Wednesday, December 07, 2005
Disasters!
Earthquakes!
Earthquakes are sudden rolling or shaking events caused by movement under the Earth’s surface. FACT: An earthquake is the ground shaking caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An earthquake occurs when plates grind and scrape against each other. In California, for example, there are two plates the Pacific Plate (which extends from western California to Japan, including much of the Pacific Ocean floor) and the North American Plate (which is most of the North American continent and parts of the Atlantic Ocean). The Pacific Plate grinds northwestward past the North American Plate along the San Andreas Fault at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other parts, strain can build up for hundreds of years, producing great earthquakes when it finally releases. Large and small earthquakes can also occur on faults not previously recognized; recent earthquakes in Alabama and Virginia are good examples.
