A megatsunami is an exceptionally large tsunami, often most extreme close to the source. Some originate when a huge landslide abruptly displaces water in a fjord, bay or lake. The name is descriptive: scientists do not use a single universally agreed wave-height threshold for every possible event.
One of the most striking examples is Lituya Bay, Alaska, in 1958, where water reached approximately 524 meters (1,720 feet) above sea level as it ran up a mountainside. That figure is the maximum run-up elevation, not a 524-meter-high wall of water traveling across the ocean.
What causes a megatsunami?
Large landslides entering water
Rock and ice entering a narrow bay at high speed can displace a huge volume of water. The local topography can amplify the resulting surge near the source, as happened at Lituya Bay.
Volcanic eruptions and flank collapses
Explosive submarine eruptions and large volcanic mass movements can generate dangerous tsunamis. The January 2022 Hunga Tonga-Hunga Haʻapai eruption produced a complex tsunami involving near-source water displacement and atmospheric pressure disturbances.
Other extreme scenarios
Very large impacts or exceptional slope collapses could theoretically create extreme waves. Observed historical events should be clearly separated from geological reconstructions and hypothetical scenarios.
Documented examples: Lituya Bay and Tonga
| Event | Cause | Reported value | Interpretation |
|---|---|---|---|
| Lituya Bay, Alaska (1958) | Earthquake-triggered landslide | Approx. 524 m | Maximum hillside run-up, not a uniform traveling wave |
| Hunga Tonga-Hunga Haʻapai (2022) | Submarine volcanic eruption | Around 20 m at several field survey sites; higher modeled near-source wave estimates | Do not confuse field run-up with modeled initial wave heights |
How did the 1958 Lituya Bay megatsunami happen?
On July 9, 1958, an earthquake triggered a giant rockslide into Gilbert Inlet. The impact sent water surging up the opposing slope, stripping vegetation to an elevation of approximately 524 meters. The giant run-up was highly localized; waves declined in height as they traveled through the bay. See the Alaska Earthquake Center’s historical analysis.
What happened in Tonga in 2022?
The January 15, 2022 Hunga Tonga-Hunga Haʻapai eruption generated destructive local tsunami waves and signals observed at great distances. Field surveys documented wave run-up around 19–20 meters at some sites. High-resolution simulations estimated substantially larger initial wave heights close to the volcano. Modeled source-wave height is not the same measurement as coastal run-up. Read the field study and Science Advances research.
Megatsunami vs. tsunami: what is the difference?
Tsunamis are sequences of waves caused by sudden displacement of water, often due to undersea earthquakes. A megatsunami describes an unusually extreme example, frequently involving localized landslide displacement. Even a tsunami not described as a megatsunami can be deadly.
Can a megatsunami cross an ocean?
It depends on the source. Very high landslide-generated waves in restricted bays often diminish rapidly away from the impact zone, while major earthquake-generated tsunamis can travel across entire ocean basins. Tonga also demonstrated the role of atmospheric pressure disturbances in a far-reaching tsunami.
Safety: what should you do?
If you are near the sea and feel long or strong earthquake shaking, see an unusual sea-level change or hear a loud ocean roar, immediately move inland and uphill without waiting for an official warning. Follow local evacuation advice. Learn how tsunamis form, check tsunamis today, and see recent earthquakes.
Frequently asked questions
What is the tallest megatsunami ever recorded?
The 1958 Lituya Bay event produced the highest documented run-up, approximately 524 meters.
Did Tonga experience a megatsunami in 2022?
Yes. Scientific publications describe the 2022 eruption-generated near-field tsunami as a megatsunami, but modeled heights and field measurements represent different quantities.
Can an earthquake trigger a megatsunami?
Yes. An earthquake can trigger a huge landslide into a body of water, as it did at Lituya Bay.
