Seismic Waves

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Seismic waves
One of the most destructive phenomena of the nature is the earthquake and its terrible effects. Earthquakes are caused mostly when the edges of two tectonic plates get stuck on each other, or by breaking and shifting of subterranean rock, rupture of geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. A disturbance like an earthquake at any point on the Earth will produce energetic waves called seismic waves. 
Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions. The Earth's solid crust will support waves through the crust called body waves and on the surface called surface waves. An important characteristic of the waves is the wave velocity. The difference in the speed of each type of wave, is also the property that is used to determine the location of the focus of the earthquake.
There are many different seismic waves, but the most important are :
o P waves ( compressional waves);
o S waves ( transversal eaves); 
o Love waves;
o Rayleigh waves.
Near an earthquake the shaking is large and dominated by shear-waves and short-period surface waves. These are the waves that have the worse effect, from the damage point of view, to the human activities, and especially to the engineering activities: construction and exploitation of the buildings, highways. The effects of the earthquakes are influenced by several factors like: amplitude of the waves,speed of travel, time of shaking, intensity etc.
Generally, even for the large earthquakes, the time of shaking is represented by a few tens of seconds, but for the greatest ones the time can be bigger( minutes).
The distance is affecting the amplitude of the seismic waves. At farther distances the amplitude of the seismic waves decreases as the energy released by the earthquake spreads throughout a larger volume of Earth. 
The P and S types are called body waves because they travel or propagate through the body of Earth. The last two, Love and Rayleigh, are called surface waves and they travel along Earth's surface .
Traveling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake. These waves are of a higher frequency than surface waves.


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Bibliografie

http://hyperphysics.phy-astr.gsu.edu 
http://eqseis.geosc.psu.edu 
http://www.geo.mtu.edu


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