Tsunamis are a series of ocean waves generated by large-scale disturbances that displace a significant volume of water. Unlike typical wind-driven waves, these events possess immense energy and can travel across entire ocean basins. While often associated with underwater earthquakes, some tsunamis arise from other dramatic geological shifts, creating extraordinary impacts. This article explores the record-setting event that produced the tallest tsunami ever observed, delves into its specific geological origins, and differentiates it from other types of tsunamis.
The Record-Breaking Event
The tallest tsunami ever documented occurred in Lituya Bay, Alaska, on July 9, 1958. This event, often referred to as a megatsunami, produced an astonishing wave run-up height of 1,720 feet (524 meters). This run-up height indicates the maximum vertical height the wave reached on land, far surpassing the height of structures like the Empire State Building. The immense power of this wave stripped away trees and vegetation across approximately two square miles (four square kilometers) of forest, leaving clear trimlines on the surrounding hillsides.
Eyewitness accounts provide a vivid perspective of its scale. Howard Ulrich and his seven-year-old son, aboard their fishing boat Edrie, were anchored in the bay and witnessed the terrifying wall of water. Another couple, Bill and Vivian Swanson on the Badger, described being lifted over a forested spit and looking down at trees below as their boat was carried by the wave. These accounts emphasize its localized yet unparalleled destructive force.
Unraveling the Cause
The Lituya Bay megatsunami was triggered by a massive landslide, unlike most tsunamis caused by underwater earthquakes. On July 9, 1958, a 7.8 magnitude earthquake along the Fairweather Fault caused approximately 40 million cubic yards (30.6 million cubic meters) of rock and ice to plunge from an altitude of about 3,000 feet (914 meters) into Gilbert Inlet, a narrow arm of Lituya Bay. This colossal rockfall displaced an enormous volume of water instantaneously, creating the record-breaking wave.
The unique topography of Lituya Bay played a significant role in amplifying the wave’s height. The bay is a narrow, deep fjord with steep walls and a U-shaped seafloor, which effectively channeled and concentrated the energy of the displaced water. This confined geometry caused the water to slosh back and forth like a giant bathtub, contributing to the extreme run-up height observed on the opposite shore of the inlet.
Understanding Different Tsunami Types
Most tsunamis, in contrast to the Lituya Bay event, are caused by large underwater earthquakes, particularly those occurring in subduction zones where tectonic plates collide. When these earthquakes cause the seafloor to suddenly deform and vertically displace, the overlying water column is set into motion, generating a series of waves. These seismic tsunamis then propagate outward from their source across the ocean.
Typical seismic tsunamis behave differently in deep versus shallow waters. In the open ocean, they possess very long wavelengths, often hundreds of kilometers, and are barely noticeable, sometimes only rising about a foot (30 centimeters) above the normal sea surface. As these waves approach coastal areas and enter shallower water, their speed decreases, and their height dramatically increases, sometimes reaching up to 100 feet (30 meters).