Word: faults
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Dates: during 1990-1999
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When the Loma Prieta earthquake struck the San Francisco Bay area and interrupted the World Series in 1989, it was seen almost as a novelty. More than a few observers probably harrumphed that those fools got exactly what they deserved for building a city right over a fault line on a rocky cliff overlooking the ocean...
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...
Surprisingly, the hazards of thrust faults were largely overlooked until 1983, 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...
What scientists can do, with fair reliability, is estimate the magnitude of tremors likely to occur on a particular fault. In general, the longer the rupture, the more powerful the expected quake. Unlike the San Andreas, the Elysian Park system is not large enough to unleash an earthquake of magnitude 8. But some scientists believe it might be capable of a 7 or even a 7.5, especially if more than one fault segment should give way at the same time. This is what happened in 1992, when the Landers earthquake hopscotched from one fault to another, in the process gathering...