Seismic waves are waves of energy that travel through Earth’s layers, and are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions that give out low-frequency acoustic energy. The propagation velocity of seismic waves depends on density and elasticity of the medium as well as the type of wave. Velocity tends to increase with depth through Earth’s crust and mantle, but drops sharply going from the mantle to outer core.
Seismic waves are broadly classified into two main categories: body waves which travel through the Earth’s and Surface waves which travel at the Earth’s surface. Surface waves travel across the surface. Surface waves decay more slowly with distance than body waves, which travel in three dimensions. Also, particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage.
Body Waves can further be classified into two main categories:
- Primary Seismic waves
- Secondary Seismic waves.
Primary Seismic Waves
Primary waves (P-waves) are compressional waves that are longitudinal in nature. P-waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name “Primary”. These waves can travel through any type of material, including fluids, and can travel nearly 1.7 times faster than the S-waves. In air, they take the form of sound waves, hence they travel at the speed of sound. . These waves travel at 6 Km/s near the surface to 10.4 km/s near the Earth’s core about 2900 km/s below the surface.
Primary Seismic Waves
- The waves that reach the earth’s surface first once energy is emitted from the earth’s surface are referred to as primary seismic waves.
- Primary waves also referred to as P waves or Pressure waves, are longitudinal or compressional in nature.
- These waves are composed of alternating compressions and rarefactions.
- In solids, these waves generally travel almost twice as fast as S waves. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. These waves travel at 6 Km/s near the surface to 10.4 km/s near the Earth’s core about 2900 km/s below the surface.
- As the waves enter the core, the velocity drops to 8 km/s increasing to 11 km/s near the center.
- These results from increased hydrostatic pressure as well as from changes in rock composition and phase.
- Primary waves can travel through any medium (solid, liquid and gaseous medium).
- While traveling through liquid medium, primary waves change their direction.
- The waves move to and from in the direction of energy transfer.
Also Read: Difference Between Mechanical And Electromagnetic Waves
Secondary Seismic Waves
Secondary waves (S-waves) are shear waves that are transverse in nature. Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the direction of propagation, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. Shear waves can’t travel through any liquid medium, so the absence of S-wave in earth’s outer core suggests a liquid state.
Secondary Seismic Waves
- The waves that reach the Earth’s surface after the primary waves once energy is emitted from the earth’s surface are referred to as secondary waves.
- Secondary waves can travel through only solid state medium and get absorbed in any liquid state mediums.
- They have lower velocity than primary waves and scatter in all directions from the earthquake focus point.
- Secondary waves get absorbed as they enter the liquid medium.
- These waves cause the particles it passes through to move up and down.
- These waves are more damaging causing the maximum destruction during an earthquake.
Also Read: Difference Between Longitudinal And Transverse Waves
Difference Between Primary And Secondary Seismic Waves In Tabular Form
| BASIS OF COMPARISON | PRIMARY SEISMIC WAVES | SECONDARY SEISMIC WAVES |
| Description | The waves that reach the earth’s surface first once energy is emitted from the earth’s surface are referred to as primary seismic waves. | The waves that reach the Earth’s surface after the primary waves once energy is emitted from the earth’s surface are referred to as secondary waves. |
| Propagation | Primary waves can travel through any medium (solid, liquid and gaseous medium). | Secondary waves can travel through only solid state medium and get absorbed in any liquid state mediums. |
| Velocity | They have high velocity and move in a radial direction from the focus of the earthquake. | They have lower velocity than primary waves and scatter in all directions from the earthquake focus point. |
| Movement Of Particles | These waves are often referred to as forward-backward waves as particles through which these waves pass tend to move to and fro in the direction of the waves. | These waves cause the particles it passes through to move up and down. |
| Damage & Destruction | Initial waves are felt but are not damaging to a great extent. | These waves are more damaging causing the maximum destruction during an earthquake. |
Also Read: Difference Between Stationary And Progressive Waves