Listening to the San Andreas

Based in part on his gravestone research, Boatwright and his colleagues have produced a new Shake Map for the 1906 earthquake, showing in vivid color the intensities of shaking that California experienced. The results are unsettling. The Sebastopol area, for example, shook far harder than anyone would have predicted, given its distance from the quake's epicenter.

You could look at this data two ways, Boatwright says. If you live right on top of the San Andreas, you may not be in as much trouble as you fear. "Or," he adds, "if you live 30 miles away from the fault, maybe you shouldn't feel too secure."

When that next big quake will strike is the question seismologists are invariably pestered with at cocktail parties. "Everybody always wants to know when it's going to occur," Boatwright says.

But for many earth scientists, quake prediction is a touchy subject. Some feel that specific prediction may be impossible ― that working on such research is as useful as studying 1-900 psychic hotlines.

We know more than we used to. Before the 1906 earthquake, scientists had no idea how often big quakes might strike the same stretch of fault. Every year? Every thousand years? The 1906 quake helped establish the elastic-rebound theory: Stresses build onto the fault, are released by an earthquake, then build again. Since the 1960s, it's been accepted that these stresses are created by parts of the earth's surface ― in California's case, the Pacific and North American plates ― sliding past each other.

But how fast are the stresses building? To know that, you need to know how fast the plates are moving, and in what direction. With the advent of global positioning systems, scientists can make these measurements with unprecedented accuracy.

Operated by the USGS and independent agencies such as UNAVCO (University Navstar Consortium), GPS stations resemble early prototypes of R2-D2 that never made it into Star Wars. USGS physical science technician Gary Hamilton has set up stations from the Farallon Islands off San Francisco to the High Sierra. Each one receives signals transmitted from U.S. Department of Defense satellites orbiting 12,000 miles above the earth. Scientists use the data to fix precise longitude, latitude, and elevation. With these, they can determine how fast the part of California sitting on the Pacific plate is moving compared to the part sitting on the North American plate: an average of 1.67 inches a year.