Which of the following statements regarding earthquakes is/are correct? 1. The point on the Earth’s surface directly above the focus is called the epicentre of the earthquake. 2. Earthquakes generate Primary and Secondary waves that radiate outward from the earthquake focus. 3. Deep-focus earthquakes are likely to cause more damage than shallow-focus earthquakes. Select the answer using the code given below:
- (a)1 only
- (b)1 and 2 only
- (c)2 and 3 only
- (d)1, 2 and 3
Correct — B, statements 1 and 2 only. The place inside the Earth where the rupture begins and energy is first released is the focus (or hypocentre); the point on the surface vertically above it is the epicentre, so statement 1 is true. From the focus the released energy travels out as seismic body waves — Primary (P) waves, which are fast compressional waves, and Secondary (S) waves, which are slower shear waves — so statement 2 is true. Statement 3 is false: it is shallow-focus earthquakes that are the more destructive, because their energy reaches the surface over a shorter path with less spreading and absorption, whereas deep-focus quakes lose much of their intensity before the waves arrive at the surface. With only 1 and 2 correct, the answer is (b).
- (a)1 only — Drops statement 2, but earthquakes genuinely do radiate Primary and Secondary waves outward from the focus — a defining feature of seismic energy release.
- (c)2 and 3 only — Keeps the false statement 3 and drops the true statement 1. Shallow-focus quakes, not deep-focus ones, do the greater damage, and the epicentre really is the surface point above the focus.
- (d)1, 2 and 3 — Includes statement 3, which is wrong — deep-focus earthquakes are generally less damaging at the surface than shallow-focus ones of comparable magnitude.
An earthquake is the shaking of the ground caused by a sudden release of energy in the Earth's crust, usually from movement along a fault. The energy spreads out as seismic waves from the focus (hypocentre), the point of origin at depth; the surface point directly above it is the epicentre, where shaking is usually strongest.
The tempting error is statement 3. Because 'deep' sounds more powerful, students assume deep-focus quakes are worse. In fact the depth of the focus works against surface damage — the deeper the source, the more the waves weaken before reaching the surface — so shallow-focus events (source within about 70 km of the surface) are the destructive ones.
- The focus (hypocentre) is the underground point where rupture starts; the epicentre is the surface point directly above it.
- Seismic body waves are of two kinds — Primary (P) waves are faster compressional waves that travel through solids, liquids and gases, while slower Secondary (S) waves are shear waves that pass only through solids.
- Earthquakes are classified by focal depth as shallow (about 0-70 km), intermediate (70-300 km) and deep (300-700 km).
- Shallow-focus earthquakes generally cause the most surface damage; the shadow zone for S and P waves also reveals that the Earth's outer core is liquid.
- Assuming deep-focus quakes are the most damaging — the opposite is true because depth weakens the waves before they reach the surface.
- Confusing focus (underground origin) with epicentre (surface point above it), or intensity (Mercalli) with magnitude (Richter/moment).
Asked as a statement-code question mixing sound definitions (epicentre, P and S waves) with a plausible-but-false claim about focal depth and damage.
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 — intensity uses the Mercalli scale, magnitude measures energy released and is read from seismic-wave amplitude; the Richter step is not a hundred-fold in energy.
Same earthquake theme, centred on seismic waves and how their amplitude is measured — a natural companion to this NDA item's Primary and Secondary wave statement.
Consider the following geological phenomena: 1. Development of a fault 2. Movement along a fault 3. Impact produced by a volcanic eruption 4. Folding of rocks. Which of the above cause earthquakes?
- (a) 1, 2 and 3
- (b) 2 and 4
- (c) 1, 3 and 4
- (d) 1, 2, 3 and 4
Answer(a) 1, 2 and 3 — faulting and fault movement and volcanic shocks release seismic energy; slow folding of rocks does not.
Explores what triggers earthquakes and the release of seismic energy from the focus — the mechanism underlying the P and S waves referred to in this NDA question.
- practice — not a real PYQ
Which seismic wave is the fastest and can travel through solids, liquids and gases?
- (a)Secondary (S) wave
- (b)Primary (P) wave
- (c)Love wave
- (d)Rayleigh wave
Answer(b) Primary (P) wave — it is a compressional wave, the fastest seismic wave, and passes through solids, liquids and gases.
- practice — not a real PYQ
The inability of Secondary (S) waves to pass through the outer core is evidence that the outer core is:
- (a)solid
- (b)liquid
- (c)gaseous
- (d)made of ice
Answer(b) liquid — S waves are shear waves that cannot travel through liquids, so their absence beyond the outer core shows it is liquid.