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Why did this area get slammed so hard? At least part of the answer lies in the loosely consolidated sediment that sits below the surface. Seismic waves pass quickly through bedrock, but they become trapped in sediment-filled basins. "It's sort of like being in a bathtub filled with water," says USGS seismologist Thomas Brocher. "When you start splashing, the waves keep bouncing up and down and from side to side." The basin effect amplifies not only the intensity of the shaking but also its duration, which is no doubt why buildings collapsed in Santa Rosa in 1906, killing some 100 people. There are similar sedimentary basins throughout the Bay Area--around the Silicon Valley city of Cupertino, for example, and the expanding subdivisions that surround the Lawrence Livermore National Laboratory.
The biggest basin lies well east of the Bay, in the broad delta formed by the convergence of the Sacramento and San Joaquin rivers. Among the most catastrophic consequences of a big earthquake in the Bay Area, says University of California at Davis geologist Jeffrey Mount, would be the failure of the delta's aging levee system, which protects not just farmland and residential areas but also the water supply for some 23 million people. Shaken hard enough, the foundations of the levees would crumple, and in a kind of hydrological chain reaction, brackish water from the Bay would surge inland, contaminating the freshwater that aqueducts carry all the way to Los Angeles.
Earthquakes, scientists now know, occur along the San Andreas because the immense slabs of rock that make up the earth's crust are ever so slowly sliding past one another, borne by poorly understood currents that roil through a sea of semimolten rock. By keeping tabs on the position of key landmarks on either side of the fault, scientists can measure the speed at which the plates are traveling, in this case about 2 in. a year. The problem for the Bay Area boils down to this: except for one short section, the plates on either side of the San Andreas are tightly locked together. It's only when the stress becomes overwhelming that the San Andreas breaks apart, allowing the plates to lurch forward, 10 ft. to 20 ft. at a time.
In principle, this cycle of stress accumulation and release should be fairly regular, but scientists are finding it is not. Paleoseismologist Tina Niemi of the University of Missouri--Kansas City, for example, is studying a stream-fed marsh near Tomales Bay that has preserved evidence of past earthquakes in its sedimentary layers. By trenching through those layers to a depth of 15 ft., she has uncovered buried fissures formed by recurrent earth movements along the San Andreas. On average, that pattern repeats every 250 or so years, but "average" in this case covers a wide range. In one instance there appears to be a 600-year interval between quakes, in another just five decades.
Already, scientists say, there is a greater than 60% probability that one or more of the faults in the San Andreas system will unleash an earthquake of magnitude 6.7 or higher over the next three decades, and among the most likely candidates is the Hayward Fault. The last big earthquake on the Hayward occurred in 1868; it caused so much damage that it was known as the great San Francisco earthquake until 1906 displaced it. "The Hayward Fault is locked and loaded," says Brocher, "and it could fire at any time."