Tsunamis generally don't destroy ships in the open ocean because their immense wavelengths cause a gradual rise and fall of the water level, often unnoticed by those on board.
Understanding Tsunami Characteristics in Deep Water
While tsunamis are devastating near coastlines, their behavior in the deep ocean is quite different. This is primarily due to their incredibly long wavelengths.
- Wavelength: Tsunamis can have wavelengths of hundreds of kilometers. This means the distance between wave crests is enormous.
- Wave Height: In the open ocean, the height of a tsunami might only be a meter or less.
- Period: The period, which is the time between successive crests, can range from minutes to over an hour.
The Impact on Ships
Because of these characteristics, a ship in the open ocean experiences the following:
- Gradual Ascent and Descent: The exceptionally long wavelength means a ship rises and falls very slowly as the tsunami passes beneath it. The slope of the wave is so gentle that it's often imperceptible.
- Lack of Breaking Waves: Unlike wind-generated waves that break, tsunamis don't break in the deep ocean. They are long, low undulations of water.
- Minimal Turbulence: The lack of breaking and the long period prevent significant turbulence or sudden forces that could damage a ship.
Analogy
Imagine a very long, gentle ramp. If a boat is on that ramp, it will slowly rise and fall as it moves along the ramp, but it won't experience any sudden jolts or disruptive forces. A tsunami in the open ocean is similar to this ramp on a massive scale.
Why Tsunamis Are Dangerous Near the Coast
As a tsunami approaches the shore, the depth of the water decreases. This causes:
- Decreased Wavelength: The wavelength shortens significantly.
- Increased Wave Height: The energy of the wave is compressed into a smaller volume, causing the wave height to increase dramatically.
- Breaking Waves: Eventually, the wave becomes too steep and breaks, unleashing its destructive power on coastal areas.
Summary
In essence, tsunamis are imperceptible to ships in the deep ocean because of their immense wavelength and correspondingly gentle slopes. The ships simply ride the wave with a gradual rise and fall, unaware of the enormous energy passing beneath them. The danger arises only when the tsunami approaches shallow coastal waters.
