Earthquakes

                                           Earthquakes

Definition: - Earthquakes are caused by the passage of vibrations set up in rock body.

There are two types of earthquake. 

1) Volcanic earthquake: - Due to volcanic eruption – it effect local in nature.

2) Tectonic Earthquake: - It is due to earth movement (orogenic & Epiorogenic). It held very longer distance.

Earthquake waves: -

 During the formation of tectonic earthquake the energy is released in the form of waves, which are called as earthquake waves. It is also called seismic waves.

a)     P- Waves :- (primary waves):-These are push or pull waves. These waves are longitudinal in nature & travel with very high velocity. P-waves can passes through solids , liquids & gases.

b)    S-Waves:- (secondary waves ) These are transverse in nature & their velocity is less than P-waves . S-waves can passes through solid & gasses, while in liquid media they are reflected.

c)     L- waves:- ( long waves) These waves are also transverse in nature & their velocity is less than P & S – waves. These are also called as surface waves because their journey is limited to the surface of the earth only.

 

Focus: - The place at which an earthquake originate called it’s origin or focus. The deeper, the focus, wider is the felt area.

Epicentre: - The point on the surface vertically above the focus is known as epicenter of earthquake. The intensity felt at the epicenter is maximum.

Isoseismal Lines: - The lines joining the point of equal earthquake intensity are called isoseismal lines. They may be either circular or irregular in shape. The instrument used for recording earthquake vibration is called seismograph. The scale used to measure magnitude of earthquake is Richter scale.