Earthquake waves are basically of two types — body waves and surface waves.
Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. There are two types of body waves. They are called P and S-waves.
P-waves: are the first to arrive at the surface. They travel through gaseous, liquid and solid materials. P-waves vibrate parallel to the direction of the wave.
S-waves: arrive at the surface with some time lag. S-waves can travel only through solid materials. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.Surface waves are considered to be the most damaging waves.
Surface waves: The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The surface waves are the last to report on seismograph. They cause displacement of rocks, and hence, the collapse of structures occurs. Surface waves are divided into two different categories: Love and Rayleigh.
- Love waves: have a particle motion, which, like the S-wave, is transverse to the direction of propagation but with no vertical motion. Their side-to-side motion causes the ground to twist from side to side.
- Rayleigh waves: create a rolling, up and down motion with an elliptical and retrograde particle motion confined to the vertical plane in the direction of propagation.
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The shadow zone is the area of the earth from angular distances of 105° to 145° from a given earthquake that does not receive any direct P waves or S waves. The entire zone beyond 105° does not receive S- waves. The shadow zone of S-wave is much larger than that of the P-waves.
The shadow zones have led seismologists to conclude that interior of the earth is not homogeneous. The concepts of change in medium and density has been identified with the help of shadow zones. The shadow zone of P waves led to identification of boundary between mantle and core. Further, it has also been deduced that outer core is made of liquid, as S waves cannot travel through liquid mediums and P waves are refracted by the liquid core.