A tsunami is a displacement wave, fundamentally different from a regular wind-driven wave or a tide. These massive movements of water are created by a sudden, violent shift in the seafloor, such as a large earthquake, underwater landslide, or volcanic eruption. The wave’s height changes dramatically from the open ocean to the shoreline, meaning there is no single answer to how tall tsunamis get. Their destructive power comes not just from height, but from the tremendous volume of water carried and the speed at which they travel over land.
Deep Ocean Tsunami Height
In the deep open ocean, where water depths can exceed 4,000 meters, a tsunami is nearly imperceptible. The wave height, measured from the undisturbed sea surface, is typically less than one meter, often only a few centimeters. This minimal height makes the wave undetectable by ships, and mariners rarely notice a tsunami passing beneath their hulls.
While the height is minimal, the wave possesses an immense wavelength, the distance between successive wave crests. This distance can span hundreds of kilometers, sometimes reaching over 500 kilometers. In the deep ocean, the wave can travel at speeds comparable to a jet airliner, often exceeding 800 kilometers per hour. Early warning systems use seafloor pressure sensors to detect the slight change in water pressure caused by the passing wave, rather than relying on visual observation.
The Mechanism of Coastal Run-up
The low, fast-moving wave transforms into a towering surge through a process known as shoaling as it approaches the coast. This transformation begins when the leading edge of the wave encounters the shallower water of the continental shelf. Because a tsunami’s speed is directly related to the water depth, the wave slows down significantly, often dropping to less than 80 kilometers per hour.
The rear of the wave, still traveling quickly in deeper water, continues to push the front forward. This action compresses the massive energy into a smaller area, causing the wavelength to shrink dramatically while forcing the water upward. The wave’s energy, which was primarily kinetic (motion) in the deep ocean, converts into potential energy (height) as it nears the shore.
The height of a tsunami is measured most accurately by the term run-up, which is the maximum vertical elevation the water reaches on the land above the normal sea level. Run-up is a more meaningful measurement than simple wave height at the shoreline because it accounts for the wave’s momentum carrying it up coastal slopes and cliffs. A wave that was less than a meter high in the deep ocean can easily shoal into a destructive surge reaching tens of meters in run-up height.
Geographic and Source Factors Affecting Height
The variability in tsunami run-up height, even for the same seismic event, is governed by local geography and the wave’s creation mechanism. The shape of the seafloor, known as bathymetry, plays a dominant role in determining wave amplification. Areas with a narrow continental shelf and a steep slope leading to the coast tend to force the wave energy upward more quickly, resulting in higher run-up. Conversely, a gradual, gently sloping seafloor can dissipate some energy, though the resulting wave may travel farther inland.
Coastal geometry, such as bays, harbors, and inlets, can act like a funnel, concentrating the wave’s energy. V-shaped harbors, for instance, can cause wave convergence, dramatically amplifying height as the water is squeezed into a smaller area. The source mechanism also dictates the initial energy. Waves generated by a massive earthquake cause a tectonic tsunami, while those caused by large, rapid landslides or volcanic collapse are termed megatsunamis.
Highest Recorded Tsunami Events
The most destructive tsunamis, like the 2004 Indian Ocean and 2011 Tohoku events, are tectonic tsunamis. They typically reach run-up heights of 15 to 30 meters along the most affected coastlines. These devastating waves cover vast distances and cause destruction across entire ocean basins.
The absolute highest wave ever recorded was a megatsunami that occurred in Lituya Bay, Alaska, on July 9, 1958. This event was triggered by an earthquake that caused approximately 30 million cubic meters of rock to slide into the narrow inlet. The rapid displacement created a tremendous splash, resulting in a maximum run-up height of 524 meters (1,720 feet) on the opposite mountainside. Although this wave was confined to the bay and did not travel across an entire ocean, it stands as the upper limit for tsunami height.