The correct option is (a).
Explanation
A Tsunami (Japanese for "harbour wave") is a series of large ocean waves caused by the sudden displacement of a large volume of water, typically due to earthquakes, volcanic eruptions, or underwater landslides. The 2004 Indian Ocean Tsunami was triggered by a massive undersea megathrust earthquake.
Statement-wise Analysis:
- Statement 1 is Correct: The tsunami was triggered by a massive earthquake (magnitude 9.1–9.3) on December 26, 2004. The epicenter was located off the western coast of northern Sumatra, Indonesia.
- Statement 2 is Incorrect: The earthquake was caused by plate tectonics involving the Indian Plate and the Burma Plate (a microplate part of the Eurasian Plate). Specifically, the Indian Plate subducted beneath the Burma Plate. The statement incorrectly suggests the reverse (Burma plate subducting below the Indian plate).
- Statement 3 is Incorrect: The speed of a tsunami wave is directly related to the depth of the water ($speed = \sqrt{gravity \times depth}$). In the deep ocean, tsunami waves travel at high speeds (up to 700–900 km/h). However, as they approach the coast and the water becomes shallow, friction with the seabed causes the waves to slow down significantly, while their height increases (a process known as shoaling). Therefore, the speed is not constant.
Key Takeaway:
Tsunami waves behave differently in deep versus shallow water: in deep water, they have high speed and low amplitude (height); in shallow water, speed decreases while amplitude increases drastically. The 2004 event resulted from the subduction of the Indian Plate under the Burma Plate.