Tsunamis are one of nature’s most destructive forces, generating a series of powerful ocean waves caused by the rapid, large-scale displacement of water. These events are typically triggered by underwater earthquakes, but they can also result from volcanic activity, meteor impacts, or massive landslides. Understanding the true size of these phenomena requires a precise method of measurement that captures the maximum impact of the water as it crashes ashore.
Defining Tsunami Size
The size of a tsunami is measured using two distinct but related concepts: amplitude and run-up height. Amplitude refers to the vertical distance between the crest of the wave and the normal sea level when the wave is in the open ocean. In deep water, a tsunami’s amplitude is often less than a meter and is barely noticeable to ships passing overhead.
The run-up height, however, is the measurement used to define the “biggest” tsunami. This value represents the maximum vertical height the water reaches above the sea level at the time of the event, measured at its furthest point inland. This measurement is taken by surveying the highest point where water-borne debris or the removal of vegetation, known as a trimline, is found on the shore. The localized coastal topography plays a significant role, as steep cliffs will force the water to surge much higher vertically than a gently sloping beach.
The Record Holder: Lituya Bay, Alaska
The record for the highest wave run-up ever documented belongs to an event that occurred on July 9, 1958, in Lituya Bay, Alaska. An earthquake with a magnitude estimated between 7.8 and 8.3 struck along the Fairweather Fault, triggering a massive landslide. The resulting wave washed out trees to a maximum elevation of 524 meters, or approximately 1,719 feet, on the opposite side of the bay’s Gilbert Inlet.
This colossal wave left a clear trimline where the forest was completely stripped away. The event was highly localized but profoundly destructive within the narrow, T-shaped fjord. Several fishing boats were in the bay at the time; two people died when their boat was sunk by the wave, while another boat was carried high over the trees but its occupants survived.
Mechanisms for Extreme Height
The event at Lituya Bay was not a standard tectonic tsunami but an example of a “megatsunami,” which is caused by a different physical mechanism. Typical tsunamis are generated by the vertical movement of the seafloor during a major earthquake, which displaces the entire water column above it. This creates a wave that travels across entire ocean basins, generally producing run-up heights of no more than 30 meters, even in powerful events.
The Lituya Bay megatsunami was caused by the sudden, massive displacement of material into the water. The earthquake triggered an immense rockfall of about 30 million cubic meters of rock plunging from nearly 914 meters into the confined waters of Gilbert Inlet. This violent impact acted like a splash, forcing the water straight up the steep slope of the opposite fjord wall. The unique, deep, and narrow geometry of the glacial fjord amplified the initial splash, converting the energy of the falling rock into an unprecedented vertical surge.
Notable Tectonic Tsunami Events
While the Lituya Bay event holds the record for the highest vertical run-up, the most destructive and widespread tsunamis are typically those caused by tectonic plate movement. These events, though featuring much lower run-up heights, affect vast geographic areas. The 2004 Indian Ocean tsunami, for example, was triggered by a magnitude 9.1 to 9.3 earthquake off the coast of Sumatra. The waves traveled across the Indian Ocean, causing devastating effects in 14 countries and killing an estimated 220,000 people.
Similarly, the 2011 Great East Japan tsunami, resulting from a magnitude 9.0 earthquake, generated waves that reached maximum run-up heights of over 40 meters in some areas. The damage was spread over hundreds of miles of coastline. These tectonic events are capable of crossing entire ocean basins and impacting multiple continents. This widespread nature makes them vastly more devastating in terms of human and economic cost than the highly localized megatsunami at Lituya Bay.