A tsunami is a series of powerful ocean waves generated by the sudden, large-scale displacement of a massive volume of water, typically from an underwater earthquake or landslide. Unlike ordinary wind-driven waves, a tsunami involves the movement of the entire water column, from the surface to the seafloor, making it a “long wave.” This fast, nearly invisible wave in the open ocean transforms into a destructive force as it encounters the shallow environment of the coast. The physics governing this transition dictates the hazard level for coastal communities.
Deep Ocean Context
In the deep ocean, a tsunami travels at extraordinary speeds, often comparable to a jet airplane, reaching up to 500 miles per hour (800 km/h) in depths of around 15,000 feet (4,600 meters). This rapid movement is possible because the wave’s speed is directly related to the water depth. The wave height in these deep waters is surprisingly small, usually less than three feet (one meter). This minimal height, combined with an enormous wavelength that can stretch for hundreds of kilometers, makes the tsunami imperceptible to ships at sea. It passes beneath a vessel as a gentle rise and fall of the sea surface, carrying immense energy with very little loss over vast transoceanic distances.
Transformation in Shallow Water
The wave’s behavior changes dramatically as it moves out of the deep ocean and encounters the continental shelf, a process known as shoaling. As the water depth decreases, friction between the water and the seafloor causes the leading edge of the wave to slow down significantly. The wave’s speed is governed by the square root of the product of gravity and the water depth. For example, in water 300 feet deep, the wave speed drops to about 60 mph, and in water 30 feet deep, it slows to only 20 mph.
This reduction in speed is coupled with a sharp decrease in the wave’s wavelength. The principle of conservation of energy dictates that the total energy flux must remain nearly constant. Since the speed component of the energy is decreasing due to the shallower water, the wave’s height must increase dramatically to compensate. This compression of energy causes the wave to grow from an insignificant swell into a towering mass of water. A wave that was one meter high in the open sea can easily grow into a wall of water 10 to 30 meters high upon reaching the shore.
Coastal Warning Signs
A characteristic phenomenon that precedes the arrival of the main wave crest is the rapid recession of the sea, commonly referred to as the “drawback.” Since a tsunami is a series of waves, the trough, or lowest point, of the initial wave often reaches the shoreline first. When this trough arrives, the water is pulled back from the shore, exposing areas of the seafloor, reefs, and harbors that are normally submerged.
This drawback can span hundreds of meters and serves as a natural warning sign. The drawback is immediately followed by the arrival of the wave crest, which rushes inland. The time between the maximum recession of the water and the arrival of the crest can range from a few minutes to over ten minutes, depending on the wave’s period and the local seafloor topography.
The Final Inundation
When the wave crest reaches the shore, it typically appears not as a breaking, curling surf wave, but as a quickly rising, turbulent surge of water. This is because the tsunami’s long wavelength prevents it from breaking like a wind-driven wave. In many cases, it manifests as a “bore,” which is a vertical wall of water that rushes inland without breaking, resembling a rapidly advancing tide.
The most damaging consequence is the resulting “runup,” which is the maximum vertical height the water reaches above the normal sea level on the land. Runup can be extreme, sometimes exceeding 30 meters (100 feet) near the source, and is influenced by the local topography. The horizontal distance the water penetrates inland is called the inundation, and a large tsunami can flood low-lying coastal areas for more than a mile. These powerful surges carry immense force and debris, causing destruction as the water pushes inland and again as it drains back to the sea.