Which one of the following is the other term used for 'Focus' in relation with an earthquake?
- (a)Hypocentre
- (b)Epicentre
- (c)Isocentre
- (d)Principal Point
Correct — A, Hypocentre. The focus of an earthquake is the point inside the Earth where the rupture begins and from which seismic waves start radiating outward, and hypocentre is simply its other name. The word is built to say so — 'hypo' means below, so the hypocentre is the centre below the surface, as against the epicentre, which is the point on the surface directly above it. Everything else about an earthquake is measured from these two: focal depth is the distance from the hypocentre up to the epicentre, and shallow-focus earthquakes, with the rupture close to the surface, do far more damage than deep-focus ones because the waves reach the ground with less of their energy spent.
- (b)Epicentre — The pair to the focus, not another name for it. The epicentre is on the Earth's surface, vertically above the focus, and it is where shaking is usually strongest — but it is not where the rupture happens.
- (c)Isocentre — Not a seismological term at all. Isocentre belongs to radiotherapy and to imaging geometry; the seismological words beginning with 'iso' are isoseismal lines, which join places of equal shaking intensity.
- (d)Principal Point — A term from photogrammetry and optics — the point where the optical axis meets the image plane. It has nothing to do with earthquakes.
An earthquake happens when accumulated stress along a fault is released suddenly and the rock slips. The rupture starts at one point, the focus or hypocentre, and the energy travels away from it as seismic waves — primary or P waves, which are longitudinal and travel fastest and through both solids and liquids; secondary or S waves, which are transverse and cannot pass through liquids; and the slower surface waves that do most of the damage. That S waves cannot cross the outer core is the observation from which the liquid outer core was inferred, and the shadow zones on the far side of the Earth are the evidence.
This is a vocabulary item with a built-in trap, because focus and epicentre are always taught together and the second is the more familiar word. Fix the pair by the prefixes — hypo for below, epi for upon — and the question answers itself. Depth then does the rest of the work in this topic. Earthquakes are classed as shallow when the focus is within about 70 km of the surface, intermediate between about 70 and 300 km, and deep below that; the shallow ones are the destructive ones, which is the point examiners most often invert into a false statement.
- The focus, or hypocentre, is the point within the Earth where the rupture begins and seismic waves originate.
- The epicentre is the point on the Earth's surface directly above the focus.
- Focal depth is the vertical distance between the hypocentre and the epicentre.
- Shallow-focus earthquakes cause more damage than deep-focus ones of the same magnitude.
- P waves travel through solids and liquids; S waves cannot pass through liquids, which is how the liquid outer core was identified.
- Swapping focus and epicentre; the epicentre is on the surface and the focus is below it.
- Assuming deep-focus earthquakes are the most damaging — shallow-focus ones are.
- Confusing magnitude, which measures energy released and is a single number, with intensity, which varies with distance and is mapped by isoseismal lines.
As a one-word synonym question, as a statements item pairing focus with epicentre and focal depth with damage, or as a diagram-label question.
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 next layer of the same vocabulary. Having fixed what the focus is, the distinction that earns marks is between intensity, which is felt effect and is read on the Mercalli scale, and magnitude, which is energy released and is computed from wave amplitude — exactly what that item separates.
Point of Origin of Earthquake Wave is known as
- (a) Epicentre
- (b) Focus
- (c) Photosphere
- (d) Seismic Zone
Answer(b) Focus
The identical fact asked from the opposite side, with the same two words as the tempting pair. NDA gives the definition and asks for the term; CDS gives the term and asks for its synonym.
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
Answer(b) 1 and 2 only
The full set of consequences that follow from knowing what the focus is — the surface point above it, the waves that leave it, and the depth rule that examiners like to invert.
- practice — not a real PYQ
The point on the Earth's surface vertically above the focus of an earthquake is called the
- (a)Hypocentre
- (b)Epicentre
- (c)Seismic origin
- (d)Fault plane
Answer(b) Epicentre — the focus or hypocentre lies below it, inside the Earth, and the vertical distance between the two is the focal depth.
- practice — not a real PYQ
The existence of a liquid outer core in the Earth was inferred chiefly from which observation about seismic waves?
- (a)P waves travel faster than S waves
- (b)S waves do not pass through the outer core
- (c)Surface waves cause the most damage
- (d)Seismic waves refract at the Mohorovicic discontinuity
Answer(b) S waves do not pass through the outer core — transverse waves cannot travel through a liquid, which creates the S-wave shadow zone beyond about 105 degrees from the epicentre.