Earthquake & Seismic Zones — Set 4

Correct Answer: C. Sunda Subduction Zone — Indian plate subducting under Burmese plate

The 2004 Indian Ocean Tsunami was generated by a massive 9.1 magnitude megathrust earthquake at the Sunda Subduction Zone, where the Indian-Australian plate subducts under the Burmese plate. The earthquake caused a 1,200 km long fault rupture and vertical seafloor displacement of several meters, displacing a massive volume of water. The resulting tsunami waves traveled across the Indian Ocean affecting 14 countries.

Correct Answer: C. About 59%

The correct answer is 'About 59%'. Approximately 59% of India's total land area is prone to earthquake hazards of moderate to very high intensity (Zones III, IV, and V). India sits at the junction of the Indian and Eurasian tectonic plates, making it inherently earthquake-prone. This large earthquake-prone area means that a significant portion of India's population and infrastructure is exposed to seismic risk.

Correct Answer: C. Zone III

The 1993 Latur Earthquake occurred in Seismic Zone III, which is considered a moderate seismic hazard area. This earthquake was particularly surprising because it occurred in the stable Deccan Plateau region of Maharashtra, which was not considered highly earthquake-prone. The Latur earthquake demonstrated that even areas in Zone III can experience devastating earthquakes and prompted revision of India's seismic hazard assessment.

Correct Answer: B. Earthquakes occurring within the interior of a tectonic plate, away from plate boundaries

Intraplate seismicity refers to earthquakes that occur within the interior of a tectonic plate, far from the plate boundary zones where most earthquakes happen. The 1993 Latur Earthquake was a classic example of intraplate seismicity in the stable Deccan Plateau region. These earthquakes are less predictable than plate-boundary earthquakes and can catch communities off guard as they occur in areas not traditionally considered earthquake-prone.

Correct Answer: B. Main Himalayan Thrust (MHT)

The Main Himalayan Thrust (MHT) is the major fault system associated with the highest earthquake risk in the Himalayan region. This thrust fault system runs along the entire length of the Himalayas where the Indian plate is being thrust under the Eurasian plate. Great earthquakes (magnitude 8+) are known to have occurred on this fault system and the region is considered due for another great earthquake.

Correct Answer: B. A smaller earthquake that follows the main earthquake in the same region

An aftershock is a smaller earthquake that follows the main earthquake (mainshock) in the same general region, occurring as the fault continues to adjust after the main rupture. Aftershocks can be dangerous because they can cause further collapse of already damaged structures. The largest aftershock is typically about 1.2 magnitude units smaller than the mainshock, and aftershocks can continue for days, weeks, or even months after the main event.

Correct Answer: A. A smaller earthquake that precedes the main earthquake

A foreshock is a smaller earthquake that precedes the main earthquake (mainshock) in the same area. However, foreshocks are only identified in retrospect — at the time they occur, it is impossible to know if a small earthquake is a foreshock of a larger event or a standalone earthquake. Some devastating earthquakes like the 1934 Bihar earthquake had identifiable foreshock sequences.

Correct Answer: B. Segments of active fault zones that have not recently experienced major earthquakes and are considered due for one

The seismic gap theory identifies segments of active fault zones that have not recently experienced large earthquakes while adjacent segments have. These quiet sections are considered to have accumulated tectonic stress and are 'overdue' for a major earthquake. Seismologists have identified several seismic gaps in the Himalayan region, particularly in Uttarakhand and parts of northeast India, as areas where future large earthquakes are considered likely.

Correct Answer: B. Three tectonic plates meeting — Indian, Eurasian, and Burmese plates

Northeast India experiences exceptionally high seismicity because it lies at the junction of three tectonic plates — the Indian plate, the Eurasian plate, and the Burmese plate. This triple junction creates complex tectonic stress patterns and multiple fault systems. The region has experienced several of India's largest earthquakes including the 1897 Shillong earthquake (8.1 magnitude) and the 1950 Assam earthquake (8.6 magnitude).

Correct Answer: D. Zone V

The Andaman and Nicobar Islands are in Seismic Zone V, the highest risk zone, because they lie along the Andaman-Sunda subduction zone. This is the same tectonic boundary that generated the 2004 Indian Ocean Tsunami. The islands are among the most seismically active territories of India, experiencing frequent earthquakes as the Indian-Australian plate subducts under the Burmese plate.