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Published on 05 Sep 2012 | over 4 years ago

This is a social science video that answers
- What is earthquake?
- How does earthquake occur?
- What are the different types of waves?



An unexpected movement of the Earth's surface is called an earthquake. When two parts of the earth's surface move suddenly in relation to each other along a fault line, due to tectonic forces, an earthquake occurs. A tremendous amount of energy is released in the form of tremors and vibrations, such earthquakes are called 'Tectonic Earthquakes'. The point of origin of an earthquake is called the 'focus', while the point directly above it, on the surface of the earth is called the 'epicentre'. Owing to the strong vibrations around the point of origin, the degree of destruction is greater around the epicentre. The branch of knowledge that deals with the study of earthquakes is called Seismology, and the experts who study seismic waves are called Seismologists.
There are three types of Seismic waves that spread in all directions from the focus. They are the P. S, and L waves. The compressional waves are the first to reach the earthquake recording station as they travel 1.7 times faster than the shear waves and are therefore called 'P' (Primary) waves, while the shear waves are called 'S' (Secondary) waves.
The primary waves cause the particles of rock to vibrate forwards and backwards in the direction of the waves. Secondary waves, on the other hand cause the particles to oscillate at right angles to the direction of the waves. When P and S reach the surface of the earth, they get converted into 'L' long waves that travel along the surface, vibrating horizontally at right angles to the direction of the waves called 'Love waves' or Rayleigh waves as they also travel like sea waves. The most violent shocks and subsequent destruction is caused by the L waves.
While the S waves cannot pass through liquids, the P waves can pass through any medium. This helped seismologists to learn more about the interiors of the earth.
The instrument used to detect and record seismic waves is called a Seismograph. The recorded material appears like a graph and is called a Seismogram.
Based on the depth of their foci, earthquakes are classified into tree types. They are:
(a) Deep-focus earthquakes that occur at depths exceeding 300 Kms, the reasons for which have not yet been identified.
(b) Intermediate focus earthquakes with depths of foci ranging between 55 and 300 Kms.
(c) Shallow-focus earthquakes with depths lesser than 55 Kms.
The area on the surface of the earth increases along with the increase in depth of the focus, while the intensity increases with the decrease in depth of the focus.
To measure the magnitude of the earthquakes, a Richter Scale is used. The point-scale is arranged in such a way that each of the units on the scale is equivalent to 30 times the energy released by the previous unit. While a magnitude of 2 is seldom felt, a magnitude of 7 indicates the lower limit of an earthquake that has a devastating effect over a large area.

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