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?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2 ✓ UPSC's answer
- (d) Neither 1 nor 2
Why the answer is (c)
• Earthquakes generate body waves of two kinds: P (primary) waves and S (secondary) waves.
• Statement 1 is correct: P waves travel faster (about 6 km/s in the crust versus 3.5 km/s for S waves), so they reach a seismograph first — hence 'primary'.
• Statement 2 is correct: P waves are longitudinal, compressing and expanding material along the direction of travel like sound; S waves are transverse, moving particles at right angles to the direction of propagation.
• Because S waves cannot pass through liquids, their absence in the 'shadow zone' revealed that the outer core is liquid.
• Hence both 1 and 2, option (c).
Why the other options are wrong
- (a) 1 only
- Statement 2 on particle motion is also correct.
- (b) 2 only
- Statement 1 on P waves arriving first is also correct.
- (d) Neither 1 nor 2
- Both statements are correct.
Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2023, held on 28 May 2023. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.