Why Earthquakes occur?

• Energy is released along a fault line, which is a break in the earth’s crustal rock layer.
• Rocks along the fault line move in opposite directions, but friction from the overlying rock prevents movement.
• Over time, pressure builds up and the rocks overcome the friction, causing an abrupt movement and releasing energy.
• The energy waves travel in all directions and the point of release is called the focus (or hypocentre) of an earthquake.
• The energy waves reach the surface and the point closest to the focus is called the epicenter.
• The epicenter is directly above the focus and is the first to experience the energy waves.

Propagation of Earthquake Waves

• Earthquakes produce different types of waves that travel and cause vibrations in rocks.
• P-waves (Primary waves) vibrate parallel to the direction of the wave, creating pressure and density differences in the material.
• S-waves (Secondary waves) vibrate perpendicular to the direction of propagation in the vertical plane, causing troughs and crests in the material.
• Both P-waves and S-waves cause stretching and squeezing of the material through which they pass.

The Emergence of Shadow Zones

• Earthquake waves are recorded by seismographs located far from the epicenter.
• There is a specific area called the “shadow zone” where waves are not recorded.
• Each earthquake has a different shadow zone:
  • Seismographs within 105° of the epicenter record the arrival of both P and S-waves.
  • Seismographs beyond 145° from the epicenter only record the arrival of P-waves.
  • A zone between 105° and 145° from the epicenter is the shadow zone for both P and S-waves.
• The shadow zone of S-waves is much larger and covers over 40% of the earth’s surface
• The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicenter.

How these properties of ‘P’ and ‘S’ waves help in determining the earth’s interior? 

• Reflection causes waves to rebound and refraction causes waves to move in different directions.
• The variations in wave direction are determined by their recordings on seismographs.
• Changes in wave velocity indicate changes in density, which helps to estimate the density of the earth.
• Observing changes in wave direction (such as the emergence of shadow zones) helps to identify different layers in the earth.

Why S-waves cannot travel through liquids?

• S-waves are shear waves that move particles perpendicularly to their direction of propagation.
• They can propagate through solid rocks because they have enough shear strength to hold the rock together.
• Liquids do not have the same shear strength: that is why, if you take a glass of water and suddenly remove the glass, the water will not keep its glass shape and will just flow away.
• S-waves need a medium that is rigid enough to propagate, which is why they do not propagate through liquids.