CDS (I) 2021 General Knowledge · Question 56 of 120

Correct — B, They can travel only through solid materials. That is the defining property of the S-wave, not the P-wave, and the paper has simply moved it across. The school text is unambiguous on both: 'P-waves move faster and are the first to arrive at the surface. These are also called primary waves. The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials' — which is options (d), (a) and (c) word for word — and then, of the secondary wave, 'an important fact about S-waves is that they can travel only through solid materials'. The physics behind the difference is that a P-wave is a compressional wave, vibrating parallel to its direction of travel, and any material can be compressed; an S-wave is a shear wave, vibrating across its direction of travel, and fluids have no rigidity to resist shear. Note also that options (b) and (c) contradict each other outright, which by itself tells a careful reader that one of them must be the answer.

• (a)They are similar to sound waves. — True. Sound in air is also a compressional wave, with the medium vibrating back and forth along the line of travel, which is exactly how a P-wave moves.
• (c)They travel through gaseous, liquid and solid materials. — True, and it is the property the answer denies. Because a P-wave compresses the medium rather than shearing it, it passes through all three states — which is why P-waves cross the earth's liquid outer core and S-waves do not.
• (d)They move faster and are the first to arrive at the surface. — True, and it is where the name comes from — primary because they arrive first, secondary because they arrive with a time lag. The gap between the two arrivals is what a seismologist uses to work out the distance to the epicentre.

An earthquake sends out body waves, which travel through the interior, and surface waves, which travel along the surface and do most of the damage. The body waves are P and S. Their different behaviour is the main evidence for the structure of the earth's interior: seismographs within 105 degrees of an epicentre record both P and S waves; beyond 145 degrees they record P waves but no S waves; and the belt between 105 and 145 degrees is a shadow zone for both. The S-wave shadow zone covers a little over forty per cent of the earth's surface, and the only explanation that fits is a liquid outer core through which shear waves cannot pass.

This item can be solved twice over, and both routes are worth practising. The logical route: options (b) and (c) are direct opposites, so exactly one of them is false, and the question reduces to a two-way choice. The physical route: name the wave type. Primary means compressional, and a compression can be transmitted by anything that can be squeezed, so gases and liquids are included. Secondary means shear, and a shear needs the medium to resist sideways displacement, which only a solid does. Every consequence follows from that one distinction — the P-wave's higher speed, its passage through the outer core, and the shadow zones that let geologists map an interior no one has seen.

• P-waves move faster than S-waves and are the first to arrive at the surface, which is why they are called primary waves.
• P-waves are similar to sound waves and travel through gaseous, liquid and solid materials.
• S-waves, the secondary waves, can travel only through solid materials.
• P-waves vibrate parallel to the direction of the wave, while S-waves vibrate across it.
• Beyond 145 degrees from an epicentre seismographs record P-waves but not S-waves; the S-wave shadow zone covers a little over forty per cent of the earth's surface, which is the evidence for a liquid outer core.

• Transferring the solids-only property from S-waves to P-waves; it is the single most reused error in this topic.
• Assuming surface waves are the fastest because they are the most destructive; they are the last to be recorded.
• Reading 'similar to sound waves' as loose analogy; it is a precise statement that both are compressional.

Asked as a which-is-not-correct item in which two options directly contradict each other, and as an assertion-reason pair on the liquid outer core.

• practice — not a real PYQ

  The existence of a liquid outer core in the earth is inferred chiefly from the observation that

  • (a)P-waves do not travel through the core
  • (b)S-waves are not recorded beyond 145 degrees from the epicentre
  • (c)surface waves die out at great depth
  • (d)the earth's magnetic field reverses periodically

  Answer(b) S-waves are not recorded beyond 145 degrees from the epicentre — shear waves cannot pass through a liquid, so the S-wave shadow zone maps the outer core.

• practice — not a real PYQ

  Which of the following earthquake waves cause the greatest damage to buildings?

  • (a)Primary waves
  • (b)Secondary waves
  • (c)Surface waves
  • (d)All three equally

  Answer(c) Surface waves — the last to be recorded on a seismograph, but the ones that displace rock and bring structures down.