A tsunami is a series of ocean waves generated by the sudden displacement of a large volume of water, most commonly associated with powerful underwater earthquakes. While a standard tsunami can be immensely destructive upon reaching the shore, a megatsunami is a separate and far more extreme class of event. The term “mega” describes a wave event that exceeds the scale and resulting wave height of any typical, seismically-generated wave. This distinction is based on the magnitude of the wave and the fundamentally different geological processes that create it.
Defining the “Mega” Difference
A traditional tsunami is typically a trans-oceanic event, meaning the wave travels across vast distances in deep water, often unnoticed by ships. In the open ocean, the height of a seismic tsunami is usually shallow, sometimes only a few centimeters, but its wavelength can span hundreds of kilometers. The wave only achieves its destructive height through a process called shoaling, where the wave energy compresses as it enters shallower coastal waters. The maximum run-up height for even the most powerful seismic tsunamis rarely exceeds 30 meters (about 100 feet).
A megatsunami, by contrast, is characterized by its localized but immense wave height, which can reach hundreds of meters almost immediately at the source. This type of event is defined by the sheer volume and speed of the material falling into the water, generating an impulsive splash rather than a gradual uplift of the water column. While the term lacks a precise scientific definition, it is generally applied to events that produce a wave amplitude exceeding 100 meters (328 feet) near the source. These waves tend to dissipate energy rapidly, making them devastating to nearby coastlines but less likely to travel across entire ocean basins.
Primary Mechanisms of Formation
The immense and immediate water displacement required for a megatsunami is only achievable through non-seismic forces that cause a massive volume of material to fall into a water body. Massive coastal or submarine landslides are the most common mechanism for generating these extreme waves. A large, rapid rockfall from a steep slope into a confined body of water, like a fjord or bay, transfers a tremendous amount of kinetic energy directly to the water. The sudden impact forces the water to surge upward and outward, creating the colossal run-up heights that define the event.
Another generating mechanism involves the catastrophic failure of a volcano’s flank, where a significant portion of the mountain collapses into the sea. This event is a type of massive landslide, often triggered by internal pressure or gravitational instability. The rapid displacement of a large debris avalanche into the ocean can create waves that travel greater distances than a localized rockfall. The energy release is sufficient to displace water across a wide area, leading to widespread coastal devastation.
The ultimate, though currently theoretical, mechanism for a megatsunami is the impact of a large asteroid or comet into an ocean basin. Such a hyper-velocity impact would instantaneously vaporize and displace an enormous volume of water and rock, generating an initial wave that could reach over a kilometer in height. Geological evidence suggests that the impact 66 million years ago that formed the Chicxulub crater produced an initial wave estimated to be over 1.5 kilometers (nearly a mile) high in the deep water near the impact site. This demonstrates the upper limit of the “mega” scale.
Documented Historical Occurrences
The most definitive and widely studied example of a megatsunami in modern times occurred in Lituya Bay, Alaska, on July 9, 1958. A magnitude 7.8 earthquake along the nearby Fairweather Fault triggered a massive rockfall into the narrow fjord. Approximately 30.6 million cubic meters of rock plunged from an altitude of over 900 meters into the water of Gilbert Inlet.
The resulting wave surged up the opposite slope of the bay, achieving a record-breaking run-up height of 524 meters (1,719 feet). This is the highest wave run-up ever reliably recorded, leaving a clear trimline where all trees and soil were stripped from the mountainside. The wave traveled the length of the bay, carrying a fishing boat with eyewitnesses over the trees and into the outer bay. This event provided evidence that non-seismic, impulsive mass displacement is the cause of megatsunamis.
A prehistoric example of a massive landslide-generated event is the Storegga Slide, which occurred off the coast of Norway approximately 8,200 years ago. This submarine landslide involved the collapse of an estimated 3,500 cubic kilometers of sediment from the continental shelf. The resulting paleotsunami swept across the North Atlantic, with evidence suggesting waves reached run-up heights of 10 to 20 meters along the Norwegian coast. The Storegga event demonstrates the capacity of large-scale submarine landslides to generate powerful, trans-oceanic waves.