Word: faults
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...more ways than one, the earthquake that rumbled through this desolate region on June 28 was an ominous force. In a few fearsome seconds, it rerouted roads, realigned parking lots and reconfigured the landscape in countless capricious ways, miraculously taking only one life. Rather than rupture a single fault line, it swiped a 70-km (45-mile) diagonal slash through several, at one point heaving up a raw ridge of rock roughly the size and shape of a stegosaurus' spine...
...kilometers away. But most alarming of all, this quake, the largest to hit Southern California in 40 years, appears to have substantially altered subterranean stress fields. In the process, it may have awakened a fitfully sleeping dragon -- the mighty San Andreas, the nation's biggest and most dangerous fault...
...southernmost section of the San Andreas has made scientists jumpy for some time now. Between 1948 and 1986, the region adjacent to the fault experienced only one earthquake of magnitude 5.8 or higher.* Since then there have been seven, including the Landers quake, which weighed in at an impressive 7.5. Moreover, this surge in seismicity appears to be occurring on a worrisome schedule. Excavations of old lake-bed sediments by Caltech paleoseismologist Kerry Sieh in the mid-1980s indicate that large earthquakes have roared through this section of the San Andreas at not quite 300-year intervals. The last such...
About 1,300 km (800 miles) long, the San Andreas Fault system separates two sections of the earth's crust known as plates. Like giant rafts, these plates glide across an expanse of superheated rock, viscous as tar, that surrounds the planet's molten outer core. At the rate of nearly 5 cm (2 in.) a year, the Pacific plate to the west of the San Andreas is slowly pushing north, past the North American plate on the east. One possible result: 60 million or so years from now, a sliver of the California coast that includes the megalopolis...
Getting there, however, will not be fun. The slip of the plates is not constant along the fault. The southern San Andreas bends like a river and splits into multiple branches. Because of this contortion, the Pacific and North American plates cannot slip in a straightforward way but must strain against each other like two sumo wrestlers. The battle of the plates has created numerous smaller fault lines along the San Andreas, giving the region the look of a smashed windshield. Over the millenniums, the Mojave shear zone to the east may offer a path of less resistance...