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Earthquakes

At 5:04 on the afternoon of October 17, 1989, an earthquake, without warning, struck Loma Prieta, California (60 miles south of San Francisco), lasted for six to ten seconds, killed 63 people, injured 3,757 others, caused $6 billion in damages, registered 6.9 to 7.1 on the Richter scale, and produced 6,000 aftershocks. The Loma Prieta landscape came to life when subsurface tension in one of the shifting chunks of earth's crust, the Pacific Plate, snapped beneath the San Andreas fault line. Satellite observations show that the Pacific Plate- southwest of the San Andreas fault line- slipped more than six feet closer to the North Pole, relative to the North American Plate northeast of the fault. In comparison, the 1906 San Francisco earthquake had an average slip of 15 feet over almost 300 miles of the fault line, had released about 50 times more energy, and had lasted almost 80 seconds.

In theory, the Bay Area should not have been so badly shaken because the epicenter was more than 100 kilometers away, and the seismic waves should have weakened considerably as they spread away from the source. The secret of Loma Prieta's long reach, however, is believed to lie deep in the crust, where Loma Prieta's waves were focused on San Francisco by deep crustal layers acting like mirrors. It is believed that the deep crusts near the epicenter could have efficiently reflected seismic waves from the event back toward the surface. Key to the reflection process would have been the layers of denser rock detected in the lower crust between the depths of 10 to 21 kilometers. These reflecting layers, it is believed, were well positioned to relay seismic energy from the epicenter to the Bay Area. The realization that the deep crust plays an important role in exacerbating earthquake damage gives engineers, planners, and homeowners yet another worry: "Not only must they consider soil conditions and the proximity and orientation of faults, but they have t...

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Earthquakes. (1969, December 31). In LotsofEssays.com. Retrieved 08:28, March 29, 2024, from https://www.lotsofessays.com/viewpaper/1704006.html