Earthquake & Seismic Zones — Set 6

Correct Answer: C. Ahmedabad

Ahmedabad, located approximately 300 km from the Bhuj earthquake epicentre, experienced significant building collapses despite the distance. Several multi-storey residential buildings pancaked (floors collapsed on each other) killing hundreds. Post-earthquake investigations found that the buildings had fundamental periods that resonated with the earthquake ground motion frequency and were poorly constructed, violating seismic design codes. This event highlighted the vulnerability of high-rise buildings to distant large earthquakes.

Correct Answer: C. Indian National Centre for Ocean Information Services (INCOIS)

The Indian Tsunami Early Warning Centre (ITEWC) is operated by the Indian National Centre for Ocean Information Services (INCOIS) in Hyderabad. Established after the 2004 Indian Ocean Tsunami, ITEWC monitors seismic activity and ocean water levels using seismometers and Deep-ocean Assessment and Reporting of Tsunami (DART) buoys. ITEWC can issue tsunami warnings for the Indian Ocean region within minutes of detecting a potential tsunami-generating earthquake.

Correct Answer: B. Inserting flexible isolators between building foundation and superstructure to reduce earthquake forces transmitted to building

Base isolation is an advanced seismic protection technique where flexible isolators (rubber-lead bearings, sliding isolators) are inserted between the building's foundation and its superstructure. These isolators allow the building to move independently of the ground motion, significantly reducing the earthquake forces transmitted to the structure. Base isolation is used for critical facilities like hospitals, nuclear plants, and government buildings in high seismic zones.

Correct Answer: C. 2004 Sumatra-Andaman earthquake (which generated the Indian Ocean tsunami)

The 2004 Sumatra-Andaman Earthquake (also known as the 2004 Indian Ocean earthquake) with a magnitude of 9.1 severely affected the Andaman and Nicobar Islands and all coastal areas around the Bay of Bengal. The earthquake ruptured approximately 1,200 km of fault along the Sunda subduction zone causing the Indian Ocean Tsunami. The Andaman and Nicobar Islands were closest to the epicentre and experienced not only the tsunami but also severe direct earthquake shaking.

Correct Answer: B. Shallower earthquakes generally cause more surface damage as energy is less spread out

Shallower earthquakes (depth less than 70 km) generally cause more surface damage because the seismic energy travels a shorter distance and is less spread out when it reaches the surface, resulting in more concentrated and intense ground shaking. The 2001 Bhuj Earthquake had a relatively shallow focus of about 23 km, contributing to its devastating impact. Deep focus earthquakes (below 300 km) rarely cause significant surface damage despite having high magnitudes.

Correct Answer: B. Earthquakes in the stable interior of Peninsular India — Maharashtra, Karnataka, Andhra Pradesh

Deccan Plateau intraplate seismicity refers to earthquakes occurring within the stable interior of Peninsular India — primarily in Maharashtra, Karnataka, and Andhra Pradesh. These earthquakes occur along ancient, reactivated fault zones in the Deccan volcanic rocks. The 1993 Latur, 1997 Jabalpur, and 2011 Sikkim earthquakes were notable events in or near this region. This seismicity is considered more hazardous because it occurs in areas where earthquake risk is underestimated.

Correct Answer: B. Recording near-field ground motion data from large earthquakes to improve design parameters

Strong motion seismology involves recording near-field ground motion data from moderate to large earthquakes using accelerographs placed near earthquake sources. This data is used to understand how ground shaking varies with earthquake magnitude, distance, and local site conditions. In India, the National Strong Motion Instrumentation Network records strong motion data that improves seismic design parameters in IS 1893 and helps engineers better design earthquake-resistant structures.

Correct Answer: B. Ductility — allowing controlled deformation without sudden collapse

Ductility is the key principle in earthquake-resistant design that allows structures to deform significantly without sudden catastrophic collapse, giving occupants time to evacuate. A ductile structure absorbs earthquake energy through controlled deformation in designated plastic hinge zones rather than sudden brittle failure. IS 1893 and IS 13920 (ductile detailing code) specify requirements to ensure ductile behavior of reinforced concrete structures in earthquake zones.

Correct Answer: B. Local soil conditions amplifying earthquake ground motion beyond that of bedrock

Site amplification occurs when local soil conditions amplify earthquake ground motion beyond what would be experienced on bedrock at the same distance from the earthquake. Soft, deep sedimentary deposits like river valleys and coastal plains can significantly amplify ground shaking — sometimes by factors of 5-10 times. This explains why some areas far from epicentres suffer more damage than closer areas on bedrock. Areas on soft soil in Delhi, Mumbai, and Kolkata have high site amplification potential.

Correct Answer: D. Long-period surface waves that match the natural period of tall buildings

Long-period surface waves are most damaging to tall buildings because the natural period of vibration of tall buildings matches the longer periods of these surface waves, causing resonance. When a building's natural period coincides with the dominant period of earthquake ground motion, resonance occurs causing the building to sway with increasing amplitude. This explains why some tall buildings in cities like Mexico City and Ahmedabad collapsed during distant large earthquakes due to resonance effects.