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From the air, the San Andreas stands out as a linear gash in the earth's surface that is easy to spot. On the ground, however, it is often hard to read, particularly north and south of San Francisco where it strays offshore, runs through dense redwood forests and even disappears beneath houses and streets. In many populated areas, it's impossible to tell just where the active strands of the fault lie because so many features have been filled in or bulldozed away.
Thanks to the efforts of USGS paleoseismologist Carol Prentice and her colleagues, however, residents of the Bay Area will have a much better sense of the precise path the earthquake took. Working with old photographs, Prentice has found a number of the missing signs of 1906--abrupt jogs in fences that once straddled the rupture zone, for example--and located them on aerial photos. Among the communities bisected by the fault break is San Bruno, a city of 40,000 that borders San Francisco international airport.
Luckily there are not too many structures located within the strip, about 1,000 ft. wide, that defines the San Andreas Fault zone. The same cannot be said about the nearby Hayward Fault. Along with the Calaveras, San Gregorio and Rogers Creek faults, the Hayward forms part of what scientists refer to as the San Andreas system, and it runs for 60 miles along the hills of the East Bay, cutting through the University of California, Berkeley, football stadium and skimming uncomfortably close to the Caldecott Tunnel, through which 153,000 cars pass daily. Major highways, including Interstate 80, cross the Hayward Fault, as do the pipelines that bring water down from the snow-clad Sierra. There are hundreds of privately owned structures in the fault zone, virtually all built before the state passed a tough earthquake-zoning law in 1972.
The hazards posed by earthquakes do not stop at the fault zone. Most of the damage caused by a quake comes not from the rupturing of the ground underfoot but from seismic waves that propagate out from the fault at 8,000 or more m.p.h. While the punch packed by these waves tends to diminish as the distance from the fault increases, that's not always the case. From historical accounts, USGS seismologist Jack Boatwright has assembled a ShakeMap for 1906--a map that displays the intensity of shaking in different areas. For San Francisco and other communities close to the San Andreas, it was quite severe. But even more severe was the shaking that occurred in the city of Santa Rosa, more than 15 miles away from the fault. On a scale of 1 to 10, Santa Rosa stands out as a 9-plus, somewhere between "violent" and "extreme."