Earthquak

In the interior of earth when magma(magma is molten rocks) is hot it goes up and when it is cool it goes down this process make a plate to move. The friction between the plates prevents plates to move and this cause the storage of kinetic energy in the plates. A point reached when these plates move and all the stored kinetic energy is released .When stored kinetic energy of plates is released than waves produced in all directions. The point where the energy is released is called the ‘focus’ or
‘hypocentre’, this point is beneath the surface, the point on the surface, above the focus is called ‘epicenter’. Epicenter is the first to experience the waves of earthquake. All natural earthquakes take place in the lithosphere. An instrument called ‘seismograph’ records the waves reaching the surface. Fault (fractures between two blocks of rock) is formed where two plates moves alongside. 

There are basically 2 types of waves produced in earthquake.

1. Body waves  

2. Surface waves

1. Body waves: - 

They move through the interior of the earth. They are generated due to the release of energy. There are two types of body waves. They are called P and S-waves.

           P-waves move faster and are the first to arrive at the surface. These are also called ‘primary waves’. The P-waves are similar to sound waves. They can travel through gaseous, liquid and solid materials.

          S-waves arrive at the surface with some time lag. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials. This characteristic of the S-waves is quite important. It has helped scientists to understand the structure of the interior of the earth.

      Reflection causes waves to rebound (bounce back) whereas refraction makes waves move in different directions.

2. 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 velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities. The surface waves are the last to report on seismograph. These waves are more destructive. They cause displacement of rocks, and hence, the collapse of structures occurs.

Propagation of Earthquake Waves: Different types of earthquake waves travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass.

P waves:  p waves vibrate parallel to the direction of waves. This exerts pressure on the material in the direction of the propagation. As a result, it creates density differences in the material leading to stretching and squeezing of the material. When a material is denser it squeezes and when it is less dense then is stretch.

S waves and surface waves: The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. They create more damage.

Shadow Zone: Earthquake waves get recorded in seismographs located at far off locations. however there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’.

Measuring of earthquake:

The earthquake is measured either according to it magnitude or intensity. Richter scale is used to measure magnitude of earthquake while the intensity of earthquake is measured with Mercalli scale. Richter scale ranges from 0-10 and Mercalli scale ranges from 1-12. ‘Magnitude’ is energy released during earthquake and ‘intensity’ is visible damage caused by earthquake.

Tsunami:

When seismic waves occur in sea or the earthquake, the large waves of water produced called tsunami.