A tsunami is a series of powerful waves caused by the displacement of a large volume of water, most commonly triggered by an underwater earthquake. The reported height of a tsunami can seem confusingly inconsistent, ranging from barely noticeable to hundreds of meters. This wide variation exists because a tsunami’s height changes drastically, transforming dramatically as it moves from the deep ocean to the shallow coastline.
The Physical Transformation of Tsunami Waves
A tsunami is often a subtle event in the open ocean, typically standing less than one meter tall. The water is deep, allowing the wave to travel at incredible speeds, sometimes over 800 kilometers per hour, similar to a jet airplane. The wave’s energy is spread across an extremely long wavelength, which can stretch for hundreds of kilometers, making it virtually imperceptible to ships passing overhead.
This profile changes fundamentally through a process called shoaling as the wave approaches the shore and the seafloor rises. The decreasing water depth creates friction with the wave’s base, forcing the wave to slow down significantly. As the leading edge of the wave slows, the massive volume of water following behind compresses the wave’s energy into a much smaller, taller column. This transformation converts the wave’s kinetic (motion) energy into potential (height) energy, causing the wave’s amplitude to grow enormously before reaching the coast.
Defining Tsunami Height Measurements
The reported height of a tsunami depends entirely on the specific measurement used by scientists, which often leads to confusion in public reports. One measurement is the Wave Height, which is the vertical distance between the crest (peak) and the preceding trough (lowest point) of the wave, measured at the shoreline. This figure is typically the height of the turbulent water mass as it first reaches the immediate coast.
The figure most frequently reported is the Run-up, which is the maximum vertical height above the normal sea level that the water reaches on the land. This is often measured post-event by finding water marks, debris lines, or vegetation damage on coastal structures and terrain. Because it includes the final surge of water traveling up a slope, the run-up is almost always the largest reported height number and provides the best measure of the total area affected.
The third measurement, Inundation Distance, is the horizontal extent the water travels inland from the coastline. While not a height measurement, it is directly related to the run-up height and the slope of the land. A lower run-up on a very flat coast may still result in a massive inundation distance, flooding areas miles inland with relatively shallow water.
Local Geography and Coastal Amplification
Even with the same wave approaching from the deep ocean, the final run-up height is heavily influenced by the local topography, a phenomenon known as coastal amplification. Coastal features such as V-shaped inlets, narrow bays, and harbors can funnel the incoming wave’s energy. This concentration of water forces the wave to pile up, leading to a much higher run-up in these specific, confined locations compared to nearby open beaches.
Conversely, a shallow, gently sloping coastline may experience a lower vertical run-up because the wave’s energy is spread out over a longer distance, allowing for greater inundation distance. Steep sea cliffs or coasts with natural barriers like elevated sand dunes can force the wave energy upward, creating a high run-up while simultaneously limiting the inland inundation distance. Offshore features, such as fringing reefs, can also help dissipate the energy before the wave reaches the shore.
Documenting Historical and Record Heights
Most tsunamis generated by large, subduction-zone earthquakes typically result in maximum run-up heights in the range of 10 to 30 meters near the source. For instance, the 2004 Indian Ocean tsunami, one of the deadliest ever recorded, produced waves that reached up to 50 meters in isolated spots near Sumatra. The 2011 Tohoku tsunami in Japan had a recorded maximum run-up of 40.5 meters on the coast of Miyako.
The absolute record holders are not typical tectonic tsunamis but are classified as mega-tsunamis, caused by massive landslides or rockfalls into a confined body of water. The highest wave ever documented occurred in Lituya Bay, Alaska, in 1958, following a magnitude 7.8 earthquake. The resulting rockfall into the narrow fjord caused a colossal surge of water that stripped trees and soil up to a maximum run-up height of 524 meters (1,720 feet) on the opposite mountainside.