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) :

Question

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 :

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (b) 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) 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) 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.

Concept: 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.

State PCS 2019 Q122 — Geography PYQ with Answer

State PCS 2019 Q122 — Geography PYQ with Answer