■ HOW ARE EARTHQUAKE WAVES PRODUCED ?

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◇ The abrupt release of energy along a fault (sharp break in the crustal layer) causes earthquake waves. Rock layers along a fault tend to move in opposite directions due to the force excreted on them but are held in place by counteracting frictional force exerted by the overlying rock strata.

◇ The pressure on the rock layers builds up over a period and overcomes the frictional force resulting in a sudden movement generating shockwaves (seismic waves) that travel in all directions.

◇ The point where the energy is released is called the focus or the hypocentre of an earthquake. The point on the surface directly above the focus is called epicentre. An instrument called ‘seismograph’ records the waves reaching the surface.

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■ TYPES OF SEISMIC WAVES OR EARTHQUAKE WAVES.

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The seismic waves or earthquake waves are basically of two types —

◇ Body waves and

◇ Surface waves.

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■ BODY WAVES

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Body waves are generated due to the release of energy at the focus and move in all directions travelling through the interior of the earth. Hence, the name body waves.

There are two types of body waves:-

◇ Primary - waves.

◇ Secondary - waves.

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WHY DO P-WAVES TRAVEL FASTER THAN S-WAVES ?

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◇ P-waves are about 1.7 times faster than the S-waves.

◇ P-waves are compression waves that apply a force in the direction of propagation and hence transmit their energy quite easily through the medium and thus travel quickly.

◇ On the other hand, S-waves are transverse waves or shear waves (motion of the medium is perpendicular to the direction of propagation of the wave) and are hence less easily transmitted through the medium.

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■ SECONDARY WAVES (S-WAVES)

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◇ They are recorded second on the seismograph or S-waves are also called as transverse waves or shear waves or distortional waves.

◇ They are analogous to water ripples or light waves.

◇ Transverse waves or shear waves mean that the direction of vibrations of the particles in the medium is perpendicular to the direction of propagation of the wave. Hence, they create troughs and crests in the material through which they pass (they distort the medium).

◇ S-waves arrive at the surface after the P-waves.

◇ These waves are of high frequency and possess slightly higher destructive power compared to P-waves.

◇ The trembling on the earth’s surface caused due to these waves is from side to side (horizontal).

◇ S-waves cannot pass through fluids (liquids and gases) as fluids do not support shear stresses.

◇ They travel at varying velocities (proportional to shear strength) through the solid part of the Earth.

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■ SURFACE WAVES ( L-WAVES)

#interiorofearth

◇ The body waves interact with the surface rocks and generate new set of waves called surface waves (long or L-waves). These waves move only along the surface.

◇ Surface Waves are also called long period waves because of their long wavelength.

◇ They are low-frequency transverse waves (shear waves).

◇ They develop in the immediate neighbourhood of the epicentre and affect only the surface of the earth and die out at smaller depth.

◇ They lose energy more slowly with distance than the body waves because they travel only across the surface unlike the body waves which travel in all directions.

◇ Particle motion of surface waves (amplitude) is larger than that of body waves, so surface waves are the most destructive among the earthquake waves.

◇ They are slowest among the earthquake waves and are recorded last on the seismograph.

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