A tsunami is a series of waves created by the sudden displacement of a large volume of water. This displacement is typically caused by geologic events like underwater earthquakes, volcanic eruptions, or massive landslides. Unlike wind-generated waves, a tsunami involves the movement of the entire water column from the surface to the seafloor. In the deep ocean, these waves possess an extremely long wavelength and can travel at speeds comparable to a jet airliner, over 700 kilometers per hour. As a tsunami approaches the coast and enters shallower water, its speed drops drastically, but the wave energy compresses, causing the wave height to surge dramatically.
The Highest Measured Run-Up
The tallest wave height ever reliably measured occurred on July 9, 1958, in Lituya Bay, Alaska. This event produced a wave with a record-shattering “run-up” height of 524 meters (1,720 feet). The run-up measurement is the maximum vertical height the water reached on the adjacent slope above sea level, not the height of the wave itself while at sea.
The catastrophic event began when a powerful magnitude 7.8 to 8.3 earthquake struck the nearby Fairweather Fault. The shaking caused approximately 30 million cubic meters of rock and glacial material to detach from a cliff face. This immense mass plunged into the confined waters of Gilbert Inlet, creating a massive, localized surge that stripped the forest cover and soil from the steep slopes opposite the rockfall.
Evidence of the unprecedented height was left behind in the form of a clear trimline, marking where the wave had completely obliterated the vegetation. The enormous wave traveled the length of the 11-kilometer-long bay before dissipating into the Gulf of Alaska. Despite the extreme height, the event was highly localized, resulting in the loss of two fishing boats and five lives.
The Unique Mechanics of Megatsunamis
The Lituya Bay event is classified as a megatsunami, a term reserved for waves generated by mechanisms other than the vertical movement of the seafloor during a major earthquake. This classification is due to the localized, high-energy source: the massive subaerial rockfall. The landslide acted like a massive stone dropped into a bathtub, displacing an enormous volume of water instantaneously and causing a splash-up effect.
Typical tectonic tsunamis are caused by the vertical shift of the Earth’s crust beneath the ocean, displacing the entire water column and generating waves that travel across an ocean basin. These waves have a relatively small height in the deep ocean, gaining size only as they near a distant shoreline. In contrast, the megatsunami mechanism creates a wave born with tremendous initial height and energy but largely confined to the immediate area of the impact.
The narrow, deep, fjord-like shape of Lituya Bay contributed significantly to the record run-up height. The immense volume of rock fell into a confined inlet, which focused the displacement energy upward and onto the opposite slope. This unique combination of a massive, sudden rockfall and the bay’s confining geometry generated the colossal wave run-up.
Other Extreme Tectonic Events
While the Lituya Bay event holds the record for the tallest measured run-up, it was a hyper-localized disaster. The largest and most destructive tsunamis are generated by tectonic forces, impacting entire ocean basins and causing widespread devastation. These tectonic tsunamis, while having lower run-up heights, are far more significant in terms of global impact and loss of life.
The 2004 Indian Ocean Tsunami, triggered by a magnitude 9.1 to 9.3 megathrust earthquake off Sumatra, Indonesia, is a prime example. That event killed over 227,000 people across 14 countries, making it the deadliest tsunami in recorded history. Although maximum wave heights reached around 30 meters, the sheer geographic scale demonstrated a destructive power that dwarfed the localized Alaskan event.
Similarly, the 2011 Tohoku Tsunami in Japan, caused by a magnitude 9.0 offshore earthquake, generated waves up to 40 meters in height along the coast. This event resulted in nearly 20,000 deaths and missing persons, and is considered the costliest natural disaster in world history. The widespread impact of these ocean-spanning waves represents a more globally significant threat than the extreme but confined nature of a landslide-generated megatsunami.