Earthquake & Seismic Zones — Set 12

Correct Answer: B. Small earthquakes are far more frequent than large ones — following the Gutenberg-Richter relationship

The frequency-magnitude relationship of earthquakes follows the Gutenberg-Richter law: the number of earthquakes decreases exponentially with increasing magnitude. For every magnitude 7 earthquake, there are approximately 10 magnitude 6 earthquakes, 100 magnitude 5 earthquakes, and 1,000 magnitude 4 earthquakes. This relationship explains why large destructive earthquakes are rare but small tremors are constantly occurring at seismically active zones like the Himalayas and northeast India.

Correct Answer: B. Real-time seismic network with automated earthquake detection and public alerting similar to Japan's system

India is in the process of developing a real-time earthquake early warning (EEW) system using the existing and expanded National Seismic Network. The Ministry of Earth Sciences and NCS have been working on deploying more seismograph stations with real-time telemetry to reduce earthquake detection and alerting time. While India's system is still developing compared to Japan's highly advanced EEW system, the goal is to provide seconds to tens of seconds of warning to cities distant from earthquake epicentres.

Correct Answer: B. Widespread landslides in Himalayan terrain, building collapses, and damage to mountain roads

The 2011 Sikkim Earthquake (magnitude 6.9, September 18, 2011) caused widespread landslides in the steep Himalayan terrain of Sikkim, Darjeeling, and Nepal, as well as building collapses and severe damage to mountain roads and bridges. The earthquake killed approximately 111 people and caused significant infrastructure damage. The event highlighted the cascading hazards associated with Himalayan earthquakes — direct building collapse combined with earthquake-triggered landslides that blocked roads and caused further casualties.

Correct Answer: B. The curved seismic belt along the Himalayan mountain range where the Indian plate collides with Eurasian plate, generating the highest earthquake risk in India

The Himalayan arc is the curved seismic belt extending from Jammu & Kashmir through Himachal Pradesh, Uttarakhand, Nepal, Sikkim to Arunachal Pradesh where the Indian plate is colliding with the Eurasian plate, creating the world's highest mountain range. This arc concentrates the highest earthquake risk in India because the tectonic collision is ongoing, building up enormous stress that is periodically released in large to great earthquakes. The Himalayan arc is considered one of the most seismically dangerous regions on Earth.

Correct Answer: B. Most of Rajasthan is in Zone II-III (lower hazard) but has experienced moderate earthquakes including along the Aravalli fault zones

Most of Rajasthan falls in Seismic Zones II and III, representing lower to moderate earthquake hazard. However, the state has experienced moderate earthquakes, particularly along ancient fault zones associated with the Aravalli mountain ranges. The northwestern parts of Rajasthan near the Pakistan border and Gujarat are in higher seismic zones due to proximity to the Kutch fault system. While not as seismically active as the Himalayas or Northeast India, Rajasthan's earthquake risk should not be ignored.

Correct Answer: B. It created a 6-meter scarp (Allah Bund = God's dam) across the Rann of Kutch and was one of the first scientifically documented major earthquakes in India

The 1819 Allah Bund earthquake (magnitude ~7.8) in Gujarat's Kutch region is historically significant because it created a 6-meter high and 80 km long scarp (ground uplift) across the Rann of Kutch — this feature was called 'Allah Bund' meaning God's dam. It was one of the first scientifically documented surface rupturing earthquakes in South Asia, providing early geological evidence of active faulting in the Kutch region. The same fault system ruptured again in the 2001 Bhuj earthquake.

Correct Answer: B. Collapse concentrated in a floor with fewer/weaker columns or walls compared to other floors — often the ground floor with large openings

Soft storey failure occurs when a particular floor (most commonly the ground floor) in a multi-storey building has significantly less lateral stiffness and strength than the floors above it — typically because the ground floor has large openings for shops or parking with few structural walls while upper floors have more walls. During earthquakes, the soft storey collapses while upper floors remain relatively intact, causing catastrophic building failure. This failure mode caused many deaths in Ahmedabad during the 2001 Bhuj earthquake.

Correct Answer: B. High seismic zone location, buildings on steep unstable slopes, old construction with poor materials, and limited access for rescue teams

Himalayan cities like Shimla, Dehradun, and Shillong are particularly vulnerable due to their high seismic zone location (Zone IV-V), buildings constructed on steep, unstable slopes, predominantly old construction with traditional materials, narrow mountain roads limiting rescue team access, and dense construction in constricted valleys. An earthquake affecting these cities would be challenging to respond to because of access difficulties and the combined hazard of earthquake shaking plus landslides.

Correct Answer: B. A magnitude 6.8 earthquake that killed 768 people, demonstrating the ongoing seismic threat to the Himalayan region of Uttarakhand

The 1991 Uttarkashi Earthquake (magnitude 6.8, October 20, 1991) killed 768 people in the Garhwal region of Uttarakhand and caused significant building damage and landslides. Coming just two years before the 1993 Latur earthquake, it further highlighted India's earthquake vulnerability. The Uttarkashi earthquake occurred in the same Main Central Thrust zone that poses ongoing seismic hazard to the Himalayan region, and the pattern of destruction from this event informed subsequent earthquake preparedness measures.

Correct Answer: B. To ensure all new buildings comply with IS 1893, retrofit critical infrastructure, and progressively reduce the vulnerable building stock

India's national goal for earthquake-resistant construction is to ensure all new buildings comply with IS 1893 seismic design standards, retrofit critical infrastructure (hospitals, schools, government buildings), and progressively reduce the large stock of vulnerable non-engineered buildings through awareness, capacity building, and regulatory enforcement. NDMA's guidelines emphasize that achieving earthquake safety requires coordinated action across building regulations, construction practice, engineering education, and community awareness.