Lawson, Reid, and their colleagues had no way of understanding the ultimate source of the forces behind earthquakes. But by the late 1960s, scientists had come to realize that the Earth is divided into about 15 plates of crust, constantly shifting as new rock forms at mid-ocean ridges and old crust dives into the Earth's interior at subduction zones in the deep sea. Suddenly the Himalaya were revealed as a crash site, with India slamming into Asia. And the San Andreas was not just a long strike-slip fault: It was a plate boundary, where the North American and Pacific plates grind slowly past each other at a rate—precisely measured by GPS—of two inches (five centimeters) a year.

But except for a section called the "creeping zone" in central California, the San Andreas is locked. Around San Francisco, the fault hasn't budged since 1906. North of Los Angeles, a long stretch of the fault has been stuck since 1857. Near Palm Springs, there's been no action on the fault since about 1680.

At some point the San Andreas will have another relaxation event. When that happens, despite all the forecasts, all the measurements, all the scientific conferences, nearly everyone will be caught by surprise.

Although it is probably the most famous fault on the planet, the San Andreas is often strangely hard to find. It slices an enigmatic path through wildly varied topography. Sometimes it's obvious—viewed from above on the Carrizo Plain in south-central California, for example, where it looks like a zipper, or at Thousand Palms in the Mojave Desert, where fan palms line up neatly to drink water percolating upward through the fault. But usually the San Andreas lurks in the landscape, a shadowy presence. When you search for the fault you spend a lot of time thinking: Is this it? Or is that it? Is this the boundary between two enormous tectonic plates, one stretching to Japan and the other to the middle of the Atlantic Ocean? Or am I standing in a random ditch?

A century after Lawson et al. rambled across California, researchers are still pinpointing the fault's active trace. I tagged along with Carol Prentice, a geologist with the U.S. Geological Survey in Menlo Park, who has been stomping through the dense redwood forests of northern California. She is aided by a new technology called LIDAR, which uses aircraft-borne lasers to trace the contours of the land. Photos and maps in hand, she hikes through the woods, noting every feature that might reveal the exact location of the fault: sag ponds, offset streams, displaced fences. She has even found what appears to be a redwood stump literally ripped apart by the great quake. Prentice takes you into brush so thick and tangled you have to crawl. What we couldn't see on foot, we saw on knee.

I asked her what would happen if the fault broke right under us, out here in the boondocks. "That'd be so cool, if we were right here," she said. "Oh! I would love it. You wouldn't be able to stand up. It'd knock you on your butt. Presumably you'd see the 'rending and heaving of the sod.'" She was quoting from the Lawson report.