A tsunami is a large series of waves in a body of water caused by the rapid displacement of a massive volume of water. The term “mega tsunami” refers to an extremely rare, localized event characterized by wave run-up heights that far exceed those of typical seismic tsunamis. These extraordinary waves can reach heights of hundreds of meters near the source, demonstrating a distinct geological mechanism of generation. This article explores the unique phenomena required to generate such extreme waves, which are fundamentally different from more common tsunamis.
Distinguishing Mega Tsunamis from Typical Events
The vast majority of tsunamis are generated by large earthquakes occurring in subduction zones, where one tectonic plate slides beneath another. This action causes the seafloor to suddenly shift vertically, displacing the entire column of water above it over a very large area. These seismic tsunamis have immense volume and can travel across entire ocean basins, though their wave height in the deep ocean is often only a few tens of centimeters. The maximum run-up height for even the most powerful seismic tsunamis rarely exceeds 30 meters.
A mega tsunami, by contrast, is not generated by slow, large-area seafloor movement but by the near-instantaneous and massive displacement of material directly into a body of water. This rapid, concentrated injection of matter causes the water to be violently “splashed” outward and upward. This mechanism transfers energy with extreme efficiency, leading to initial wave heights and localized run-up measurements many times greater than those of a typical seismic event. The sheer speed and volume of the material’s entry creates the localized, catastrophic scale associated with the “mega” designation.
Massive Submarine and Subaerial Landslides
The most common geological phenomenon capable of creating a mega tsunami is the rapid failure of a massive terrestrial or submarine slope. This involves the sudden movement of rock, sediment, or ice into a body of water, generating a wave through direct momentum transfer. Submarine landslides, occurring entirely underwater, displace large volumes of water but often generate tsunamis that dissipate energy quickly as they travel.
Subaerial landslides, where a large mass of material slides from above sea level directly into the water, are significantly more efficient at generating extreme waves. The most famous example is the 1958 Lituya Bay event in Alaska, where an earthquake-triggered rockfall of approximately 40 million cubic meters plunged into a narrow inlet. This instantaneous displacement generated a wave that achieved a documented run-up height of 524 meters on the opposing slope.
This efficiency is maximized when the slide’s velocity is close to the wave’s propagation speed, a condition easily met by a rapidly accelerating subaerial slide. The immense potential energy of the falling material is converted directly into wave energy, creating a towering wave front immediately at the impact zone. While the energy of these waves tends to decay rapidly away from the source, they present an extreme, localized hazard unmatched by most other tsunami mechanisms.
Volcanic Flank Collapse
Another geological mechanism that can generate a mega tsunami is the catastrophic flank collapse of a volcanic island. Volcanic structures, especially those rising steeply from the seafloor, are inherently unstable due to internal pressure, hydrothermal alteration, and gravitational forces. Over time, large sections of the volcano’s side can become prone to detachment.
The failure occurs when an enormous volume of the volcano’s flank, potentially hundreds of cubic kilometers of material, detaches and slides rapidly into the ocean. This massive, sudden displacement creates a powerful initial wave. For example, the 1792 Mount Unzen event in Japan involved a flank collapse and landslide that generated a wave with run-up heights of 55 meters in the nearby bay.
The 1883 eruption and collapse of Krakatoa generated one of the largest tsunamis in recorded history, with waves reaching 41 meters high. While the potential for an ocean-crossing mega tsunami from such a collapse is debated, these events undoubtedly generate devastating, multi-story waves near the source. The rapid entry of a massive rock mass represents a rare but potent source for the largest waves.