Earthquake & Seismic Zones — Set 3

Correct Answer: B. Developing and maintaining national standards including IS 1893 for seismic safety

The Bureau of Indian Standards (BIS) plays the crucial role of developing and maintaining national standards for earthquake-resistant construction, including the IS 1893 seismic design code and IS 4326 for earthquake-resistant design and construction of buildings. These standards translate scientific understanding of seismic hazards into practical engineering guidelines. BIS regularly reviews and updates these codes based on new research and lessons from major earthquakes.

Correct Answer: B. Conducting geological surveys, seismotectonic studies, and geological hazard mapping

The Geological Survey of India (GSI) conducts geological surveys, seismotectonic studies, and geological hazard mapping that form the scientific basis for earthquake risk assessment. GSI identifies fault lines, assesses tectonic activity, and produces geological maps used for seismic hazard analysis. Their work feeds into the seismic zone map and IS 1893 design parameters used by engineers and planners throughout India.

Correct Answer: A. Ministry of Earth Sciences

The National Centre for Seismology (NCS) functions under the Ministry of Earth Sciences and serves as the national agency responsible for monitoring seismic activity across India. NCS operates the National Seismic Network with seismograph stations distributed across the country. It provides earthquake location, magnitude, and depth data in near-real-time, which is critical for disaster response and long-term seismic hazard assessment.

Correct Answer: B. An instrument that records ground vibrations caused by seismic waves

A seismograph (or seismometer) is a scientific instrument that detects and records ground vibrations caused by seismic waves from earthquakes, explosions, or other sources. The recorded output is called a seismogram, which shows the arrival times and amplitudes of different seismic wave types. By analyzing seismograms from multiple stations, seismologists can determine earthquake location, magnitude, and focal depth.

Correct Answer: B. Strengthening existing structures to withstand earthquake forces

Seismic retrofitting is the process of strengthening existing structures (buildings, bridges, dams) that were built before modern seismic codes to improve their ability to withstand earthquake forces. Common retrofitting techniques include adding shear walls, steel bracing, reinforced concrete jackets around columns, and base isolation systems. India has many older buildings that pre-date modern seismic codes, making retrofitting a critical challenge in earthquake-prone areas.

Correct Answer: B. Using the faster P waves to issue warnings before the more destructive S and surface waves arrive

Earthquake Early Warning (EEW) systems work by detecting the initial, faster P waves from an earthquake and quickly analyzing them to issue warnings before the slower but more destructive S waves and surface waves arrive at populated areas. The warning time available depends on the distance from the epicentre — areas close to the epicentre receive very little warning while areas farther away have more time. Japan operates the world's most advanced EEW system.

Correct Answer: C. Intensity of ground shaking at a specific location

The Modified Mercalli Intensity (MMI) scale measures the intensity of ground shaking at a specific location based on observed effects such as human sensation, structural damage, and changes in the landscape. Unlike magnitude scales (Richter, Mw) which measure the energy at the source, the MMI varies with distance from the epicentre. India's seismic zone map uses MMI values to classify areas from Zone II (MMI VI or less) to Zone V (MMI IX and above).

Correct Answer: B. The product of the area of fault rupture, average displacement, and rigidity of rock

The seismic moment is a physical measure of the energy released in an earthquake, calculated as the product of the area of fault rupture, the average displacement along the fault, and the rigidity (shear modulus) of the rock. The Moment Magnitude Scale (Mw) was developed by Hiroo Kanamori and Thomas Hanks in 1979 and is directly calculated from the seismic moment. This makes it a more physically meaningful measure than the Richter Scale, especially for very large earthquakes.

Correct Answer: B. Jammu & Kashmir and Uttarakhand

Jammu & Kashmir and Uttarakhand are in the Himalayan region and are classified in Seismic Zone IV to V, representing high to very high earthquake hazard. These areas sit along the Main Himalayan Thrust, one of the world's most active fault systems where the Indian plate continuously collides with the Eurasian plate. The combination of high seismicity and mountainous terrain also increases the risk of earthquake-triggered landslides.

Correct Answer: C. Convergent (destructive) boundary — Indian plate colliding with Eurasian plate

Most Himalayan earthquakes occur at the convergent (destructive) boundary where the Indian tectonic plate continuously collides with the Eurasian plate, pushing up the Himalayas. This collision is occurring at approximately 5 cm per year, building up enormous tectonic stress that is periodically released as large earthquakes. The Main Himalayan Thrust (MHT) system is the primary fault zone responsible for the largest Himalayan earthquakes.