Which one of the following statements regarding S-waves is not correct?
- (a)S-waves are slower than P-waves.
- (b)During an earthquake S-waves are less destructive to structures like buildings and highway overpasses.
- (c)S-waves can travel only through solids and cannot travel through the Earth's outer core.
- (d)Seismologists have concluded that the Earth's outer core must be liquid or fluid.
Correct — B, During an earthquake S-waves are less destructive to structures like buildings and highway overpasses. That is the statement which is not correct, so it is the one the question wants. S-waves shake the ground at right angles to the direction they travel, so they move a building sideways, and that side-to-side motion is far harder for a structure to survive than the push-and-pull of a P-wave. Structural engineers design for lateral load precisely because of it. The other three statements all hold. S-waves are slower, arriving second and giving the lag between P and S arrivals that seismologists use to locate an epicentre. They pass only through solids, because a liquid has no shear strength to restore a sideways displacement. And the fact that S-waves fail to arrive on the far side of the Earth is exactly the evidence from which seismologists concluded that the outer core is liquid — one of the strongest inferences in the earth sciences, drawn entirely from what did not arrive.
- (a)S-waves are slower than P-waves. — A correct statement, so not the answer. NCERT says P-waves move faster and are the first to arrive at the surface, which is why they are called primary.
- (c)S-waves can travel only through solids and cannot travel through the Earth's outer core. — Correct as well. A shear wave needs a medium that resists change of shape, and a liquid does not.
- (d)Seismologists have concluded that the Earth's outer core must be liquid or fluid. — Correct, and it follows from the previous statement. The S-wave shadow across more than forty per cent of the Earth's surface is the evidence for it.
An earthquake sends out body waves and surface waves. Of the body waves, P-waves are compressional, pushing and pulling the material along the direction of travel, and they pass through solids, liquids and gases alike. S-waves are shear waves, displacing material at right angles to the direction of travel, and they pass only through solids. Because the P-wave is faster it arrives first, and the interval between the two arrivals grows with distance from the focus, which lets a network of stations triangulate an epicentre. The pattern of arrivals and non-arrivals across the globe maps the interior: the S-wave shadow zone covers more than forty per cent of the Earth's surface, and the P-wave shadow lies between 105 and 145 degrees from the epicentre.
Read the stem carefully — it asks which statement is not correct, so three of the four will be true and the work is to spot the odd one. The false statement here is the one that reverses a comparison, which is the commonest way of building this question type. It helps to remember why S-waves are the dangerous ones: a building resists vertical load by design, since it holds itself up, but a sideways shove is a load it was never carrying before. Surface waves, which the item does not mention, are slower still and do the most damage of all near the epicentre.
- P-waves are compressional and travel through gaseous, liquid and solid materials; NCERT notes they are similar to sound waves.
- S-waves are shear waves and can travel only through solid materials.
- P-waves arrive first, and the growing gap between the P and S arrivals with distance is used to locate the epicentre.
- The S-wave shadow zone covers more than forty per cent of the Earth's surface; the P-wave shadow zone lies between 105 and 145 degrees from the epicentre.
- The absence of S-waves beyond the shadow zone is the evidence that the outer core is liquid.
- Answering a not-correct question with a correct statement.
- Assuming the faster wave must be the more damaging one.
- Confusing the P-wave shadow zone, which is a band, with the S-wave shadow, which is a whole hemisphere-scale region.
As a not-correct item among four statements about seismic waves, or as a statements question on shadow zones and what they prove about the interior.
Consider the following statements regarding earthquakes: I. The intensity of an earthquake is measured on the Mercalli scale. II. The magnitude of an earthquake is a measure of energy released. III. Earthquake magnitudes are based on direct measurements of the amplitude of seismic waves. IV. In the Richter scale, each whole number demonstrates a hundred-fold increase in the amount of energy released. Which of these statements are correct?
- (a) I, II and III
- (b) II, III and IV
- (c) I and IV
- (d) I and III
Answer(a) I, II and III
The measurement side of the same subject. Magnitude is read from the amplitude of the very waves this item describes, while intensity records what the shaking did to people and buildings.
Which of the following statements is/are correct ? 1. Hypocenter is the point on the surface of the Earth, nearest to the focus. 2. Velocity of earthquake waves is higher in denser materials. 3. P waves move faster and are the first to arrive at the surface of the Earth. Select the correct answer using the code given below :
- (a) 1 and 2
- (b) 2 and 3
- (c) 1 and 3
- (d) 3 only
Answer(b) 2 and 3
Confirms the first statement of this item from an official key — P-waves move faster and arrive first, which is exactly why the S-wave is called secondary.
- practice — not a real PYQ
The conclusion that the Earth's outer core is liquid rests principally on which observation?
- (a)S-waves do not arrive beyond the shadow zone
- (b)P-waves travel faster than S-waves
- (c)Surface waves cause the most damage
- (d)Earthquake magnitude is measured on a logarithmic scale
Answer(a) S-waves do not arrive beyond the shadow zone — a shear wave cannot cross a liquid, so their absence maps the liquid layer.
- practice — not a real PYQ
Which of the following is used to locate the epicentre of an earthquake?
- (a)The time difference between the P-wave and S-wave arrivals at several stations
- (b)The colour of the seismogram trace
- (c)The depth of the focus alone
- (d)The magnitude recorded at a single station
Answer(a) The time difference between the P-wave and S-wave arrivals at several stations — the gap grows with distance, so three stations fix the point.