A tsunami is a series of ocean waves generated by the displacement of a large volume of water, typically caused by submarine earthquakes. A mega tsunami is a distinct and extremely rare event, representing an unusually energetic variant of this phenomenon. The term describes a wave created by an instantaneous, massive disturbance that produces an initial wave amplitude measured in the tens or hundreds of meters. Unlike the most common tsunamis, which are generated by tectonic plate shifts, a mega tsunami is characterized by a non-seismic origin and localized, but immense, wave height near the source.
Understanding the Scale of Mega Tsunamis
The distinction between a regular tsunami and a mega tsunami rests primarily on the mechanism of generation and the resulting wave amplitude near the source. Most seismically generated tsunamis are shallow-water waves with long wavelengths, which build height only as they approach the coast. Even the most powerful tsunamis caused by tectonic activity rarely produce a maximum inland water height, known as run-up, exceeding 30 meters.
Mega tsunamis, by contrast, are generated by a sudden, massive impact that effectively “splashes” the water column upwards. This rapid displacement creates an initial wave height that can be hundreds of meters tall, far exceeding the scale of typical oceanic tsunamis. The extreme energy transfer defines the “mega” classification, often proposed for events with an initial wave amplitude greater than 50 meters. This unique formation process means the event is highly localized, yet devastatingly powerful in its immediate vicinity.
Specific Triggers for Extreme Wave Events
The immense energy needed to create a mega tsunami requires specific, large-scale, non-seismic mechanisms. The most commonly cited cause is a massive landslide, either subaerial (above water) or submarine (underwater), where a colossal volume of rock or sediment rapidly enters the water body. This sudden transfer of mass and momentum into the water column is what generates the disproportionately large wave. The collapse of coastal cliffs or the failure of a steep submarine slope, often triggered by an earthquake, provides the necessary scale for this displacement.
Volcanic flank collapse is another known trigger, occurring when an unstable side of a large ocean island volcano slides into the sea. This mechanism involves the collapse of many cubic kilometers of material, as observed in the geological record of island chains like Hawaii. The impact of a large asteroid or meteorite striking an ocean represents a theoretical, albeit extremely rare, mechanism capable of causing the largest possible mega tsunamis. Events resulting from these mechanisms are typically short-lived and localized compared to the basin-wide reach of tectonic tsunamis.
How Height Translates to Coastal Destruction
The extreme height of a mega tsunami near its source translates into a catastrophic level of coastal destruction, driven by both energy and volume. The sheer mass of the water propelled by the generating event moves as a massive, turbulent surge rather than a typical breaking wave.
This enormous surge possesses such momentum that it can scour shorelines, stripping away soil and vegetation down to the underlying bedrock. The water’s energy allows it to travel to incredible elevations on opposing slopes before gravity pulls it back. This process involves the entire water column, resulting in the complete annihilation of coastal ecosystems and infrastructure across the affected area.
Documenting Past Mega Tsunami Events
Examples of mega tsunamis exist in both the modern era and the geological record, providing evidence for these extreme events. The most famous modern observation occurred in Lituya Bay, Alaska, on July 9, 1958. An earthquake triggered a massive rockfall of approximately 40 million cubic yards into the narrow inlet.
This instantaneous displacement created a colossal wave that washed up the opposing slope to a maximum run-up height of 524 meters (1,719 feet), the highest ever reliably measured. Prehistoric events include the Storegga Slide, a vast submarine landslide that occurred off the coast of Norway approximately 8,200 years ago. Evidence from this event shows it generated waves that affected coastlines across the North Atlantic. The 1883 eruption and collapse of the Krakatoa volcano generated a massive wave with a height of 41 meters.