Consider the following statements:

1. In a seismograph, P waves are recorded earlier than S waves.
2. In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation.

Which of the statements given above is/are correct?

1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2

Explanation

Statement 1 is correct

• Seismic waves are waves of energy that travel through the Earth’s layers and are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large human-made explosions. The refraction or reflection of seismic waves is used to study the structure of the Earth’s interior. The terms seismic waves and earthquake waves are often used interchangeably. Seismic waves, or earthquake waves, are basically of two types: body waves and surface waves.
• Body waves: Body waves are generated due to the release of energy at the focus and move in all directions, travelling through the interior of the Earth. Hence, the name body waves.
  • There are two types of body waves:
    • the P-waves or primary waves (longitudinal in nature ― wave propagation is similar to sound waves), and
    • the S-waves or secondary waves (transverse in nature ― wave propagation is similar to ripples on the surface of the water).
• Primary Waves (P-waves): Primary waves are called so because they are the fastest among the seismic waves and hence are recorded first on the seismograph.
  • P-waves are also called as the longitudinal waves because the displacement of the medium is in the same direction as, or the opposite direction to, (parallel to) the direction of propagation of the wave; or compressional waves because they produce compression and rarefaction when travelling through a medium; or pressure waves because they produce increases and decreases in pressure in the medium.
  • P-waves create density differences in the material, leading to stretching (rarefaction) and squeezing (compression) of the material.

Statement 2 is correct

• These waves are of relatively high frequency and are the least destructive among the earthquake waves. The trembling on the earth’s surface caused by these waves is in the up-down direction (vertical). They can travel through all media, and their velocity depends on the medium’s shear strength (elasticity). Hence, the velocity of the P-waves in Solids > Liquids > Gases. These waves take the form of sound waves when they enter the atmosphere. P-waves are about 1.7 times faster than the S-waves.

• Secondary Waves (S-waves): Secondary waves (secondary, they are recorded second on the seismograph) or S-waves are also called transverse waves, shear waves, or distortional waves. They are analogous to water ripples or light waves. Transverse waves, or shear waves, mean that the direction of vibrations of the particles in the medium is perpendicular to the direction of propagation of the wave. Hence, they create troughs and crests in the material they pass through (distorting the medium). S-waves arrive at the surface after the P-waves. These waves are high-frequency and have slightly greater destructive power than P-waves. The trembling on the earth’s surface caused by these waves is from side to side (horizontal). S-waves cannot pass through fluids (liquids and gases) as fluids do not support shear stresses. They travel at varying velocities (proportional to shear strength) through the solid part of the Earth.

Answer: (c) Both 1 and 2; Difficulty Level: Easy