Earthquake & Seismic Zones — Set 8

Correct Answer: C. 8.0-8.5+ magnitude — great earthquakes from Himalayan thrust faults

Paleoseismological studies of the Himalayan thrust fault system have identified evidence of past great earthquakes (magnitude 8.0-8.5+) that ruptured large sections of the Main Himalayan Thrust. Such events occur infrequently (hundreds to thousands of years recurrence) but are catastrophic when they do occur. The Central Himalayan seismic gap is considered capable of generating a future magnitude 8+ earthquake that would severely impact the densely populated Gangetic plains.

Correct Answer: B. Earthquake-resistant allows controlled damage to protect lives while earthquake-proof means no damage — a practical impossibility for large earthquakes

The key difference is that 'earthquake-resistant' construction is designed to protect lives by undergoing controlled and repairable damage (especially in strong earthquakes) without collapsing, while 'earthquake-proof' implies no damage — a practically unachievable goal for large earthquakes. IS 1893 adopts a performance-based design philosophy: buildings should resist moderate earthquakes without damage, resist rare strong earthquakes with repairable damage, and prevent collapse even in very rare catastrophic earthquakes to allow occupant evacuation.

Correct Answer: B. It acts as a rigid basement block that amplifies earthquake damage

The Shillong Plateau is a rigid basement block of Precambrian rocks that acts like a hard punch into the softer Himalayan terrain as the Indian plate pushes northward. This creates unique tectonic stress patterns around the plateau that are responsible for many of the large earthquakes in Assam and surrounding areas. The presence of this rigid block surrounded by active thrust and wrench faults makes northeast India one of the world's most seismically active regions.

Correct Answer: B. Historical great earthquakes show the need for updating zones — Latur earthquake in Zone III proves areas can be more dangerous than mapped

India's seismic zonation is periodically revised based on new scientific data and historical earthquake records. The 1993 Latur earthquake in Maharashtra (Zone III at the time) killed nearly 10,000 people and demonstrated that the moderate hazard classification may underestimate risk in some areas. Similarly, the Koyna Dam area was reclassified after the 1967 reservoir-induced earthquake. These examples show that seismic hazard assessment must continuously incorporate new data.

Correct Answer: B. Educating communities about earthquake risks, safe behaviors, and evacuation routes is critical to reducing casualties

NDMA guidelines emphasize that community awareness is critical to earthquake preparedness and reducing casualties. Communities need to know about local earthquake hazards, how to make their homes safer, the 'Drop, Cover, Hold On' procedure, and evacuation routes. NDMA's Aapda Mitra scheme and school safety programs specifically aim to create earthquake-aware communities. Educated communities respond more effectively during earthquakes and recover more quickly afterward.

Correct Answer: C. Over 100 stations

India's National Seismic Network operated by the National Centre for Seismology (NCS) has over 100 broadband seismograph stations distributed across the country, including in remote areas of the Himalayas, northeast India, and the Andaman and Nicobar Islands. This network provides continuous monitoring of seismic activity across India's diverse geological settings. Additional seismograph networks are operated by various state governments and research institutions.

Correct Answer: B. Scientific estimation of expected ground shaking intensity at a site for a given time period

Seismic hazard assessment is the scientific process of estimating the expected ground shaking intensity at a specific site over a given time period, based on the historical earthquake record, geological information about fault systems, and mathematical models of earthquake occurrence and wave propagation. Probabilistic Seismic Hazard Analysis (PSHA) produces hazard curves showing the probability of exceeding various ground motion levels, which form the basis for seismic design standards like IS 1893.

Correct Answer: B. Earthquake-triggered landslides blocked rivers, creating temporary lakes that then burst causing catastrophic floods

The 1950 Assam Earthquake (8.6 magnitude) triggered massive landslides across the steep Himalayan terrain that blocked multiple rivers, creating temporary landslide dams. When these landslide dams subsequently failed (breached), they released catastrophic floods that affected the Brahmaputra valley for days. This earthquake-flood sequence caused far more deaths and damage than the direct earthquake shaking, demonstrating how large earthquakes can trigger complex multi-hazard events.

Correct Answer: B. Delhi, Mumbai, Kolkata, and cities in northeast India represent high-risk combinations of seismic zone and population

Delhi (Zone IV), Mumbai (Zone III-IV), Kolkata (Zone III), and cities in northeast India (Zone IV-V) represent critical combinations of significant seismic hazard and large populations. Delhi is particularly at risk given its Zone IV classification and close proximity to the Himalayan seismic zone. Many buildings in these cities pre-date modern seismic codes and are highly vulnerable. A major earthquake affecting any of these megacities could be catastrophic for India's economy and human welfare.

Correct Answer: B. The stable interior region of India (Zone II-III) encompassing most of southern and central India — Karnataka, Kerala, Tamil Nadu, parts of Maharashtra and AP

The Peninsular India seismic zone generally corresponds to Zones II and III in India's seismic zonation, covering the stable interior of Peninsular India including Karnataka, Kerala, Tamil Nadu, large parts of Maharashtra and Andhra Pradesh. Despite being in lower seismic zones, this region experiences intraplate earthquakes including the 1993 Latur, the 2001 Bhuj (near plate boundary), and various Koyna events. The region's ancient Precambrian rocks are generally more earthquake-resistant than Himalayan sedimentary rocks.