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Seismic Waves. Types and Nature of Seismic Waves.

When a sudden break or shift occurs in the Earth’s crust, energy radiates out as seismic waves, just as the energy from a disturbance in the body of water radiates out in wave form. In every earthquake, there are several different types of seismic waves.

Body Waves:

Body waves move through the inner part of the Earth, while surface waves travel over the surface of earth. Surface waves some times called Long Waves, or simply L-waves are responsible for most of damage associated with earthquakes, because they cause the most intensive vibrations. Surface waves stem from body waves that reach the surface.

There are two main types of  body waves:

P-waves:

These are also called Primary waves or compressional waves. These waves actually travel with a velocity of about 1-5 miles/second. Their velocity depends on the material they are passing through. If the material is dense the velocity will be high, while if the material is not so dense the speed of the waves will also be lesser. As the P-waves have maximum velocity among the all the seismic wave, these waves arrive first at any surface location. They can travel through solid, liquid and gas, and so will pass completely through the body of the Earth. As they travel through rock, the waves move tiny rock particles back and forth , pushing them apart and then back together in line with the direction the waves are travelling. These waves typically arrive at the surface as an abrupt thud.

Image result for primary waves

S-Waves:

These waves are also called Secondary waves. After the arrival of Primary waves, the secondary waves arrive at a point of Earth surface. The velocity these waves is lesser than that of P-waves. Unlike P-waves these waves are not compressional waves. These waves are just like the waves produced on the surface of water by dipping something in the water. These waves usually move the earth surface up and down. By the up and down movement of the Earth surface these waves brings much destruction upon the earth surface. These wave are not much powerful as the Primary waves are. These waves do not pass throughout the body of earth. They only travel through solid material, and so are stopped at the liquid layer in the Earth’s mantle. These waves are also called Shear waves.

Image result for secondary waves animation

Surface Waves:

Surface waves are something like the waves in a body of water. They move the surface of the Earth up and down. This generally causes the worst damage because the wave motion rocks the foundations of man made structures. L-waves are the slowest moving of all waves, so the most intense shaking usually comes at the end of an Earth quakes. These waves are also called Rayleigh waves.

Image result for secondary waves animation

Note: P-waves pass through both mantle and core, but are slowed and refracted at the mantle / core boundary at a depth of 2900 km. S-waves passing from the mantle to the core are absorbed because shear waves cannot be transmitted through liquids. This is evidence that the outer core does not behave like a solid substance.

Identification of Different types of Seismic Waves:

All these waves can easily be identified, on a seismography record and by measuring the difference in the arrival times of these waves, it is possible to estimate the distance of the source from the observatory place. The foremost arriving waves which give a compressional movement to the earth and are arrive as an abrupt thud. Secondary waves arrive after some seconds the secondary waves. The movement of the waves is up and down and are some what slower in jerks. The L-waves arrive latest and these waves are longitudinal like the secondary waves. Their movement is quite slower and long lasting.

 

 

 

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