Earthquakes & Volcanism — World Geography

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Earthquakes & Volcanism

3 questions

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Q.1

WBCS Prelims 2013

Seismograph is :

A. An instrument for photographing clouds

B. An instrument for recording earthquake tremors

C. An instrument for measuring depths of water in the sea

D. A device for recording changes in the atmospheric pressure

Explanation

Why Correct: Seismographs detect and record ground vibrations from earthquakes, volcanic eruptions, or explosions.
Distractor Analysis: Photographing clouds is done by cameras or satellites, measuring sea depths uses echo sounders or bathymeters, and recording atmospheric pressure changes requires barometers.
Takeaway: The Richter scale measures earthquake magnitude, while the Mercalli intensity scale describes shaking effects.

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Q.2

WBCS Prelims 2010

Tsunamis are generated by

A. Under-sea plate movements

B. Undersea volcanism

C. Tidal change

D. None of the above

Explanation

Why Correct: Tsunamis primarily originate from sudden vertical displacement of the seafloor during underwater earthquakes at tectonic plate boundaries.
Distractor Analysis: Undersea volcanism can trigger tsunamis through landslides or caldera collapse, but this is less common than seismic activity. Tidal changes result from gravitational forces of the moon and sun, not tsunami generation.
Takeaway: The 2004 Indian Ocean tsunami was caused by a magnitude 9.1-9.3 megathrust earthquake along the Sunda Trench, with waves reaching up to 30 meters height.

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Q.3

WBCS Prelims 2008

The tsunami which devastated some of the coastal areas of India in December,2004 originated from

A. Subduction of Indian plate under the Myanmar plate

B. Subduction of Myanmar plate under the Indian plate

C. Subsidence and trench formation near Andaman Nicobar islands

D. Subduction of Myanmar plate under the Pacific plate near Hawaii

Explanation

Why Correct: The 2004 Indian Ocean tsunami originated from the subduction of the Indian Plate beneath the Burma Plate (Myanmar Plate) at the Sunda Trench, causing a magnitude 9.1–9.3 earthquake off Sumatra.
Distractor Analysis: Subduction of Myanmar plate under Indian plate reverses the actual tectonic movement. Subsidence and trench formation near Andaman Nicobar islands describes local effects, not the primary origin. Subduction of Myanmar plate under Pacific plate near Hawaii is geographically incorrect—Hawaii is in the central Pacific, unrelated to the 2004 event.
Takeaway: The 2004 tsunami was triggered by the second-largest earthquake ever recorded on a seismograph, with rupture along 1,300 km of the Sunda Trench.

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