Tsunamis are powerful ocean waves that gain destructive force as they approach shorelines. Understanding their formation and how their size is measured provides insight into their potential impact. This article explores the tallest recorded tsunami, explains how tsunami height is quantified, describes the conditions for megatsunamis, and reviews other significant historical events.
The Lituya Bay Megatsunami
The tallest tsunami ever recorded occurred in Lituya Bay, Alaska, on July 9, 1958. This megatsunami produced a record run-up height of 524 meters (1,720 feet). It was caused by a magnitude 7.8 earthquake that triggered a landslide. Approximately 30 million cubic meters of rock plunged into the narrow bay, displacing immense water.
The initial wave scoured the mountainous slopes, stripping trees and vegetation up to the extreme run-up line. While the run-up height was immense, the actual wave height in the bay was estimated closer to 100 feet, showcasing the difference between wave height and run-up.
Measuring Tsunami Height
Measuring tsunami height involves distinct metrics. “Run-up height” refers to the maximum vertical distance a tsunami wave reaches above normal sea level on land. This measurement, often determined by surveying debris or water marks, is the figure for the Lituya Bay event. “Open ocean wave height” describes the vertical distance from trough to crest in the deep ocean, where it is typically only a few feet tall. As the tsunami approaches the coast and enters shallower water, its speed decreases, and its height increases, forming “coastal wave height.”
How Megatsunamis Form
Megatsunamis, characterized by their exceptionally high run-up, typically form under specific geological and geographical conditions that amplify wave energy. The primary cause of these colossal waves is the rapid displacement of a massive volume of material, such as a large landslide, into a body of water. These landslides can be subaerial, meaning they occur on land and plunge into water, or submarine, occurring entirely underwater.
When such a large mass of rock or debris falls into a confined body of water, like a narrow bay or fjord, the displaced water has nowhere to go but up, channeling and amplifying the wave’s energy. This mechanism differs from most tsunamis, which are generated by large-scale seafloor displacement from underwater earthquakes. The unique topography of Lituya Bay, with its steep slopes and enclosed shape, played a significant role in concentrating the wave’s energy and contributing to its record-breaking height.
Other Significant Tsunami Events
While the Lituya Bay event holds the record for the tallest run-up, other tsunamis have caused far more widespread devastation and loss of life due to their broad reach and impact on densely populated areas. The 2004 Indian Ocean tsunami, triggered by a magnitude 9.1 earthquake off the coast of Sumatra, Indonesia, caused immense destruction across 14 countries, resulting in an estimated 227,000 to 275,000 fatalities. This event highlighted the need for global early warning systems.
Another significant event was the 2011 Tohoku tsunami in Japan, caused by a magnitude 9.0 earthquake. This tsunami reached coastal areas within minutes, with wave heights up to 40 meters (130 feet) in some locations, leading to widespread destruction, thousands of deaths, and the Fukushima nuclear accident.
The 1883 eruption of Krakatoa volcano also generated massive tsunamis, which claimed approximately 36,000 lives in coastal towns of Java and Sumatra. These events, although not as tall in run-up as Lituya Bay, demonstrate the catastrophic power of tsunamis originating from different geological processes.