of the USGS Northern California Earthquake Hazards Program. "But it may not have been the worst-case scenario."
Concerned about what their research is showing, Zoback and her colleagues have redoubled their efforts to raise public awareness of the hazard that lurks below. Later this month their voices will be reinforced by the more than 2,000 scientists, engineers and emergency managers gathering in San Francisco for a special 100th Anniversary Earthquake Conference.
The question is: Will Bay Area residents pay attention to what these public-spirited researchers say? The ghost of Hurricane Katrina, no less than that of 1906, will haunt the centennial as it gets under way. "Katrina has shown us what a $100 billion--plus disaster looks like, the kind of disaster no one wanted to talk about before," says Chris Poland, chief executive of Degenkolb Engineers and chairman of the conference. "It's shown us what happens when you damage a community so much that its economy stops."