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Scientists now know that the Hayward creeps—it inches along steadily, although millimeters along would be more accurate. At the rim of the stadium, a Berkeley professor named Richard Allen shows me the result of 80 years of creep: a four-inch (ten-centimeter) jog in the concrete, inelegantly bandaged with a rusty metal plate. We're both a little amused. What hubris to build a stadium on a fault!

But Allen points out the central problem: Faults don't just creep. They also "break." They "rupture." The creep happens in plain sight, but the breaking, the rupturing, the lurching—the earthquaking—will hit you blindside.

Allen teaches Berkeley's oldest course on earthquakes. He calls it Earthquakes in Your Backyard. The name couldn't be more appropriate, because the Hayward is a particularly dangerous fault. It hasn't spawned a major earthquake since 1868. Sometime soon, it could go.

Much of the stadium is built on soft ground, the kind that amplifies seismic waves. "In an earthquake," says Allen, "the entire field may liquefy." The players wouldn't sink into a jiggling vat of goo. They'd just get knocked off their pins—tackled by a temblor.

But of course no one on that field is worried about an earthquake. It's a hot summer day a few weeks before the start of the season. The players are worried about making the team. They're worried about beating Stanford.

You see right there a fundamental problem with earthquakes: They refuse to operate on human standard time. They're on their own peculiar schedule. Earthquake faults have a nasty way of combining patience with impulsiveness. Wait, wait, wait—lurch.

It's been a hundred years since the last big one in California, the 1906 San Francisco earthquake, which helped give birth to modern earthquake science. A century later, we have a highly successful theory, called plate tectonics, that explains why 1906-type earthquakes happen—along with why continents drift, mountains rise, and volcanoes line the Pacific Rim. Plate tectonics may be one of the signature triumphs of the human mind, geology's answer to biology's theory of evolution. And yet scientists still can't say when an earthquake will happen. They can't even come close.

Some of the simplest questions about earthquakes remain hard to answer. Why do they start? What makes them stop? Does a fault tend to slip a little—telegraphing its malign intent—before it breaks catastrophically? Why do some small quakes grow into bigger quakes, while others stay small?

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