A tsunami is a series of ocean waves caused by sudden disturbances on the seafloor, such as underwater earthquakes, landslides, or volcanic eruptions. Unlike wind-generated waves, tsunamis move through the entire water column. Upon reaching coastlines, these waves can cause widespread destruction.
Defining Tsunami Dimensions
Understanding a tsunami’s “size” involves several characteristics beyond simple height. In the deep ocean, a tsunami wave has a small amplitude, making it imperceptible to ships. These waves possess extremely long wavelengths, often hundreds of kilometers, and can travel across entire ocean basins at speeds exceeding 800 km/h (500 mph).
As a tsunami approaches shallower coastal waters, its speed decreases, and its wavelength shortens. This shoaling effect compresses the wave’s energy, increasing its height. The “run-up” defines the maximum vertical height a tsunami reaches on land above sea level. A large tsunami can flood low-lying coastal areas, with run-up heights potentially reaching tens of meters.
Geological Drivers of Tsunami Scale
A tsunami’s magnitude is influenced by geological and physical factors at its source. Large underwater earthquakes, magnitude 8.0 or greater, are the most common generators of destructive tsunamis. These events involve vertical displacement of the seafloor, often at subduction zones where one tectonic plate slides beneath another. This sudden movement pushes a large volume of water, initiating the tsunami.
The size of the seafloor rupture area and the amount of water displaced correlate with the tsunami’s initial energy. Deeper water depths at the source allow for more effective energy transfer to the water column, contributing to larger tsunamis. Local coastal topography and offshore bathymetry can amplify tsunami waves as they near land. Features like bays, harbors, and steep shorelines can cause waves to funnel and grow taller.
Historical Extremes in Tsunami Height
History provides examples of the scale tsunamis can achieve. The 1958 Lituya Bay event in Alaska holds the record for the highest localized run-up, reaching 524 meters (1,720 feet). This “mega-tsunami” was caused by an earthquake-triggered landslide plunging into the narrow inlet, generating an enormous splash wave that denuded trees to high elevations. This localized event is distinct from tectonic tsunamis, as direct material impact creates greater initial wave heights.
The 2004 Indian Ocean Tsunami, triggered by a magnitude 9.1 to 9.3 earthquake off Sumatra, caused widespread destruction across multiple coastlines. This event produced waves as high as 9 meters (30 feet) in some areas, reaching eastern India and Sri Lanka within two hours and affecting the Horn of Africa. The 2011 Tohoku Tsunami in Japan, caused by a magnitude 9.0 earthquake, generated run-up heights reaching 40 meters (130 feet) in Miyako city, inundating hundreds of square kilometers of coastline.
Physical Constraints on Tsunami Magnitude
Despite their power, there are physical limits to how large tsunamis can become. The maximum size of an earthquake or landslide on Earth is finite, which limits the initial energy input into a tsunami. The largest earthquake ever recorded was magnitude 9.5 in Chile in 1960, and scientific consensus suggests earthquakes of magnitude 10 or larger are not possible due to fault length limitations.
As tsunamis travel across ocean distances, they experience energy loss due to friction with the seafloor and scattering. The depth of the ocean also imposes a physical constraint, as the maximum potential wave height in the open ocean is limited by the available water depth. While coastal features can amplify waves, there are physical limits to how much run-up can occur based on wave energy and local topography.