Given below are two statements , one is labelled as Assertion (A) and the other as Reason (R). Assertion (A): This highest concentration of seismic zones lies in Himalayan region in India. Reason (R): There are many longitudinal thrust zones in Himalayas.

The correct answer is A. Both statements are true, and the reason explains the assertion. The Himalayas are young fold mountains formed by the collision of the Indian and Eurasian plates. That collision produces strong compressional stress, which is released along major thrust faults that run parallel to the arc, which is what 'longitudinal' means here. These are the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT). Slip along these faults generates most of the region's earthquakes, so the thrust zones are the cause of the high seismicity.

• Option B is wrong because the reason is the explanation, not a separate fact.
• Option C is wrong because the reason is a correct geological fact.
• Option D is wrong because the assertion is correct: the Himalayan belt has the highest seismic hazard in India.

For the Mains, this connects to disaster management and the seismic zonation of India. Note that IS 1893 once had five zones. Zone I was merged into Zone II in the 2002 revision, so India now has four zones, with Zone V covering the highest-risk Himalayan belt. Check the current code when you revise.

Trap: Aspirants often mark B because both statements look true, without asking whether the thrust zones actually cause the seismicity.