Lituya Bay, a remote fjord along the Alaskan Panhandle, is the location of the largest wave ever reliably recorded in modern history. This extraordinary event, which occurred in 1958, was a megatsunami, a phenomenon distinct from tectonic tsunamis caused by plate movements. Megatsunamis are created by massive, non-seismic displacements, such as a large landslide or rockfall, plunging into water. The unique combination of geological features and seismic activity in Lituya Bay created the conditions for this disaster. Understanding the event requires examining the bay’s setting, the initiating earthquake, and the physics of the resulting rockfall.
The Unique Geography of Lituya Bay
Lituya Bay is a deep, narrow, T-shaped fjord, a valley carved by glacial action and flooded by the sea. This morphology significantly amplified the wave’s destructive power. The bay is approximately seven miles long, reaching depths over 700 feet in its center.
The T-shape is formed where the main body intersects with two smaller, glacially-carved inlets at its head, including Gilbert Inlet. The surrounding slopes are exceptionally steep, rising sharply to elevations of several thousand feet. This confining geography acts like a funnel, making the waters susceptible to wave amplification from any large-scale disturbance. The region is also situated directly on the Fairweather Fault, a major transform boundary running along the coast.
The 1958 Earthquake Trigger
The event began on the night of July 9, 1958, when a powerful earthquake struck the region at approximately 10:16 PM local time. The seismic event registered a moment magnitude between 7.8 and 8.3, the strongest earthquake in the area in over 50 years. Its epicenter was located near the Fairweather Range, roughly 13 miles southeast of the head of Lituya Bay.
The earthquake was not the direct cause of the wave, but served as the trigger. Intense ground motions lasted for several minutes, destabilizing the precarious, steep slopes surrounding Gilbert Inlet. This seismic energy initiated the movement of a massive, unstable rock mass high above the water.
The Massive Landslide Mechanism
The earthquake’s intense vibrations caused an enormous section of rock and ice to break away from the northeastern wall of Gilbert Inlet. This landslide involved approximately 90 million tons of material, falling almost vertically from a height of 3,000 feet into the confined waters below.
The kinetic energy of this mass striking the water created a massive, localized splash displacement, differentiating it from a typical tectonic tsunami caused by seafloor movement. The rockfall acted like a giant object impacting the water, generating an impulse wave that shot upward and outward. This rapid and forceful displacement instantly created a wave of unprecedented scale, splashing water high up the opposite slope.
Measuring the Mega-Wave
The direct result of the massive rockfall was a wave run-up that reached an astonishing height of 1,720 feet on the opposite side of Gilbert Inlet. Wave run-up is the maximum vertical height reached by the water above sea level on a slope. This measurement, the tallest ever recorded for a wave, was determined by physical evidence left on the mountainside.
The wave stripped all trees and soil from the slope, leaving a distinct boundary known as a trimline. This line of devastation, visible as a pale scar against the older forest, provided the precise elevation of the water’s maximum reach. The destructive wave continued to surge through the main body of the fjord, reaching heights of up to 700 feet in lower areas. Eyewitness accounts confirmed the wave’s power; two boats were lost, and one remarkably rode the wave.