Tsunamis are a series of waves generated by the sudden, large-scale vertical displacement of a massive volume of water. This phenomenon is most commonly triggered by powerful undersea earthquakes, but it can also be caused by volcanic eruptions or significant submarine landslides. While tsunamis are one of nature’s most destructive forces near the coast, they are frequently completely unnoticeable to ships and people in the open ocean.
The Origin and Characteristics of Tsunamis
Tsunamis are fundamentally different from wind-created waves, which only disturb the ocean’s surface layer. Originating events, such as the sudden movement of a tectonic plate, displace the entire water column from the seabed to the surface. This massive energy transfer distinguishes a tsunami wave from a surface wave.
Because the wave involves the full depth of the ocean, it carries immense energy across the entire basin. Unlike wind waves, a tsunami has an extremely long wavelength, often ranging from 50 to over 300 miles between crests. This means the tsunami behaves as a “shallow-water wave,” even in the deepest parts of the ocean, because the depth is small compared to the wave’s length.
Deep-Ocean Behavior The Unseen Wave
The primary reason a ship will not notice a passing tsunami is how the wave’s energy is distributed over its enormous wavelength. In deep water, the vertical height, or amplitude, of the wave is very small. The wave crest is often less than 3 feet (1 meter) higher than the surrounding sea surface.
This small height spread across hundreds of miles means the rise and fall of the ocean surface is gradual and imperceptible to a vessel. A ship experiences a gentle, slow swell easily masked by ordinary wind-driven waves. Furthermore, the wave travels at extraordinary speeds in the deep ocean, often exceeding 500 miles per hour.
Because the wave is so long and fast, the time it takes for a single crest to pass a stationary point (the wave period) can be between 10 minutes and two hours. A vessel simply rides this long, low, fast-moving bulge of water without any dramatic pitch or roll. The energy is kinetic (expressed in speed) rather than potential (expressed in height), making the wave invisible and harmless far from shore.
The Shoaling Effect Transformation Near Shore
The wave only becomes a threat when it leaves the deep ocean and enters shallower coastal waters, a process known as shoaling. As the water depth decreases, friction between the moving water and the seabed causes the tsunami to slow down dramatically. The wave’s speed can drop from over 500 mph to less than 50 mph near the shore.
Because the wave’s energy must be conserved, this decrease in speed forces the wave’s length to compress, converting kinetic energy into potential energy. The long wavelength bunches up, and the wave’s height increases or “shoals” dramatically. A wave less than a meter high in the deep ocean can quickly grow to 30 feet (9 meters) or more as it approaches the coast.
This transformation creates the devastating wall of water that rushes inland, contrasting sharply with the nearly undetectable gentle swell that passed silently beneath ships miles offshore. This destructive nature is a direct result of energy conservation as the wave enters a changing environment.