About Earthquake:

  • An earthquakeis the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth’s lithosphere that creates seismic waves.
  • Earthquakes can range in intensity, from those that are so weak that they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, and wreak destruction across entire cities.
  • The location below the earth’s surface where the earthquake starts is called the hypocentre or focus.
  • The location directly above it on the surface of the earth is called the epicenter.

Types of Earthquakes:

  • There are three basic types of seismic waves – P-waves, S-waves and surface waves.
  • P-waves and S-waves are sometimes collectively called body waves.

P waves:

  • They are the first waves to hit the seismographs when an earthquake strikes.
  • They are similar longitudinal waves which means that the direction of motion and propagation are the same.
  • These waves can travel through solid, liquid, and gas.
  • These waves travel in the speed range of 1.5-13 km/s.

S waves:

  • S wavesalso called secondary wavesand shear waves, are the second waves to hit the seismographs after the P waves.
  • They are transverse waves, which means that the motion is perpendicular to the direction of wave propagation.
  • These waves travel in a transversal direction.
  • These waves travel through only solids.

Surface waves:

  • Surface waves are those waves that travel on the surface of the earth.
  • The destruction by these waves is huge.
  • They are basically last to arrive after P Waves and S Waves.

Shadow zone of Earthquake:

  • A seismic shadow zoneis an area of the Earth’s surface where seismographs cannot detect direct P-waves and/or S waves from an earthquake.
  • This is due to liquid layers or structures within the Earth’s surface.
  • The most recognized shadow zone is due to the core-mantle boundary where P waves are refracted and S waves are stopped at the liquid outer core.

  • Shadow zone of S waves is larger than P waves shadow zone.

Syllabus: Prelims