Model Answer

Propagation of Earthquake Waves

Earthquake waves are disturbances that travel through the Earth, transferring energy from the focus of an earthquake. There are two main types of earthquake waves: body waves and surface waves.

• Body Waves: These waves travel through the Earth’s interior. There are two types:
  • P-waves (Primary waves): These are compressional waves that travel fastest, pushing and pulling the material in the direction of propagation.
  • S-waves (Secondary waves): These are shear waves that move perpendicular to the wave direction, causing the ground to move up and down or side to side.
• Surface Waves: Generated by the interaction of body waves with the Earth’s surface, surface waves travel along the ground. The two types are:
  • Love Waves: These move the ground horizontally, similar to S-waves, but without vertical displacement.
  • Rayleigh Waves: These create rolling motion, similar to ocean waves, causing vertical and horizontal ground movement.

Role in Studying the Earth’s Interior

Seismic waves provide crucial insights into the Earth’s interior. The way these waves travel through different materials helps scientists understand the Earth’s structure. For example, S-waves cannot travel through the liquid outer core, confirming the presence of a molten layer beneath the Earth’s solid crust. P-waves can travel through both solid and liquid, but they bend (refract) when they pass through the outer core. By studying these wave patterns and their velocities, scientists can map the Earth’s layers, including the crust, mantle, and core. The creation of seismic “shadow zones”—areas where waves do not reach—further confirms the liquid nature of the outer core.

Through this method, seismic data have revealed detailed information about Earth’s interior composition and structure.