Earthquakes and Tsunamis

The governing concept is subduction at an active plate boundary. Many ocean trenches form where one tectonic plate bends and descends beneath another. Friction can lock the plates temporarily, allowing strain to accumulate; sudden rupture or movement then produces a submarine earthquake. If the seafloor is displaced vertically, the overlying water is disturbed and tsunami waves may be generated. Thus option A gives the correct reason. Calm water does not prevent earthquakes, and option C contradicts the active movement at a trench. Option D is not the causal explanation: distance from the coast may affect impacts, but it does not create the hazard.

A tsunami is a series of long-period waves produced by the sudden displacement of a large volume of seawater. The displacement may result from an undersea earthquake, submarine landslide, volcanic eruption or, more rarely, another major disturbance. In deep water, a tsunami may have a small visible height but can travel rapidly across an ocean. Near the coast, decreasing depth slows the wave and concentrates its energy, causing the wave height and flooding potential to increase greatly. Ordinary wind waves are generated mainly by wind acting on the sea surface and usually affect a much shallower surface layer. A tsunami is not a neap tide, light-wind wave or colour. Hence B is correct.