A tsunami is a powerful series of ocean waves generated by large-scale disturbances that displace a significant volume of water. These disturbances commonly include undersea earthquakes, but also encompass phenomena like large landslides or volcanic eruptions. Understanding these events involves delving into specific measurement criteria and the geological conditions that can produce such an extraordinary event.
Understanding Tsunami Magnitude
The “size” of a tsunami can be described in several ways, reflecting different aspects of its power and impact. One common metric is wave height, which measures the vertical distance from the normal sea level to the wave’s peak, either in the open ocean or closer to shore. In deep ocean, tsunami waves have small heights and long wavelengths, often passing unnoticed.
Another significant measurement is run-up height, which refers to the maximum vertical height above sea level that the tsunami water reaches on land. This measurement is taken by observing the highest point of debris or watermarks left by the wave as it inundates the coastline. The total energy released by a tsunami also contributes to its magnitude, though this is a more complex calculation often tied to seismic scales. Different events may be considered “largest” depending on which of these criteria is applied, with run-up height being particularly relevant for extreme, localized tsunamis.
The Record-Breaking Event
The world’s largest tsunami, based on recorded run-up height, occurred in Lituya Bay, Alaska, on July 9, 1958. This extraordinary event saw water surge to an astounding height of 1,720 feet (524 meters).
The immediate impact within the narrow fjord was severe. The wave washed out trees and cleared all vegetation from large areas of the surrounding slopes. Several fishing boats were in the bay at the time; some were sunk, resulting in casualties, while others rode the wave. This localized event altered the landscape and provided insights into the potential power of tsunamis.
The Science Behind the Giant Wave
The run-up height of the Lituya Bay tsunami was a direct consequence of a massive landslide triggered by an earthquake. On July 9, 1958, an earthquake with a moment magnitude between 7.8 and 8.3 struck along the Fairweather Fault. This seismic activity caused approximately 30 million cubic meters (40 million cubic yards) of rock and ice, equivalent to about 90 million tons, to detach from a mountainside. The mass plunged from an altitude of roughly 3,000 feet (914 meters) into the deep waters of Gilbert Inlet, an arm of Lituya Bay.
The unique geography of Lituya Bay played a substantial role in amplifying the wave. The bay is a T-shaped fjord, a long, narrow inlet carved by glaciers, with steep, nearly vertical cliffs. When the volume of rock plunged into Gilbert Inlet, it created a splash wave. The confined shape of the fjord acted like a funnel, concentrating the wave’s energy and forcing it to surge upward as it encountered the opposite slope, leading to the record run-up height.