Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R). Assertion (A) : The highest concentration of Seismic zones lies in Himalayan region in India. Reason (R) : There are many longitudinal thrust zones in Himalayas. Codes :
- (a)Both (A) and (R) are true and (R) is the correct explanation of (A)
- (b)Both (A) and (R) are true, but (R) is not the correct explanation of (A)
- (c)(A) is true, but (R) is false
- (d)(A) is false, but (R) is true
Correct — A, both (A) and (R) are true and (R) is the correct explanation of (A). Take the two halves in turn. The Assertion is true: on the seismic zoning map of India (IS 1893, four zones numbered II to V), the highest-hazard Zone V is made up almost entirely of the Himalayan arc and its extensions — the Kashmir Valley, parts of Himachal Pradesh and Uttarakhand, the whole of the North-East, north Bihar, along with the Rann of Kutch and the Andaman and Nicobar Islands — while the rest of the Himalayan belt and the adjoining Indo-Gangetic foredeep fall in Zone IV. No other part of India carries so much high-hazard ground. The Reason is also true: the Himalaya is built of great thrust faults that run longitudinally, that is along the strike of the arc from Kashmir to Arunachal — from south to north, the Himalayan Frontal Thrust, the Main Boundary Thrust and the Main Central Thrust, all of which flatten at depth into a single detachment, the Main Himalayan Thrust. And the Reason genuinely explains the Assertion, because an earthquake is nothing other than the sudden slip of rock along a fault. The Indian plate is still driving northwards into Eurasia at roughly five centimetres a year; that convergence is absorbed by locking and periodic rupture on exactly these thrusts. The 2015 Gorkha earthquake in Nepal, magnitude about 7.8, was a rupture on the Main Himalayan Thrust itself. The thrusts are therefore not a coincidental feature of a shaky region — they are the machinery that produces the shaking.
- (b)Both (A) and (R) are true, but (R) is not the correct explanation of (A) — Both statements are indeed true, but the link between them is causal, not incidental. Seismicity is defined by slip on faults, and the Himalayan thrusts are the faults on which the India-Eurasia convergence is released. Choosing (b) treats the thrusts as a separate curiosity of Himalayan structure when they are the direct mechanism of the earthquakes.
- (c)(A) is true, but (R) is false — The Reason is factually correct. The Main Central Thrust, the Main Boundary Thrust and the Himalayan Frontal Thrust are standard, mapped, arc-parallel thrust zones that define the Greater, Lesser and Outer (Siwalik) Himalaya respectively. 'Longitudinal' here means running along the length of the range, which is precisely how these thrusts are oriented.
- (d)(A) is false, but (R) is true — The Assertion is not false. India's most severe seismic zone is dominated by the Himalayan belt and the North-East, and the region's record bears it out — the 1905 Kangra, 1934 Bihar-Nepal and 1950 Assam-Tibet great earthquakes, the 2005 Kashmir earthquake and the 2015 Gorkha earthquake all lie along this arc.
The Himalaya is a collision orogen: the Indian plate has been under-thrusting the Eurasian plate since the closure of the Tethys, and the shortening is taken up on a stack of low-angle thrust faults that run parallel to the mountain arc. Working northward from the plains, the Himalayan Frontal Thrust carries the Siwaliks over the Indo-Gangetic alluvium, the Main Boundary Thrust carries the Lesser Himalaya over the Siwaliks, and the Main Central Thrust carries the crystalline Greater Himalaya over the Lesser Himalaya; at depth all three merge into the Main Himalayan Thrust, the plate-boundary detachment. Strain accumulates on locked patches of this detachment for decades or centuries and is released as large earthquakes. India's seismic zoning map (IS 1893) grades the resulting hazard from Zone II, the stable peninsular shield, to Zone V, the very-high-hazard belt that traces this collision front and its north-eastern syntaxis.
Assertion-Reason items are decided in three separate steps, and candidates lose marks by collapsing them. Step one: is (A) true on its own? Step two: is (R) true on its own? Only if both survive does step three arise — does (R) supply the cause of (A)? Here the deciding insight for step three is the definition of an earthquake: sudden slip on a fault. Because the Himalayan thrusts are the faults, saying 'the Himalaya has many longitudinal thrust zones' is the same as saying 'the Himalaya has many active earthquake sources'. The commonest error is option (b), chosen by candidates who know both facts but never connect faults to seismicity; the second commonest is (c), from misreading 'longitudinal' as 'north-south' and concluding, wrongly, that Himalayan thrusts do not run that way. Longitudinal in structural geology means along the strike of the range.
