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Earthquakes, how earthquake formed

Earthquakes

A sudden release of stored energy in earth’s crust that creates seismic waves is termed as Earthquake.
Cause 
  • Most earthquakes are related to compressional or tensional stresses built up at the tectonic plates which make up the earth’s surface.
  • This usually happens when the tectonic plates get locked up and are not able to slide past each other.
  • It is estimated that only 10 percent or less of an earthquake’s total energy is radiated as seismic energy, the remaining energy is lost due frictional heat
Measuring the Giant
  • The moment magnitude scale is a very sophisticated scale used now a days which for measuring the intensity or size of earthquake i.e. the amount of energy released.
  • Earlier Richter scale was used for measuring the intensity of the earthquake.
  • Seismologists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves.
  •  An earthquake measuring 3 to 5 is considered minor or light and is mostly imperceptible; 5 to 7 is moderate to strong; 7 to 8 is major; and 8 or more is catastrophic.
  •  The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011
  • Intensity of shaking is measured on the Mercalli scale.
  • There are two types of seismic waves viz; P waves and S waves.

P waves

S waves

Primary waves (P-waves) are compressional waves that are longitudinal in nature.Secondary waves (S-waves) are waves that are transverse in nature.
Motion is along or parallel to the direction of wave propagationMotion is perpendicular to the direction of wave propagation
P waves travel faster through the earth to arrive at seismometer  first, hence the name “Primary”.They arrive at seismometer after the faster-moving P-waves, hence the name “Secondary”
P-waves travel at around 6–7 km per secondS-waves travel at around 3.5–4 km per second.
P Waves  can travel through any type of material, including fluids, and can travel at nearly twice the speed of S waves.S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses.
   



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Terminology
1. Epicenter-That point on the Earth’s surface directly above the hypocenter (focus) of an earthquake.
2. Hypocenter-The point on the earth’s surface where the first sign of disturbance is reported i.e the site from where the waves originate.