Word: crusts
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Dates: during 1990-1999
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...monster that inhabits the San Andreas Fault, a spectacular, 800-mile-long slash through the earth's surface. But last week's earthquake was a sobering reminder that the mighty San Andreas is not the state's only seismic menace. A web of smaller cracks crisscrosses the fragile California crust. Many of these faults are well known. But others lie hidden deep underground, like the one that gave Los Angeles its latest disaster. Until the earth moved, the residents of the northwestern suburb of Northridge had no idea that a deadly fault lay right below them, nine miles down...
California is an earthquake zone because it lies on the boundary, marked by the San Andreas Fault, between two huge sections of the earth's crust, known as plates. Gliding atop a sea of superheated rock that surrounds the planet's molten outer core, the Pacific plate -- a thick slab to which Los Angeles is attached -- is very slowly pushing its way north and west, past the North American plate to the east, which is moving in the opposite direction. Most of the time, in most places, the two plates are snagged; they block each other's progress, and tremendous...
...similar process goes on along smaller cracks in the crust outside the main fault line. But while the earth slides horizontally along the San Andreas, many of the other fissures, including the one under Northridge, are called thrust faults because they cause the ground to move vertically. Given enough time, they help form mountains and valleys...
...when a fierce temblor hit the small central California town of Coalinga. The culprit turned out to be a deeply buried fault (four to 10 miles down) that no one had known about. Its only sign on the surface had been a fold, or buckling, in the earth's crust. Many scientists had thought such folds were harmless, formed by an imperceptibly gradual lifting of the ground. But when Ross Stein, a geophysicist with the U.S. Geological Survey, and geologist Robert Yeats of Oregon State University examined the seismic record of fold belts all around the world, they uncovered...
Hidden Faults: More disasters lurk under California's crust...