A tsunami is a series of powerful ocean waves generated by large-scale disturbances that displace a significant volume of water. These waves can travel across entire ocean basins, often unnoticed in the open sea, but they gain immense height and destructive power as they approach coastal areas. The arrival of a tsunami can lead to widespread flooding, significant structural damage, and considerable loss of life.
The Lituya Bay Megatsunami
The highest tsunami ever recorded occurred on July 9, 1958, in Lituya Bay, a deep, T-shaped fjord in southeastern Alaska. This megatsunami was triggered by a magnitude 7.8 earthquake along the Fairweather Fault. The earthquake caused a massive rockslide from the mountainside above the bay, displacing an extraordinary volume of water.
Approximately 90 million tons of rock and ice plunged into Gilbert Inlet at the head of Lituya Bay. This sudden displacement generated an enormous splash wave that surged up the steep slopes on the opposite side. The wave reached an astonishing run-up height of 1,720 feet (524 meters) on the spur of land directly across from the rockslide.
The immense wave propagated down the bay, sweeping away trees and vegetation hundreds of feet up the shoreline. Evidence of its passage, a distinct trimline where forests were stripped, remains visible. Despite its extreme height in the immediate vicinity, the wave diminished significantly as it traveled out of the bay, still posing a danger to fishing vessels.
How Tsunami Heights Are Measured
Tsunami height can be described in several ways. “Wave height” refers to the vertical distance from the crest to the trough of a wave in the open ocean, where tsunamis are often only a few feet high. As a tsunami approaches shallower coastal waters, its speed decreases, and its height increases, sometimes forming a “bore,” a breaking wave front.
“Run-up height” is the most commonly cited measurement for a tsunami’s impact on land, representing the maximum vertical height a wave reaches above sea level on the shore. This measurement is taken from the highest point of water penetration inland. The Lituya Bay event is recognized for its record-breaking run-up height. This extreme run-up is a direct consequence of localized, massive water displacement in a confined space.
What Triggers a Tsunami
Tsunamis are generated by geological events that cause rapid, large-scale displacement of ocean water. The most common cause is underwater earthquakes, especially those in subduction zones where one tectonic plate slides beneath another. These quakes can uplift or depress the seafloor, displacing the overlying water column.
Large-scale landslides, whether submarine or entering the water from coastal areas, are another significant trigger. The Lituya Bay event is a prime example of a tsunami generated by a massive coastal rockslide. Volcanic eruptions can also cause tsunamis, particularly when a volcanic flank collapses into the ocean or when powerful underwater explosions or caldera collapses displace water.
Other Significant Tsunami Events
While Lituya Bay holds the record for the highest run-up, other tsunamis are notable for widespread devastation and vast reach across ocean basins. The 2004 Indian Ocean Tsunami, triggered by a massive earthquake off Sumatra, Indonesia, generated waves that traveled across the Indian Ocean, affecting 14 countries. It caused immense loss of life due to extensive coastal inundation.
The 2011 Tohoku Tsunami, following a magnitude 9.1 earthquake off Japan, produced waves over 130 feet (40 meters) high in some areas, causing widespread destruction along the Japanese coastline. This tsunami also led to the Fukushima Daiichi nuclear disaster. The 1883 Krakatoa eruption also generated tsunamis that devastated coastal regions around the Sunda Strait, causing tens of thousands of fatalities.