Thursday, July 2, 2009

EARTHQUAKES

To understand earthquakes and volcanic eruptions the fundamental role of tectonic plates needs to be examined. At their points of collision with a neighbor, or when tension or pressure builds up through encounters with obstructions, earthquakes occur and the area affected by the interference is shaken. Waves of different types and strengths are radiated from the shaken location, referred to as the epicenter. The fastest and most widely distributed of these waves are known as “P” waves. They transmit shock waves of alternating compression and dilation that can pass through gases, liquids, and solids and so reach most of the earth at all levels of depth depending on their strengths. There are other, slower types of waves that emanate from the epicenter. One of them, known as an “L” wave, moves rapidly in and out of the outermost layer of the earth and is responsible for much of the physical damage caused by earthquakes.



The Alaskan earthquake of 1964 was one of the strongest to be recorded 
since seismograph records began. Its “L” or long waves lasted for five minutes and, since their strength was very high, they were felt in places all over the world. Their strength was so high that the instruments at that time were unable to measure the maximum shocks. Later, as evidences of its destructive power were examined in detail, its strength according to the Richter Scale was known to be 9.2.  Because of the strength of its long waves, the Alaskan quake made the surface of the earth shake up and down like the ringing of a bell. Three thousand miles away, in the Great Lakes on the U.S./Canada border, the same long waves set up several series of surface waves known as seiches and, on the other side of the globe in South Africa, there were seiches in lakes and on the ocean. The same long waves lifted up the surface of the surrounding earth surface by more than forty feet and pulled it down ten feet below its normal level. This is still regarded as the greatest deformation ever known from an earthquake.

Image Source : learnontheinternet

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