The Case of the Displaced Quake - Brief Article

What should have been a fortunate near-miss in some way became a direct hit. The 1994 California quake shook apart dwellings and offices in Santa Monica, flat 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 originate a giant block of bedrock dropp down and sediments filled the long tray Then the stress from the crystal plates revers and started squeezing the area, which tilted and buckl the northern wall of the long tray ultimately shaping it into a protuberant curve.

Davis and colleagues at UCLA and Kansas State University lay opened a computer simulation that prov the bend acts like a giant earthquake len if the seismic waves hit from the right direction. When the waves race into the sediments, they moderate down. Meanwhile, the crescent shape of the boundary focuses the waves toward the center Together, these processe 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 circle of times a second, which happen to be greatest in quantity 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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