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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Megatsunami | 2/10 | https://en.wikipedia.org/wiki/Megatsunami | reference | science, encyclopedia | 2026-05-05T07:35:36.838495+00:00 | kb-cron |
A megatsunami is a tsunami with an initial wave amplitude (height) measured in many tens or hundreds of metres. The term "megatsunami" has been defined by media and has no precise definition, although it is commonly taken to refer to tsunamis over 100 metres (328 ft) high. A megatsunami is a separate class of event from an ordinary tsunami and is caused by different physical mechanisms. Normal tsunamis result from displacement of the sea floor due to movements in the Earth's crust (plate tectonics). Powerful earthquakes may cause the sea floor to displace vertically on the order of tens of metres, which in turn displaces the water column above and leads to the formation of a tsunami. Ordinary tsunamis have a small wave height offshore and generally pass unnoticed at sea, forming only a slight swell on the order of 30 centimetres (12 in) above the normal sea surface. In deep water it is possible that a tsunami could pass beneath a ship without the crew of the vessel noticing. As it approaches land, the wave height of an ordinary tsunami increases dramatically as the sea floor slopes upward and the base of the wave pushes the water column above it upwards. Ordinary tsunamis, even those associated with the most powerful strike-slip earthquakes, typically do not reach heights in excess of 30 m (100 ft). By contrast, megatsunamis are caused by landslides and massive earthquakes that displace large volumes of water, resulting in waves that may exceed the height of an ordinary tsunami by tens or even hundreds of metres. Underwater earthquakes or volcanic eruptions do not normally generate megatsunamis, but landslides next to bodies of water resulting from earthquakes or volcanic eruptions can, since they cause a much larger amount of water displacement. If the landslide or impact occurs in a limited body of water, as happened in Lituya Bay (1958) and at the Vajont Dam (1963), then the water may be unable to disperse and one or more exceedingly large waves may result. Submarine landslides can pose a significant hazard when they cause a tsunami. Although a variety of different types of landslides can cause tsunami, all the resulting tsunami have similar features such as large run-ups close to the tsunami, but quicker attenuation compared to tsunami caused by earthquakes. An example of this was the 17 July 1998 Papua New Guinean landslide tsunami, in which waves up to 15 metres (49 ft) high struck a 20-kilometre (12.4-mile) section of the coast, killing 2,200 people, yet at greater distances the tsunami was not a major hazard. This is due to the comparatively small source area of most landslide tsunami (relative to the area affected by large earthquakes) which causes the generation of waves with shorter wavelengths. These waves are greatly affected by coastal amplification (which amplifies the local effect) and radial damping (which reduces the distal effect). The size of landslide-generated tsunamis depends both on the geological details of the landslide (such as its Froude number) and also on assumptions about the hydrodynamics of the model used to simulate tsunami generation, thus they have a large margin of uncertainty. Generally, landslide-induced tsunamis decay more quickly with distance than earthquake-induced tsunamis, as the former, often having a dipole structure at the source, tend to spread out radially and have a shorter wavelength (the rate at which a wave loses energy is inversely proportional to its wavelength, so the longer the wavelength of a wave, the more slowly it loses energy) while the latter disperses little as it propagates away perpendicularly to the source fault. Testing whether a given tsunami model is correct is complicated by the rarity of giant collapses. Recent findings show that the nature of a tsunami depends upon the volume, velocity, initial acceleration, length, and thickness of the landslide generating it. Volume and initial acceleration are the key factors which determine whether a landslide will form a tsunami. A sudden deceleration of the landslide may also result in larger waves. The length of the slide influences both the wavelength and the maximum wave height. Travel time or run-out distance of the slide also will influence the resulting tsunami wavelength. In most cases, submarine landslides are noticeably subcritical, that is, the Froude number (the ratio of slide speed to wave propagation) is significantly less than one. This suggests that the tsunami will move away from the wave-generating slide, preventing the buildup of the wave. Failures in shallow waters tend to produce larger tsunamis because the wave is more critical as the speed of propagation is less. Furthermore, shallower waters are generally closer to the coast, meaning that there is less radial damping by the time the tsunami reaches the shore. Conversely tsunamis triggered by earthquakes are more critical when the seabed displacement occurs in the deep ocean, as the first wave (which is less affected by depth) has a shorter wavelength and is enlarged when travelling from deeper to shallower waters. Determining a height range typical of megatsunamis is a complex and scientifically debated topic. This complexity is increased by the two different heights often reported for tsunamis – the height of the wave itself in open water and the height to which it surges when it encounters land. Depending upon the locale, this second height, the "run-up height," can be several times larger than the wave's height just before it reaches shore. While there is no minimum or average height classification for megatsunamis that the scientific community broadly accepts, the limited number of observed megatsunami events in recent history have all had run-up heights that exceeded 100 metres (300 ft). The megatsunami in Spirit Lake in Washington in the United States generated by the 1980 eruption of Mount St. Helens reached 260 metres (853 ft), while the tallest megatsunami ever recorded (in Lituya Bay in 1958) reached a run-up height of 520 metres (1,720 ft). It is also possible that much larger megatsunamis occurred in prehistory; researchers analyzing the geological structures left behind by prehistoric asteroid impacts have suggested that these events could have resulted in megatsunamis that exceeded 1,500 metres (4,900 ft) in height.