# Earthquakes

An earthquake is a vibration or oscillation of the surface of the earth caused by the elasticity or the isostatic adjustment of the rocks, beneath the surface of the earth. It may be caused by human as well as natural activities. Before the earthquake waves hit a region, the amount of 'Radon' gas increases in the atmosphere of that region. Therefore, rise in the concentration ofRadon gas' over the atmosphere of a region indicates that the region is going to be hit by an earthquake. The point, below the surface of the earth, from where the seismic (earthquake) waves originate is called the 'Focus' of the earthquake. The place, perpendicularly above the focus on the surface.

The waves generated during an earthquake are called Seismic Waves, which arc classified into 3 types:

1. Primary or Longitudinal Waves. These are simply known as P-Waves. These are longitudinal waves analogous to the sound waves. These waves have the maximum velocity among the three types of seismic waves. These waves can pass through the solid as well as liquid mediums, though their velocity gets slowed down in the liquid medium.
2. strong>Secondary or Transverse Waves. These are also called as S-Waves. These are transversal waves analogous to the light waves. These waves can travel only through the solid medium and disappear in the liquid medium. Since these waves do not pass through the core of the earth, they give an idea about the core being in liquid state.
3. Surface or Long Period Waves. These are also known as 'L' waves which originate when 'P' wave hits the surface. These waves affect only the surface of the earth. These are the most destructive and cover the longest distance among the three types of waves.

#### The Sismographs

The instruments sensitive to the seismic waves, which help us to measure the intensity of an earthquake are called 'Seismographs' -Different scales are used to measure the intensity of earthquakes such as

1. Rossy-Feral Scale - This scale measures the earthquakes between 1 to 11 units.
2. Mercalli Scale - It is an empirical scale. It is divided into 12 units.
3. Richter Scale - It is a mathematical (logarithmic) scale, which measures the intensity of an earthquake between 0 to 9. For each unit of increase in the Richter Scale, the amplitude of the earthquake wave increases by a factor of 10.

The lines joining the regions of same seismic intensity are called as Isoseismal Lines. The lines joining the places which experience the earthquake termors at the same time called Homoseismal Lines.

#### Tsunami

'Tsu-na-mi' is a Japanese word which means oncoming oceanic waves. These waves are very long and with less oscillation which originate in the oceans due to earthquakes that occur on the ocean-bed. The movement of water with the Tsunami waves is upto complete depth which makes them more caastrophic.

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