A tsunami is a series of ocean waves generated by the sudden, large-scale displacement of water, typically caused by underwater earthquakes, landslides, or volcanic activity. These waves are vastly different from ordinary wind-generated surface waves because they affect the entire water column, from the surface down to the seafloor. A tsunami carries energy across entire ocean basins, traveling at speeds comparable to a jet airplane in the deep ocean. The height of a tsunami is highly variable and often goes unnoticed in deep water, but it can dramatically amplify as it approaches land. The maximum vertical reach is only revealed once the wave interacts with the shallow coastal environment.
Defining Tsunami Height and Run-Up
Understanding the maximum reach of a tsunami requires differentiating between two measurements: wave height and run-up. In the deep ocean, the wave height, or amplitude, is the vertical distance between the wave crest and the normal sea level. This height is usually quite small, often less than one meter, which is why ships at sea rarely notice a tsunami passing beneath them.
The height that people are generally concerned with, the maximum vertical reach, is known as the run-up. The run-up is the maximum height above sea level that the water reaches. This measurement is taken at the furthest point of inland penetration, often marked by the line of debris or the destruction of vegetation.
The run-up can be dramatically larger than the offshore wave height because the tsunami’s energy is compressed as it enters shallow water. For major tectonic events, the run-up is the measurement that quantifies the destructive power on the coast. The momentum of the entire water column causes the water to surge far up the slope of the land, even if the wave crest itself is only a few meters high at the shoreline.
Factors Determining Maximum Vertical Reach
The process that transforms a small deep-ocean wave into a towering surge on land is known as shoaling. As the tsunami wave approaches the shore and the seafloor depth decreases, the wave’s speed diminishes. The trailing portion of the wave continues to move at a faster speed in deeper water, causing the wave to compress and its energy to pile up. This compression results in a dramatic increase in the wave’s height, converting its tremendous speed into vertical energy.
The shape of the coastline and the surrounding topography also play a significant role in this amplification. Narrow, V-shaped bays or inlets can act like funnels, concentrating the water’s energy and forcing the run-up to much greater elevations than on a straight coastline.
The slope of the land where the wave makes landfall is another major factor determining the final run-up height. A steep coastal slope forces the surging water upward, resulting in a high run-up over a short distance inland. Conversely, a gently sloping coast or a broad, shallow continental shelf causes the energy to be dissipated over a longer distance, resulting in a lower run-up but greater horizontal inundation far inland.
Record Heights and Extreme Cases
For tsunamis generated by major tectonic plate shifts, the maximum run-up height typically ranges between 10 and 30 meters (approximately 33 to 98 feet) above sea level. The geological record also contains instances of far more extreme run-up heights, which are categorized separately as megatsunamis.
The record for the highest wave run-up ever documented was set in Lituya Bay, Alaska, on July 9, 1958. This event was caused by a massive rockfall plunging into the narrow fjord following an earthquake, rather than a typical tectonic tsunami. The colossal impact displaced the water, generating a localized wave that stripped trees and soil from the opposite slope up to a staggering maximum run-up height of 524 meters (1,719 feet).
Megatsunamis are distinct because they are caused by a sudden, massive displacement of material directly into the water, resulting in a localized splash-up effect. While the Lituya Bay event demonstrates the absolute maximum vertical reach a wave can achieve under specific, rare geographic conditions, the heights from typical major oceanic tsunamis are generally limited to the 30-meter range.