Earthquake & Seismic Zones — Set 10

Correct Answer: B. A major thrust fault separating the Lesser Himalayas from the Outer Himalayas (Sub-Himalayas)

The Main Boundary Thrust (MBT) is a major thrust fault system separating the Lesser Himalayas from the Sub-Himalayas (Outer Himalayas/Siwaliks). It is one of the major active fault systems of the Himalayan fold-thrust belt and is associated with significant seismic activity. The MBT runs roughly parallel to the Himalayan front across northern India from Jammu through Uttarakhand to Arunachal Pradesh, representing a zone of tectonic activity that poses earthquake hazard to nearby communities.

Correct Answer: B. IIT and NITs (engineering institutes) along with NDMA

Indian Institutes of Technology (IITs) and National Institutes of Technology (NITs) are primarily responsible for research on engineering aspects of earthquake damage and safety in India, working in coordination with NDMA, BIS, and state governments. IIT Roorkee, IIT Delhi, IIT Bombay, and IIT Madras have dedicated earthquake engineering centers. Research from these institutions informs IS 1893 revisions, seismic microzonation studies, and retrofit guidelines for existing buildings.

Correct Answer: B. Earthquakes that occur within the subducting Indian plate at depth in the Andaman subduction zone

Intraslab earthquakes occur within the subducting slab (plate) at depth, as the plate bends and is subjected to internal stresses during the subduction process. In the Andaman subduction zone, intraslab earthquakes occur at intermediate depths (70-300 km) within the Indian-Australian plate as it subducts under the Burmese plate. These earthquakes can be large (magnitude 7+) and occur at depths where they may be widely felt but cause relatively less surface damage than shallow earthquakes.

Correct Answer: B. They have no lateral load-resisting system and fail catastrophically in brittle manner during horizontal earthquake forces

Unreinforced masonry (URM) buildings are most vulnerable to earthquake damage because they have no lateral load-resisting system and exhibit brittle failure — they collapse suddenly without warning when horizontal earthquake forces exceed the masonry's weak tensile and shear capacity. The mortar between stones and bricks has very little tensile strength, so horizontal earthquake forces cause walls to crack and collapse. A large proportion of India's rural and semi-urban housing is URM construction, representing the country's greatest earthquake vulnerability.

Correct Answer: B. Detailed mapping of local site conditions at neighborhood level to assess how ground shaking varies across a city

Seismic microzonation is the detailed mapping of local site conditions (soil type, depth to bedrock, groundwater level) at the neighborhood level to assess how earthquake ground shaking varies across a city. This is important for Indian cities because it helps identify specific neighborhoods with high amplification potential that require stricter building regulations or retrofitting programs. NDMA has funded microzonation studies for several major Indian cities to guide risk reduction planning.

Correct Answer: B. Operating the Indian Tsunami Early Warning Centre for ocean earthquakes and tsunami detection

INCOIS (Indian National Centre for Ocean Information Services) operates the Indian Tsunami Early Warning Centre (ITEWC) which monitors seismic activity in the Indian Ocean region and issues tsunami warnings when large submarine earthquakes are detected. ITEWC uses a combination of seismograph data and ocean buoys to detect tsunami generation and can issue warnings within minutes of a significant earthquake. This system was established after the 2004 Indian Ocean Tsunami to prevent similar loss of life in future events.

Correct Answer: B. Intense ground shaking that can cause significant damage to structures — measured by peak ground acceleration

Strong ground motion refers to intense ground shaking during earthquakes that is capable of causing significant damage to structures and infrastructure. It is typically measured by peak ground acceleration (PGA), peak ground velocity (PGV), or spectral acceleration. Strong motion accelerographs record these parameters and the data is used to develop ground motion prediction equations (GMPEs) that form the basis of seismic hazard assessment and earthquake-resistant design standards in India.

Correct Answer: C. 2002 with further revisions in 2016

IS 1893 (Criteria for Earthquake Resistant Design of Structures) was last comprehensively revised in 2002 following the devastating 2001 Bhuj earthquake, and further revised in 2016 incorporating lessons from the 2001 Bhuj earthquake, new scientific understanding, and alignment with international codes. The 2002 revision reduced the number of seismic zones from 5 (I to V) to 4 (II to V) by merging Zones I and II. Regular revisions ensure the code incorporates new scientific and engineering knowledge.

Correct Answer: B. The fault breaking through to the Earth's surface, potentially destroying any structure built across it

Ground rupture (surface rupture or surface faulting) occurs when an earthquake fault breaks through to the Earth's surface, creating a zone of displaced, cracked, and heaved ground along the fault trace. Any structure built directly across the fault trace (roads, buildings, pipelines, dams) can be severely damaged or destroyed by the differential ground displacement. The 2001 Bhuj earthquake caused significant surface ruptures in the Kutch region that destroyed roads and structures.

Correct Answer: B. High proportion of non-engineered masonry and earthen construction that cannot resist earthquake forces

The biggest challenge in earthquake preparedness for rural India is the overwhelming proportion of non-engineered masonry, stone, and earthen construction that cannot resist earthquake forces. These buildings make up over 70% of India's rural housing stock and are highly vulnerable to even moderate earthquakes. The 1993 Latur earthquake demonstrated this with traditional stone wada construction causing most deaths. Promoting simple, low-cost earthquake-resistant construction techniques for rural communities is a major challenge for NDMA and state governments.