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UPSC Prelims 2023 · World Geography · Rocks, Quakes and Landforms
Consider the following statements: Which of the statements given above is/are correct?
- In a seismograph, P waves are recorded earlier than S waves.
- 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.
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Show the answer and explanation
Answer: (c) Both 1 and 2
Both are standard facts.
Statement by statement
1. ✓ CORRECT P waves are fastest, so they arrive first on the seismograph.
2. ✓ CORRECT P waves are longitudinal (push–pull); S waves are transverse (particles move at right angles).
The idea
A wave carries energy through matter, while individual particles oscillate around their positions. In a compression wave the oscillation follows the travel direction; in a shear wave it is across that direction. Their different speeds give the characteristic order on a seismic record.
Earthquake releases stored strain → P and S waves leave the source → Compression travels faster than shear → P arrival is recorded → S arrival follows → Time gap increases with distance from the source
Why the others are wrong
(a) Drops the particle-motion difference.
(b) Drops the arrival order.
Also know
Liquids cannot sustain the shear motion needed by S waves. The absence of direct S waves beyond the core helps reveal a liquid outer core. Love and Rayleigh waves are surface waves, distinct from these body waves. Earthquake location requires records from several stations, not just the first arrival at one station.
🔒 P: fastest, all media. S: slower, solids only → S-wave shadow zone shows a liquid outer core.
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