UPSC Prelims 2023 Q65: P-waves and S-waves — Explained

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

Answer: (c) Both 1 and 2

Statement-wise explanation

  • 1: correct — P-waves (primary waves) are the fastest seismic waves. S-waves travel at roughly 60% of P-wave speed. Both leave the focus together, so the P-wave always reaches a seismograph first and is the first signal on the record. The gap between P and S arrival grows with distance from the focus, and seismologists use it to work out how far away the earthquake was.
  • 2: correct — P-waves are longitudinal, or compressional. Each particle moves to and fro along the direction the wave is travelling, squeezing and stretching the material. S-waves are transverse, or shear. Particles move at right angles to the direction of travel. The NCERT Class XI text describes this as vibration perpendicular to propagation in the vertical plane, which matches the 'up and down' wording here.

How to eliminate

You cannot guess your way to this answer, because both statements are basic textbook facts. Two things settle it. First, 'P' stands for primary: these are the first waves to arrive, so Statement 1 is true almost by definition. Second, P-waves are longitudinal, like sound waves, and S-waves are transverse, like a rope being shaken. That makes Statement 2 true. A careful candidate may hesitate over 'up and down' in Statement 2, since S-waves can also shake the ground side to side. UPSC took its wording from NCERT, which describes S-wave vibration as perpendicular to the direction of travel in the vertical plane. If a statement repeats the NCERT definition, don't mark it wrong for leaving out details. Rule to remember: P = primary = push-pull (longitudinal). S = secondary = shear (transverse).

Know more

Earthquake waves start at the focus (hypocentre). They come in two families.

  • Body waves travel through the Earth's interior: P-waves and S-waves.
  • Surface waves form when body waves reach the surface and interact with surface rocks. They arrive last and do the most damage.
Feature P-wave (primary) S-wave (secondary)
Wave type Longitudinal (compressional) Transverse (shear)
Particle motion Parallel to direction of travel Perpendicular to direction of travel
Speed Fastest; first to arrive About 60% of P-wave speed; arrives second
Medium Solids, liquids and gases Solids only

Why S-waves stop at liquids: a liquid cannot resist shear, so it cannot carry a shear wave.

Shadow zones: a shadow zone is a part of the surface where seismographs record no direct waves from a given earthquake.

  • P-waves are not recorded between about 105° and 145° from the epicentre. The liquid outer core refracts (bends) them.
  • S-waves are not recorded anywhere beyond about 105°. This is the main evidence that the outer core is liquid.

Reading a seismogram: the P-wave arrives first, then the S-wave, then the surface waves. The longer the S–P time gap, the farther the station is from the earthquake. With S–P gaps from three or more stations, the epicentre can be located by triangulation.

Speed changes with material: seismic waves change speed and direction as they pass into layers of different density. This reflection and refraction is how scientists mapped the crust, mantle and core without drilling into them.

India: the National Centre for Seismology (Ministry of Earth Sciences) runs the national seismograph network and publishes earthquake reports on seismo.gov.in.

NCERT Concepts to Revise

  • P-waves: Both statements turn on how P-waves move and how fast they travel compared with other earthquake waves.
  • S-waves: Statement 2 asks how particles vibrate in S-waves compared with the direction the wave travels.
  • Body waves: P- and S-waves are the two types of body wave. Knowing this family frames both statements.
  • Shadow zone: Shadow zones come from the different ways P- and S-waves behave, a common follow-up to this question.
  • P-wave shadow zone: The P-wave shadow zone (about 105°–145°) tests whether you know how P-waves behave in the liquid outer core.
  • Reflection and refraction of seismic waves: Changes in seismic wave speed explain why arrival times differ on a seismograph.

Read more