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All that changed at 5:12 the next morning, when the bars had finally emptied. Something happened deep under the seafloor just off the Golden Gate, out near the shipping channel. Along an ancient crack in the Earth, two slabs of rock began moving in opposite directions.

An earthquake will unzipper a fault at two miles per second. This one broke north and south. In some places the slip was just 6 or 7 feet (2 meters), but elsewhere the ground lurched fully 16 feet (4.9 meters) in a snap. The fault broke for 270 miles (335 kilometers), from Shelter Cove, way up in the redwood country of northern California, all the way south to the old mission town of San Juan Bautista.

It wasn't the worst earthquake in history by a long shot, but it was sensational. Not only did it heave the ground and topple buildings, it ruptured the water mains, leaving San Franciscans helpless as their Victorian homes and bustling shopping districts and warehouses and opera burned to the ground. No one knows how many people died, but about 3,000 is the consensus.

It inspired a kind of war on earthquakes, using the weapons of science. Until the San Francisco earthquake, geologists weren't sure how earthquakes and faults were connected. Many believed that faults were the by-products of earthquakes, not their source. The great Berkeley geologist Andrew Lawson had discovered the San Andreas Fault more than a decade earlier, naming it after the San Andreas Valley—and possibly himself (Andreas equals Andrew). But he thought it was just a little sniffle of an earth crack, a trivial thing not much more than a dozen miles in length, responsible for the narrow valley that holds San Andreas Lake and Crystal Springs Reservoir on the San Francisco Peninsula.

But earthquakes are teachable moments. When the fires died down and San Francisco started to rebuild, Lawson and a team of colleagues set out to solve the mystery of the Great Earthquake. They literally walked the "mole tracks" where the fault rupture had churned across barnyards and meadows. Then they continued south for 600 miles (970 kilometers), reading the landscape, discovering the unbroken sections of the fault. This fault just kept going and going, all the way down past Los Angeles. In 1908 the team published the fabled Lawson report, which showed this rip in the Earth in vivid photographic detail.

In the course of the investigation, a scientist named Harry Fielding Reid figured out why earthquakes happen. Reid studied all the reports of ground motion, of roads and fence lines offset by the fault, and came up with the key concept of "elastic rebound." The surface of the Earth isn't perfectly stiff. It bends. Land at some distance from a locked fault will slowly stretch in opposite directions, but the fault itself will remain locked, under increasing strain. Finally the fault breaks, and the land springs back violently, releasing accumulated strain. An earthquake, says Bill Ellsworth of the U.S. Geological Survey in Menlo Park, California, is "a relaxation process" —from the standpoint of the planet at least.

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