CDS (II) 2021 General Knowledge · Question 95 of 120

Correct — D, Climate of the area. Damage from an earthquake is the product of three separate things: how hard the ground shakes, how much is standing in the way of that shaking, and how well the things standing there resist it. Strength of the earthquake covers the first — magnitude measures the energy released, and intensity, on the Modified Mercalli scale, measures the shaking and the destruction actually felt at a place. Population density covers the second: the same shock passing under an empty stretch of country and under a crowded town produces very different losses, which is why casualty figures track settlement and not magnitude alone. Type of building covers the third, and it is the factor engineers say matters most — heavy unreinforced masonry with poor connections collapses at shaking levels that a ductile reinforced-concrete frame designed for the local seismic zone rides out. Climate does not enter that chain. A given ground motion under a given building does the same thing to it in a hot dry plain as in a cold wet one, which is why the key marks the climate of the area as the factor that does not influence the extent of damage.

• (a)Strength of earthquake — The most direct influence of the four. Magnitude sets how much energy is released and, with depth and distance, how strongly the ground moves at a site — and the Richter scale is logarithmic, so one whole number up is roughly thirty-one times more energy.
• (b)Population density — Decides exposure. Two earthquakes of the same magnitude can kill in very different numbers depending on how many people and buildings are within reach of the strong shaking.
• (c)Type of building — Decides vulnerability, and it is the one factor engineering can change. The standard line that earthquakes do not kill people, badly built buildings do, is a statement about exactly this variable.

Disaster studies separate the hazard from the disaster. The hazard is the natural event, described by magnitude, depth, epicentral distance and the local ground conditions. The disaster is what that event does to people and property, and it depends on exposure — how many people and structures are in the shaken area — and on vulnerability, meaning how badly those structures and communities cope. Reducing risk works almost entirely on the second and third of these, because the first cannot be altered.

The item is testing whether you can tell the three risk terms apart from a factor that merely sounds environmental. Be honest about the edge of the claim rather than overstating it: climate is not completely absent from the story, because it shapes local building traditions and because saturated or frozen ground behaves differently under shaking — liquefaction, for instance, needs loose water-saturated sandy soil. But that is ground condition and construction practice doing the work, not climate directly, and among the four choices offered climate is plainly the odd one out. Where India is concerned the practical version of this question is the seismic zoning map, which puts the whole Himalayan belt, the Kutch region and the north-east in the highest hazard zones, and the building codes that go with it.

• Magnitude measures the energy released by an earthquake; intensity, on the Modified Mercalli scale, measures the shaking and damage experienced at a particular place.
• The Richter scale is logarithmic — a rise of one whole number corresponds to roughly thirty-one times more energy released.
• Risk in disaster studies is treated as a combination of hazard, exposure and vulnerability; damage rises with all three.
• India's seismic zoning divides the country into four zones, II to V, with V the most hazardous — it includes parts of the Himalaya, Kutch and the north-east.
• The great majority of earthquake deaths are caused by the collapse of buildings and by secondary effects such as landslides and fire, not by the ground motion itself.

• Treating magnitude alone as the measure of a disaster; a large offshore shock under an empty sea can kill fewer than a moderate one under a dense town.
• Confusing intensity with magnitude — intensity varies from place to place for the same earthquake, magnitude does not.
• Reading 'climate' as 'terrain' or 'soil'. Ground conditions do matter; climate as such is what the question rules out.

As a 'which one does not influence' item on damage factors, or as statements distinguishing hazard, exposure and vulnerability.

• practice — not a real PYQ

  The intensity of an earthquake, as distinct from its magnitude, is measured on which one of the following scales?

  • (a)Richter scale
  • (b)Modified Mercalli scale
  • (c)Beaufort scale
  • (d)Saffir-Simpson scale

  Answer(b) Modified Mercalli scale — it records the shaking and damage felt at a place, so it varies across the affected area, while magnitude on the Richter scale is a single figure for the event.

• practice — not a real PYQ

  In disaster studies, the vulnerability component of earthquake risk refers principally to

  • (a)the amount of energy released at the focus
  • (b)the number of people living in the affected area
  • (c)the ability of buildings and communities to withstand the shaking
  • (d)the distance of the site from the epicentre

  Answer(c) the ability of buildings and communities to withstand the shaking — energy released is hazard, and the number of people exposed is exposure.