UPSC Prelims 2023 Q65: Geography – Physical Geography & Geomorphology
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer: (c) Both 1 and 2
Explanation
Seismic Waves — P and S Waves
| Statement | Claim | Verdict | Detail |
|---|---|---|---|
| 1 | In a seismograph, P waves are recorded earlier than S waves | TRUE | P waves (Primary waves) travel faster (~6–7 km/s in crust) than S waves (Secondary waves) (~3–4 km/s). Therefore P waves arrive first and are recorded earlier on a seismograph — hence the names Primary and Secondary. |
| 2 | P waves: particles vibrate to-and-fro in direction of propagation; S waves: particles vibrate perpendicular | TRUE | P waves are longitudinal waves (compressional) — particle motion is parallel to wave direction. S waves are transverse waves (shear) — particle motion is perpendicular to wave direction. This is the fundamental difference between the two wave types. |
Both statements are correct.
Answer: (c) Both 1 and 2.
Why each option is right or wrong
- (a) Incorrect. Statement 1 alone being correct — Statement 2 also accurately describes the wave motion of P and S waves.
- (b) Incorrect. Statement 2 alone — Statement 1 is equally correct; P waves definitely arrive before S waves at any seismographic station.
- (c) CORRECT. Both statements are correct: P waves arrive first (faster velocity) and P waves are longitudinal while S waves are transverse.
- (d) Incorrect. 'Neither' — both are fundamental, well-established facts about seismic wave behaviour.
Key point
P waves: faster (primary), longitudinal (to-and-fro). S waves: slower (secondary), transverse (perpendicular). Both statements are correct. The P-S arrival time gap is used to calculate earthquake epicentre distance.
Remember
P waves are recorded earlier than S waves because they travel faster. P waves are longitudinal (particle motion parallel to propagation), while S waves are transverse (particle motion perpendicular to propagation). Both statements are correct.
Common mistakes
- Confusing longitudinal and transverse descriptions — P waves = longitudinal (compression/rarefaction like sound). S waves = transverse (shear, like light).
- Not knowing P waves travel through liquids but S waves cannot — this is why the Earth's outer core (liquid iron) blocks S waves.
- Forgetting why P comes first — P stands for 'Primary' because it arrives first at the seismograph station.
Exam tip
P = Primary = faster = Longitudinal (to-fro). S = Secondary = slower = transverse (side-to-side). P can travel through solid + liquid. S = solid only. Seismograph time gap between P and S arrival = used to find earthquake distance.
Sources
- NCERT Geography Class 11 — Chapter 3: Interior of the Earth
- Geological Survey of India — Seismology Basics