Which of the following statements with reference to shadow zones in an event of an earthquake is/are correct? 1. Zone between 105° and 145° from epicentre was identified as the shadow zone for both P-waves and S-waves 2. The shadow zone of P-waves is much larger than that of the S-waves Select the answer using the code given below :
- (a)1 only
- (b)2 only
- (c)Both 1 and 2
- (d)Neither 1 nor 2
Correct — A, statement 1 only. Seismographs within 105 degrees of the epicentre record both P and S waves, while those beyond 145 degrees record P waves but not S waves, so the belt between 105 and 145 degrees receives neither and is the shadow zone for both types of wave — statement 1 is true. Statement 2 reverses the real relationship: the S-wave shadow zone is much larger than the P-wave shadow zone, because S waves cannot pass through the liquid outer core and are missing across the whole far side beyond 105 degrees, whereas P waves only skip the narrow 105 to 145 degree band. So statement 2 is false and only statement 1 is correct.
- (b)2 only — Statement 2 is false — it inverts the fact that the S-wave shadow zone is the larger one — while statement 1 is true, so choosing statement 2 alone is doubly wrong.
- (c)Both 1 and 2 — This wrongly counts statement 2 as correct. The P-wave shadow zone is the smaller of the two, not the larger, so both cannot be right.
- (d)Neither 1 nor 2 — This rejects statement 1, which is true: the 105 to 145 degree belt is indeed the shadow zone for both P and S waves.
When an earthquake occurs, seismographs around the world do not all record its waves. A shadow zone is a belt of the Earth's surface that receives no direct P or S waves. Its existence, and the fact that S waves disappear over a much wider area, is the key evidence that the Earth has a liquid outer core.
P waves are longitudinal and can travel through solids and liquids, but they bend sharply (refract) on entering the liquid outer core, leaving a ring-shaped gap roughly between 105 and 145 degrees. S waves are transverse and cannot travel through liquids at all, so they vanish everywhere beyond about 105 degrees. That is why the S-wave shadow zone covers the whole far hemisphere and is much larger than the P-wave band — the reverse of what statement 2 claims.
- P waves are recorded within 105 degrees and again beyond 145 degrees, leaving a P-wave shadow band between 105 and 145 degrees.
- S waves are recorded only within 105 degrees; the liquid outer core stops them from reaching anywhere beyond.
- The belt from 105 to 145 degrees is therefore a shadow zone for both P and S waves.
- The S-wave shadow zone (all of the surface beyond 105 degrees) is much larger than the P-wave shadow band, which is how the liquid outer core was discovered.
Statement 1 is true and statement 2 is false, so the answer is 1 only (option A).
- The S-wave shadow zone is the larger one, not the P-wave shadow zone.
- S waves do not just travel slowly; they are stopped completely by the liquid outer core.
Asked through statement-correctness on the shadow zone, the behaviour of P and S waves, and what they reveal about the Earth's interior.
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
- (d) Neither 1 nor 2
Answer(c) Both 1 and 2 — P waves are faster and so recorded first, and P waves are longitudinal while S waves are transverse.
UPSC tests the same P-wave and S-wave behaviour that produces the shadow zones asked about in Q115.
- practice — not a real PYQ
Which type of seismic wave cannot pass through the liquid outer core of the Earth?
- (a)P waves
- (b)S waves
- (c)Both P and S waves
- (d)Surface waves
Answer(b) S waves — being transverse, they cannot travel through liquids, so they are blocked by the liquid outer core.
- practice — not a real PYQ
The seismic shadow zone provides the main evidence for which feature of the Earth's interior?
- (a)A solid inner core
- (b)A liquid outer core
- (c)The thickness of the crust
- (d)The presence of the mantle
Answer(b) A liquid outer core — S waves stop and P waves bend at it, creating the shadow zones.