The Main Boundary Thrust and Main Central Thrust of the Himalaya are
Answer: A
The collision has stacked the Himalaya on a series of thrusts that dip beneath the range, and slip on them is what produces its great earthquakes. Naming the thrusts is naming the geological sources of that seismicity.
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The Himalayan Frontal Thrust is significant because it is
Answer: B
Correct answer: the youngest and southernmost of the thrust system
The Himalayan thrusts young southward, each new one forming in front of the last as the deformation front advances into the foreland. The Frontal Thrust is the present front of that system, carrying the range over the Ganga plain, and it is where the surface rupture of a great Himalayan earthquake would be expected.
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The Kutch region's seismicity is associated geologically with
Answer: C
The Mesozoic rift's normal faults are being compressed and reactivated as reverse faults under the present stress field. That inversion of an old structure is why a region far from any plate boundary suffers large earthquakes.
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The Narmada-Son lineament is important seismotectonically as
Answer: D
Correct answer: an ancient crustal weakness capable of reactivation
The Narmada-Son lineament is a Precambrian structure deep within the Indian plate, far from any present boundary, and it is significant because such old zones of weakness are the paths along which intraplate stress is relieved. The Jabalpur earthquake of 1997 was associated with it, which is the practical demonstration.
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Peninsular India suffers damaging earthquakes despite lying far from a plate boundary because
Answer: A
Correct answer: ancient faults within the plate are being reactivated
Intraplate earthquakes occur where old faults, formed in some earlier tectonic episode, are favourably oriented to the present stress field and slip again. Latur in 1993 and Bhuj in 2001 are the striking Indian examples, and they matter disproportionately because such regions have long recurrence intervals and correspondingly little seismic tradition in their building.
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The Shillong plateau is seismically notable because it is
Answer: B
Bounding structures such as the Dauki and Oldham faults have lifted the block clear of the surrounding plains and remain capable of large events. The 1897 earthquake there produced some of the largest accelerations ever inferred.
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The Andaman and Nicobar islands owe their seismicity to
Answer: C
Correct answer: subduction of oceanic lithosphere beneath the arc
The Andaman and Nicobar islands sit on the Sunda arc, where the Indian plate is subducting eastward beneath the Burma plate. That is a plate boundary setting, quite unlike the intraplate seismicity of peninsular India, and it produced the great Sumatra-Andaman earthquake and tsunami of 2004.
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A fault is classed as active if it has displaced ground during the
Answer: D
Movement within so recent an interval implies that the fault remains part of the present stress system and may move again. The precise threshold differs between codes, but the principle of recent displacement is common to all.
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A capable fault, in the terminology used for critical facilities, is one that
Answer: A
Correct answer: could produce significant surface displacement
A capable fault is one judged able to rupture the ground surface within the period that matters for the facility, and the term is used in nuclear siting because surface displacement cannot be designed against. The judgement rests on evidence of movement in the recent geological past, usually within the Holocene or Quaternary.
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Deciding whether a fault is active requires above all
Answer: B
Correct answer: dating the youngest deposits it displaces
Activity is a statement about time, so it is settled by finding the youngest material the fault has cut and the oldest that lies undisturbed across it. That brackets the last movement. The fault’s length, dip and gouge tell you about its size and mechanism but nothing about when it last moved.
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