Earthquakes

Earthquakes are like vibrations in the Earth, which separate the ground. The energy is created by a sudden dislocation of parts in the crust, by volcanic eruptions, or even by explosions caused by man. But most of the time, great earthquakes happen by the dislocation of the crust. The crust first bends and then, when the stress surpasses the power of the rocks, break into a new position. When the ground breaks, vibrations called seismic waves are created. These waves travel out from the source where they started on the surface and through the ground at a certain speeds depending on what the waves travel through. Sometimes the vibrations are of low frequency or high frequency. This causes the whole planet to shake.

 Faults are fractures in the Earth’s crust in which two blocks of the crust slip with respect to each other. There are three main groups of faults. Normal faults happen in response to tension or tugging, the overlying block moves down the slope of the fault plane. Thrust faults happen in response to squeezing or firmness, when the overlying block moves up the slope of the fault plane. Strike-slip faults happen in response to either kind stress, the blocks move horizontally past each other. Strike-slip is more along transform faults, thrust in subduction zones and the other along spreading zones. 

New Madrid-a great earthquake was followed by a large aftershock within 6 hours on December 6, 1811.

Mexico City*-a powerful earthquake shook west-central Mexico Tuesday, January 21st 2003. Nineteen people were reported dead. According to this news report, the first estimate from Mexico’s national seismological service put the quake’s magnitude at 7.6. The agency said it struck at 8:07 p.m. in Colima, a small state that includes the port state of Manzanillo, about 3000 miles west of Mexico City. But a spokesman for the U.S. Geological Survey, Butch Kinerney, said that scientists there calculated that the real magnitude was 7.8.  

Try this site for more information:

http://pubs.usgs.gov/gip/earthq1/how.html

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