The destructive power of a tsunami often leads people to wonder about the largest wave ever recorded. Understanding the biggest examples requires looking beyond the typical image of a breaking ocean wave. Measuring the true height of a tsunami is a complex scientific endeavor that focuses not on its appearance in the open ocean, but on the maximum elevation it reaches when it collides with land. This approach reveals a record-shattering event that dwarfs the waves of even the most devastating seismic tsunamis.
Understanding Tsunami Measurement
Tsunamis are not measured like standard wind-generated waves with crests and troughs in the deep ocean, where they often travel unnoticed. In the deep sea, a powerful tsunami might only be a few centimeters to a meter high, yet it moves at the speed of a jet airliner. The wave’s destructive potential is measured only once it reaches the shore and is forced upward by the shallow seabed and coastal topography.
The standard scientific metric for gauging a tsunami’s intensity is the run-up height. This measurement is defined as the maximum vertical elevation the water reaches above the normal sea level at the time of the event. Post-event survey teams determine this height by identifying the furthest inland and highest points where the water left clear evidence, such as debris lines, scour marks, or the “trimline” where vegetation was stripped away.
The run-up height is the definitive figure used when comparing the scale of different tsunamis. This land-based measurement captures the vertical surge of water, which is sensitive to the local shoreline’s steepness. A steep cliff face or a narrow inlet can dramatically amplify the water’s momentum, resulting in a run-up height far exceeding the wave’s initial size.
The Record-Breaking Wave of Lituya Bay
The highest tsunami run-up ever documented occurred in a remote location: Lituya Bay, Alaska, on July 9, 1958. The wave reached an astounding maximum run-up height of 1,720 feet (524 meters). This figure, which is taller than the Empire State Building, was not a conventional wave but a massive, localized surge of water that splashed up the steep mountainside.
Scientists established this record-setting elevation by studying the trimline left on the opposite side of the bay from the source of the event. The powerful surge of water completely stripped the forest of trees and soil up to that staggering height, leaving a visible boundary between the devastated area and the untouched, older forest above it. This unique physical evidence allowed for a precise post-event measurement of the maximum water elevation.
The event, triggered by a magnitude 7.8 earthquake along the Fairweather Fault, occurred in a bay shaped like a narrow, deep fjord. While the earthquake itself was the catalyst, the record wave was generated by a colossal landslide. The displacement of water was so immense that it created a momentary, monumental surge, essentially a massive splash, rather than a propagating wave that traveled across the entire ocean.
The human impact of this record-breaker was surprisingly limited due to the bay’s isolated location; only a few fishing boats were anchored there. Eyewitness accounts described a sudden, violent motion of the water and an incomprehensible wall surging up the cliff face before the resulting wave sped down the length of the bay.
The Mechanics Behind Megatsunamis
The event at Lituya Bay is scientifically classified as a megatsunami, a term reserved for waves generated by mechanisms other than large-scale seismic seafloor displacement. These waves are typically caused by massive, rapid movements of material into the water, such as landslides, rockfalls, volcanic activity, or even meteorite impacts. The key characteristic of a megatsunami is its extremely localized, but proportionally enormous, run-up height.
The physics of the Lituya Bay event involved the sudden entry of approximately 30.6 million cubic meters of rock and glacial debris into Gilbert Inlet. This immense volume of mass, plunging from an elevation of about 3,000 feet, instantaneously displaced a proportionate volume of water. The narrow, deep, and steep-sided geometry of the fjord played a crucial role.
Because the water was contained within a confined basin, the energy from the rockfall had nowhere to dissipate horizontally like an open-ocean tsunami. Instead, the force was directed upward and outward, violently surging up the slope opposite the impact point. This rapid, focused displacement allowed the water to briefly reach an elevation of over 1,700 feet, far exceeding the scale of any earthquake-generated wave.
Comparing Landslide and Seismic Tsunami Scale
The Lituya Bay megatsunami represents the maximum vertical height a wave has ever reached, but it is fundamentally different from the widespread, ocean-crossing tsunamis that cause regional devastation. Tectonic tsunamis, like those generated by the 2004 Indian Ocean event or the 2011 Tohoku earthquake in Japan, are caused by the vertical movement of the seafloor during an earthquake. This movement lifts or drops the entire water column above it, creating a wave that propagates across vast ocean basins.
While seismic tsunamis can produce run-up heights of tens of meters—the 2011 Japan event reached around 40 meters in some areas—they rarely approach the localized vertical scale of a megatsunami. The critical difference is the affected area and total energy. Tectonic tsunamis carry vastly more total energy and can affect coastlines thousands of miles away, leading to catastrophic regional inundation and high death tolls.
Landslide-generated megatsunamis are short-wavelength, high-amplitude events that quickly lose energy once they leave the confined space of their origin. They are characterized by extreme run-up heights near the source, but their destructive power diminishes rapidly over distance. This contrast shows that the “biggest” tsunami, measured by maximum vertical run-up, is a localized event, while the most destructive tsunamis are those generated by seismic activity that threaten entire coastlines.