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The Restless Earth: Earthquakes
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Hell's Crust: Our Everchanging Planet
The Restless Planet: Earthquakes
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 Damage in Santa Monica.

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Quake Prediction

Learning from Earthquakes

by Daniel Pendick

It's hard to imagine that anything good could come out of an earthquake. But after the shaking stops, the planet sometimes reveals some of its secrets or confronts seismologists with entirely new questions.

The 1994 Northridge earthquake in Southern California continues to drive research more than four years after the magnitude-6.7 earthquake, which killed 61 people and caused more than $40 billion in damage. The direct cause was a previously unknown fault nine miles beneath Northridge, a town in the San Fernando Valley. It was a "blind" fault, one that doesn't break the surface and make itself visible. (See animations below.) To seismologists, the quake drove home the point that the thicket of known and unknown local faults in the Los Angeles Basin may represent a hazard at least as great as the main San Andreas fault to the east.

Though Northridge would normally be considered "moderate" on the earthquake magnitude scale, it did tremendous physical damage. In fact, it was the costliest natural disaster in U.S. history. And, inexplicably, the shaking was unusually strong in certain spots. Sherman Oaks and Santa Monica, for instance, suffered greater damage than some towns much closer to the earthquake's epicenter.

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Damage from the Northridge, CA, quake in Sherman Oaks, which suffered more damage than communities closer to the epicenter.

Seismologists in California have proposed that a bowl-shaped dip in the bedrock beneath the northwestern Los Angeles Basin may be responsible for the damage pattern. Using oil company geologic maps, they found traces of a 1.8-mile-wide depression beneath the southern edge of the Santa Monica Mountains. Like a lens focusing sunlight into a burning dot, the depression might have focused passing seismic waves and caused stronger groundshaking at the surface in Santa Monica. The focusing effect contributed to the unusually strong shaking in Sherman Oaks, but part of the blame also rests with soft sediments deposited there in the past by the LA River. The sediments jiggle like a bowl of Jell-O, amplifying the groundshaking.

Animation: Blind Thrust Fault
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Animation: Normal Fault
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In the fall of 1998, scientists from UCLA, the U.S. Geological Survey, and the California Institute of Technology will put the theory to the test. They plan to set off 60 explosive charges underground in a line stretching from the Malibu Coast to the Mojave Desert. Vibrations from the explosions -- essentially, small artificial earthquakes -- will travel through any buried geologic structures, like blind faults or dips, which will bend and reflect the waves. The returning signals, picked up by a network of 1,000 seismometers, should contain traces of the bowl's effects on the waves -- if the bowl is there. If so, locating similar structures elsewhere could point to areas more at risk than others from nearby earthquakes.
 

Photo Credit: J. Dewey, U.S. Geological Survey

Article: All Stressed Out | Sidebar One: Learning from Earthquakes | Sidebar Two: Quake Prediction | Sidebar Three: Build Smart | ANIMATION
Hell's Crust: Our Everchanging Planet  |  The Restless Planet: Earthquakes
Out of the Inferno: Volcanoes  |  Waves of Destruction: Tsunamis
 

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