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Environment

The Case of the Displaced Quake

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What should have been a fortunate near-miss somehow became a direct hit. The 1994 California quake shook apart homes and offices in Santa Monica, even though the epicenter was 15 miles to the northeast in Northridge. "It was surprising— the density of damaged buildings in northern Santa Monica was just as great as it was right up near the earthquake," says UCLA geophysicist Paul Davis. Now he knows why.

Santa Monica sits at the focal point of an unusual seismic magnifying glass. The setup for disaster began millions of years ago, when Earth's crust shifted and stretched in the region between Santa Monica and Northridge. As a result, a giant block of bedrock dropped down and sediments filled the trough. Then the stress from the crustal plates reversed and started squeezing the area, which tilted and buckled the northern wall of the trough, ultimately shaping it into a convex curve.

Davis and colleagues at UCLA and Kansas State University developed a computer simulation that proved the curve acts like a giant earthquake lens if the seismic waves hit from the right direction. When the waves run into the sediments, they slow down. Meanwhile, the crescent shape of the boundary focuses the waves toward the center. Together, these processes concentrated the seismic waves from Northridge. The amplification was especially strong— about a factor of six— for those waves having a period of about 10 cycles a second, which happen to be most damaging to the one- and two-story buildings that abound in northern Santa Monica. Davis and his colleagues are now creating three-dimensional maps of the Southern California region to identify other areas that might be susceptible to a similar geological surprise.

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