Seismic Waves
Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.
There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves. Body waves can travel through the earth's inner layers, but surface waves can only move along the surface of the planet like ripples on water. Earthquakes radiate seismic energy as both body and surface waves.
BODYWAVES
They are of two types:
P WAVESThe first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to 'arrive' at a seismic station. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves. P waves are also known as compressional waves, because of the pushing and pulling they do. Subjected to a P wave, particles move in the same direction that the wave is moving in, which is the direction that the energy is traveling in, and is sometimes called the 'direction of wave propagation
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| FIGURE 1 - A P WAVE TRAVELS THROUGH A MEDIUM BY MEANS OF COMPRESSION AND DILATION. PARTICLES ARE REPRESENTED BY CUBES IN THIS MODEL |
S WAVES
The second type of body wave is the S wave or secondary wave. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium. It is this property of S waves that led seismologists to conclude that the Earth's outer core is a liquid. S waves move rock particles up and down, or side-to-side--perpindicular to the direction that the wave is traveling in (the direction of wave propagation).
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| FIGURE 2 - AN S WAVE TRAVELS THROUGH A MEDIUM. PARTICLES ARE REPRESENTED BY CUBES IN THIS MODEL. |
SURFACEWAVES
They are of two types:
LOVE WAVES
The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side. Confined to the surface of the crust, Love waves produce entirely horizontal motion.
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| FIGURE 3 - A LOVE WAVE TRAVELS THROUGH A MEDIUM. PARTICLES ARE REPRESENTED BY CUBES IN THIS MODEL. |
RAYLEIGH WAVES
The other kind of surface wave is the Rayleigh wave, named for John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.
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| Figure 4 - A Rayleigh wave travels through a medium. Particles are represented by cubes in this model. |
The method of epicentral determination makes use of the time interval between the arrival of P and S waves at each station due to difference in their velocity. This interval so calculated from a prior knowledge of their velocities gives corresponding distances based upon the local or regional earth structure. The distance from three stations determines the epicenter based on the theorem of geometry. For this purpose, either a map showing the location of the stations is taken or better a big sheet globe may be used. Using the distance scale, one can take a compass and draw three arcs of a circle, with the three observatories as centers. By the theorem, the arcs will intersect at some point. This point of intersection on earth’s surface is the epicenter.
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