Earthquake & Seismic Zones — Set 7

Correct Answer: B. The southernmost thrust fault of the Himalayan system, marking the boundary of active mountain building

The Himalayan Frontal Thrust (HFT), also called the Main Frontal Thrust (MFT), is the southernmost active thrust fault of the Himalayan fold and thrust belt where the Himalayan mountain front meets the Indo-Gangetic Plains. It is one of the most active fault segments in India and is associated with the risk of a future great earthquake. Earthquakes on the HFT could cause massive damage to cities in the Gangetic plains including Delhi, Lucknow, and Patna.

Correct Answer: C. 19,000

The 1905 Kangra Earthquake killed approximately 19,000 people in the Kangra Valley of present-day Himachal Pradesh. It struck on April 4, 1905 at 6:20 AM local time with a magnitude of approximately 7.8. The earthquake destroyed the historic town of Kangra, most other towns in the valley, and caused widespread landslides in the Himalayan terrain.

Correct Answer: B. Northern Bihar — Terai and foothills region bordering Nepal

Northern Bihar, particularly the Terai and foothill regions bordering Nepal, is most earthquake-prone because it lies closest to the active Himalayan seismic zone. The 1934 Bihar-Nepal earthquake caused catastrophic damage in this region. Parts of northern Bihar are classified in Seismic Zone IV and even Zone V in some areas near the Nepal border. The densely populated Gangetic plains of Bihar are also vulnerable to seismic amplification.

Correct Answer: B. Permanent ground uplift or subsidence caused by earthquake fault displacement — observed in Andaman Islands during 2004 earthquake

Co-seismic uplift refers to permanent ground uplift or subsidence that occurs simultaneously with an earthquake due to the fault displacement. During the 2004 Indian Ocean earthquake, the Andaman and Nicobar Islands experienced dramatic co-seismic changes — some areas rose by 1-2 meters while others subsided by similar amounts. These changes permanently altered the coastlines of some islands and affected coral reef habitats.

Correct Answer: B. The Himalayan Main Central Thrust (MCT) and other Himalayan fault systems

Uttarakhand's high earthquake risk is associated with the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Himalayan Frontal Thrust (HFT) — a system of major thrust faults running through the Himalayan region. Uttarakhand experienced several major historical earthquakes including the 1999 Chamoli earthquake (6.8 magnitude) and numerous smaller events. The state is classified primarily in Seismic Zone IV with parts in Zone V.

Correct Answer: B. Continuously monitoring seismic activity across India and providing rapid earthquake alerts

The National Seismic Network operated by the National Centre for Seismology (NCS) serves to continuously monitor seismic activity across India's diverse geological settings and provide rapid earthquake location and magnitude data. The network comprises broadband seismograph stations distributed throughout India. Rapid earthquake alerts from this network are critical for activating disaster response and for tsunami warning systems in the Bay of Bengal and Arabian Sea.

Correct Answer: B. Kashmir Himalaya fold-thrust belt and associated seismic zones

Kashmir Valley seismicity is associated with the Kashmir Himalaya fold-thrust belt — a complex system of thrust faults and fold structures where intense tectonic activity occurs due to the collision between the Indian and Eurasian plates. The 2005 Kashmir earthquake (7.6 magnitude, primarily in Pakistan-occupied Kashmir) killed over 73,000 people. Indian-administered Kashmir is in Seismic Zone V, the highest risk zone.

Correct Answer: B. Seismometer (seismograph)

A seismometer (or seismograph) specifically measures the velocity or displacement of ground motion during earthquakes by detecting the relative movement between a suspended mass (inertial mass) and the Earth's surface. Modern digital seismometers can record ground motions with periods ranging from milliseconds to hundreds of seconds with very high sensitivity. The recorded seismogram is the basis for determining earthquake parameters such as location, magnitude, and focal mechanism.

Correct Answer: B. One of the largest intraplate earthquakes (8.1 magnitude) in the world, demonstrating the high seismicity of northeast India

The 1897 Shillong Earthquake (magnitude 8.1) is one of the largest intraplate earthquakes ever recorded and demonstrates the exceptionally high seismicity of northeast India. The earthquake struck on June 12, 1897 causing massive surface deformation, large landslides, and approximately 1,542 deaths. It was scientifically significant because it provided early evidence that the Shillong Plateau acts as a rigid block being pushed south by the Himalayan collision.

Correct Answer: B. When earthquake ground motion frequency matches building natural frequency, causing amplified swaying and potential collapse

Building resonance occurs when the dominant frequency of earthquake ground motion matches the natural frequency of vibration of a building, causing the building to sway with increasingly large amplitude, potentially leading to collapse. In Ahmedabad during the 2001 Bhuj earthquake, many 9-12 storey residential buildings had natural periods that resonated with the earthquake waves that had traveled 300 km, causing pancake collapses. This phenomenon explains why seemingly distant earthquakes can still cause catastrophic damage in cities with certain building types.