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What is an Earthquake

   Suddenly We feel we are shaking, On the ground, on an upper floor, sitting on a chair, lying on a bed, or even walking. We ask some one else near us "is it an earthquake?." Shaking may last for a while, or it may stop. If shaking continues we are afraid, switch on the radio, or the TV. There may be a news on earthquake. No news is good news. The earthquake might have been a minor one, limited to a small area. It detail news may appear in the regular news bulletin. The bulletin would give information on where it occurred, what was its magnitude, what was its intensity etc. Earthquakes do not distinguish between people, between areas, clock time, weather or any other thing. Learning to live with earthquakes requires knowing about earthquakes. Begin here to learn about earthquakes.

How Do Earthquake occur?

        Earthquakes result from sudden release of energy, which is stored in the interior of the Earth. The energy is stored because of continuous movements of different segments of the Earth relative to each other giving rise to strain build up. When the strain build up exceeds the strength  of the Earth's material (rock), the rock breaks, the moving segments try to rebound to their earlier state of equilibrium releasing the stored energy. The released energy radiates in different directions as vibrations. These vibrations are called seismic waves. 

Seismic Waves are generated when energy is released in the interior of the Earth, according to the laws of mechanics. The energy which travel from the interior in different directions and arrives on the surface of the earth is travels in the form of what are called Body Waves. The body waves are, again, of two types, The Primary, or P type waves and Secondary, or S type waves. On hitting the surface of the earth, the earthquake disturbance produces another set of waves, which are called Surface Waves, which travel along the surface of the Earth to very far distances and, at times,  have a strong damage potential.

What are P and S waves?

    At the of earthquake occurrence, adjacent surfaces of a geological fault move relative to each other, the fault breaks and energy is released. A large part of this energy release is dissipated in crushing and heating the rock in the area of the fault break. A small fraction of the released energy escapes as elastic energy, produces compression and shear stresses in the medium, thereby producing compressional and shear waves. The compression waves are called P-waves and the Shear waves are called S-wave. P-waves travel faster than the S-waves in the interior of the earth, and these are the first ones to reach a seismological observatory. For this reason P-waves are called Primary waves and S-Waves are called Secondary waves. P wave vibrations have a frequency range of 1-2 Hz at thousand kilometers or more while S-wave frequencies  are lower than this. From very strong earthquakes they can do several rounds of the earth surface. It is surface waves which cause maximum damage to objects on the earth's surface. Periods of the ground motion in surface waves is in the range of 10-30 second.

Free oscillations

Some  very large earthquakes have caused the entire earth to vibrate as one single body. Such vibrations. Periods as high as 57 seconds were observed during the great Lisbon.