Tsunamis are a series of waves generated by a large, rapid displacement of water, most commonly from an underwater earthquake. The height of a tsunami is frequently misunderstood because the wave appears vastly different depending on its location. Its destructive potential is not found in the open ocean but in the dramatic transformation it undergoes as it approaches a coastline. The actual height a tsunami reaches when it hits land is highly variable, depending on a complex interplay of physics and local geography.
Tsunami Wave Characteristics in the Open Ocean
A tsunami wave traveling across the deep ocean behaves in a counter-intuitive manner compared to the towering image often associated with it. In water depths greater than 6,000 feet, the wave height is typically very small, often less than three feet, and sometimes only a few inches above the normal sea surface. This low height makes a tsunami virtually undetectable by ships at sea.
The wave’s energy is distributed throughout the entire water column, from the surface down to the seafloor, unlike wind-generated waves that only affect the upper layer. This massive water movement occurs at tremendous speeds, often reaching 500 to 550 miles per hour, similar to the speed of a jet airplane. While traveling this fast, the tsunami possesses an extremely long wavelength, the distance between successive wave crests, which can span between 60 and 300 miles.
The Mechanism of Wave Shoaling and Amplification
The seemingly harmless wave in the deep ocean undergoes a profound change as it nears the shore through a physical process known as shoaling. As the wave moves into shallow water, friction with the rising seafloor begins to slow its immense speed; the shallower the water, the slower the wave travels.
Since the total energy carried by the wave must be conserved, the reduction in speed forces the massive volume of water to compress. This compression causes the wavelength to dramatically shorten. The energy is converted from kinetic energy (speed) into potential energy (height), causing the water mass to pile up vertically, leading to amplification. For example, a wave moving over 500 miles per hour in the deep ocean may slow to only 20 miles per hour near the coast, with a corresponding surge in height.
Coastal Wave Height and Maximum Run-Up
The final, destructive measurement of a tsunami at the coast requires a distinction between the wave height and the maximum run-up. Wave height is the vertical distance from the trough to the crest of the wave at the shoreline. Maximum run-up is the highest vertical elevation above sea level that the water penetrates inland.
For most destructive tsunamis generated by distant earthquakes, the wave height at the shore typically ranges from 10 to 50 feet. The local topography of the harbor, bay, and seafloor acts as the final amplifier, focusing the water’s energy and determining the extreme local height experienced. Extreme run-up events can be significantly higher, often reaching over 100 feet in tsunamis generated close to the source, such as the 1958 landslide event in Lituya Bay, Alaska, which produced a run-up that stripped trees and soil up to 1,722 feet above sea level.