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Focused quake waves hit Bay Area.

When the Loma Prieta earthquake struck on Oct. 17, 1989, it caused surprisingly severe damage to San Francisco and Oakland, both of which sit about 100 kilometers from the quake's epicenter in the Santa Cruz mountains. Even as the Bay Bridge and other structures were collapsing, however, some cities much closer to the quake suffered significantly less damage, a paradox that has seismologists scratching their heads. Two researchers now think they have answered the riddle of Loma Prieta's long-distance punch.

Using a powerful new computer model, Anthony J. Lomax and Bruce A. Bolt of the University of California, Berkeley, have tested how seismic waves traveled through the crust as they radiated away from the center of the magnitude 7.1 quake. The study shows that geological structures in the region caused the shaking in San Francisco and Oakland to reach double the level expected for a quake of that size.

The study supports a theory raised soon after the quake which suggests that a fraction of the extra vibrations were reflected waves (SN: 4/21/90, p.251). This idea holds that some downward-directed seismic waves bounced off the 25-km-deep Moho-the boundary between the crust and mantle--and were focused up toward the surface in the vicinity of San Francisco and Oakland.

The computer modeling suggests that a second type of focusing came into play as well. Geologic structures refracted waves that normally would have passed to the east or west of San Francisco and Oakland, funneling the vibrations so that they hit the metropolitan region. On the west, hard rocks on the Pacific side of the San Andreas fault caused seismic waves heading toward Alaska to bend toward the Bay Area. On the east, thick sediments under the Santa Clara Valley caused waves heading toward Nevada to bend toward San Francisco and Oakland, the researchers report in the Oct. 2 GEOPHYSICAL RESEARCH LETTERS.

The Moho bounce and wave refraction caused extra-strong waves to focus on a region between the two cities measuring 10 km wide by 15 km long, according to Lomax and Bolt. In a sense, these cities were hit by bad luck -- a combination of the earthquake's location and the presence of nearby geologic structures. Human factors also played a role. The researchers note that many of the damaged structures in San Francisco and Oakland were built on soft sediment and landfill, which amplified the shaking.
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Title Annotation:San Francisco area, California
Publication:Science News
Article Type:Brief Article
Date:Oct 31, 1992
Words:398
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