Which one of the following is TRUE regarding tsunamis?
1 They are started by fault-induced, horizontal shifts in the seafloor.
2 Their wave heights decrease and wavelengths increase as they move into shallower water.
3 They travel slower in the deep water of the open ocean than in the shallow coastal ocean.
4 They have very long wave length and small wave height in the open ocean.
The correct answer and explanation is :
The correct answer is:
4. They have very long wavelength and small wave height in the open ocean.
Explanation:
Tsunamis are large, powerful sea waves caused primarily by underwater disturbances, such as earthquakes, volcanic eruptions, or landslides. While they are sometimes referred to as “tidal waves,” they are not related to tides. Their behavior and characteristics vary depending on the depth of the water they pass through.
In the open ocean, tsunamis have very long wavelengths — often ranging from 100 to 200 kilometers (60 to 120 miles). However, their wave height (amplitude) is quite small, typically less than 1 meter (3 feet). Because of this, they often go unnoticed by ships at sea, despite traveling at incredible speeds — sometimes exceeding 800 km/h (500 mph), similar to the speed of a commercial airliner.
This behavior changes dramatically as the tsunami approaches shallower coastal waters. Due to a phenomenon known as wave shoaling, the wave slows down (due to friction with the seafloor), its wavelength shortens, and its height increases significantly, sometimes forming destructive waves over 10 meters (33 feet) high.
Let’s review the incorrect options:
- Option 1 (False): While tsunamis are caused by undersea earthquakes, they are generally associated with vertical displacement of the seafloor (such as uplift or subsidence), not horizontal shifts.
- Option 2 (False): In shallower water, tsunamis experience increased wave height and decreased wavelength, not the other way around.
- Option 3 (False): Tsunamis travel faster in deeper water and slower in shallower water due to changes in water depth affecting wave speed.
In conclusion, Option 4 is the only accurate statement. Tsunamis behave very differently in the open ocean compared to near-shore areas. Recognizing their small height and long wavelength in deep water is crucial for understanding early detection and tsunami warning systems.