- IS 1893 divides India into four seismic zones, II to V; Zone V, the highest hazard, covers the North-Eastern states, the Kashmir Valley, parts of Himachal Pradesh and Uttarakhand, north Bihar, the Rann of Kutch and the Andaman and Nicobar Islands.
- The three great arc-parallel Himalayan thrusts, from south to north, are the Himalayan Frontal Thrust, the Main Boundary Thrust and the Main Central Thrust; at depth they merge into the Main Himalayan Thrust.
- The Indian plate continues to converge on Eurasia at roughly five centimetres a year, and a substantial share of that shortening is stored and released on the Himalayan thrust system.
- Great and major earthquakes along this arc include 1905 Kangra, 1934 Bihar-Nepal, 1950 Assam-Tibet (about magnitude 8.6), 2005 Kashmir and 2015 Gorkha in Nepal (about magnitude 7.8, a rupture of the Main Himalayan Thrust).
- The NDMA's standard figure is that roughly 59 per cent of India's landmass is prone to earthquakes of moderate or higher intensity, that is Zones III, IV and V taken together.
Answer (a): Zone V is essentially a map of the Himalayan collision front, and the reason is structural — the arc-parallel thrusts (HFT, MBT, MCT) are the faults on which the shaking is generated.
- Reading 'longitudinal' as 'north-south'. In structural geology it means along the length or strike of the range, which for the Himalaya is roughly east-west along the arc.
- Marking (b) because both statements are known to be true but the causal link is never examined — Assertion-Reason questions are lost at this step more than at any other.
- Assuming Zone V is exclusively Himalayan; the Rann of Kutch is an intraplate zone and the Andaman and Nicobar Islands sit on a subduction zone, yet both are Zone V.
UPSC prefers the mechanism — what causes earthquakes, how magnitude differs from intensity, what P and S waves do — while UPPSC prefers the regional distribution and the Assertion-Reason format seen here, so pair the physics of faulting with a clear mental image of the Indian zoning map.
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
This is the precise link UPPSC's Reason depends on — faults and slip along faults cause earthquakes, while slow folding does not, which is exactly why a belt full of active thrusts is a belt full of earthquakes.
When you travel in Himalayas, you will see the following : 1. Deep gorges 2. U-turn river courses 3. Parallel mountain ranges 4. Steep gradients causing landsliding Which of the above can be said to be the evidences for Himalayas being young fold mountains?
- (a) 1 and 2 only
- (b) 1, 2 and 4 only
- (c) 3 and 4 only
- (d) 1, 2, 3 and 4
Answer(d) 1, 2, 3 and 4
The same structural fact seen from the surface — the 'parallel mountain ranges' a traveller notices are the fault-bounded slices separated by the longitudinal thrusts, and their youth is the tectonic activity that makes the belt seismic.
- practice — not a real PYQ
Which one of the following thrusts marks the southernmost structural boundary of the Himalaya, along which the Siwalik hills ride over the Indo-Gangetic alluvium?
- (a)Main Central Thrust
- (b)Main Boundary Thrust
- (c)Himalayan Frontal Thrust
- (d)Indus-Tsangpo Suture Zone
Answer(c) Himalayan Frontal Thrust — it is the youngest and southernmost of the arc-parallel thrusts, carrying the Siwaliks over the plains; the MBT lies north of it, the MCT further north still, and the Indus-Tsangpo Suture marks the northern limit of the Himalaya, not its southern edge.
- practice — not a real PYQ
Which one of the following regions does NOT lie in Seismic Zone V of the seismic zoning map of India?
- (a)Rann of Kutch in Gujarat
- (b)The North-Eastern states
- (c)Andaman and Nicobar Islands
- (d)Interior Deccan plateau of Maharashtra
Answer(d) Interior Deccan plateau of Maharashtra — it is part of the comparatively stable peninsular shield and falls in the lower hazard zones, whereas the Kutch intraplate zone, the entire North-East and the Andaman subduction belt are all classified as Zone V.