Megatsunami

A megatsunami is an incredibly large wave created by a substantial and sudden displacement of material into a body of water.

Megatsunamis have different features from ordinary tsunamis. Ordinary tsunamis are caused by underwater tectonic activity (movement of the earth's plates) and therefore occur along plate boundaries and as a result of earthquakes and the subsequent rise or fall in the sea floor that displaces a volume of water. Ordinary tsunamis exhibit shallow waves in the deep waters of the open ocean that increase dramatically in height upon approaching land to a maximum run-up height of around in the cases of the most powerful earthquakes. By contrast, megatsunamis occur when a large amount of material suddenly falls into water or anywhere near water (such as via a landslide, meteor impact, or volcanic eruption). They can have extremely large initial wave heights in the hundreds of metres, far beyond the height of any ordinary tsunami. These giant wave heights occur because the water is "splashed" upwards and outwards by the displacement.

Examples of modern megatsunamis include the one associated with the 1883 eruption of Krakatoa (volcanic eruption), the 1958 Lituya Bay megatsunami (a landslide which caused an initial wave of ), and the 1963 Vajont Dam landslide (caused by human activity destabilizing sides of valley). Prehistoric examples include the Storegga Slide (landslide), and the Chicxulub, Chesapeake Bay, and Eltanin meteor impacts.

Overview

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 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 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 .

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 at the Vajont Dam (1963) and in Lituya Bay (1958) 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 where waves up to 15 m high impacted a 20 km 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 shorter wavelength waves. 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 has a shorter wavelength (the rate at which a wave loses energy is inversely proportional to its wavelength, in other words the longer the wavelength of a wave, the slower 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 is dependent upon volume, velocity, initial acceleration, length and thickness of the contributing landslide. 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 slide will also influence the resulting tsunami wavelength. In most cases the 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 here. 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 due to the fact that two different heights are 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 or so-called " run-up height" can be several times larger than the wave's height just before reaching shore. While there is currently no minimum or average height classification for megatsunamis that is broadly accepted by the scientific community, the limited number of observed megatsunami events in recent history have all had run-up heights that exceeded . The megatsunami in Spirit Lake, Washington, USA that was caused by the 1980 eruption of Mount St. Helens reached , while the tallest megatsunami ever recorded (Lituya Bay in 1958) reached a run-up height of . 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 in height.

Recognition of the concept of megatsunami

Before the 1950s, scientists had theorized that tsunamis orders of magnitude larger than those observed with earthquakes could have occurred as a result of ancient geological processes, but no concrete evidence of the existence of these "monster waves" had yet been gathered. Geologists searching for oil in Alaska in 1953 observed that in Lituya Bay, mature tree growth did not extend to the shoreline as it did in many other bays in the region. Rather, there was a band of younger trees closer to the shore. Forestry workers, glaciologists, and geographers call the boundary between these bands a trim line. Trees just above the trim line showed severe scarring on their seaward side, while those from below the trim line did not. This indicated that a large force had impacted all of the elder trees above the trim line, and presumably had killed off all the trees below it. Based on this evidence, the scientists hypothesized that there had been an unusually large wave or waves in the deep inlet. Because this is a recently deglaciated fjord with steep slopes and crossed by a major fault (the Fairweather Fault), one possibility was that this wave was a landslide-generated tsunami.

On 9 July 1958, a 7.8 strike-slip earthquake in southeast Alaska caused of rock and ice to drop into the deep water at the head of Lituya Bay. The block fell almost vertically and hit the water with sufficient force to create a wave that surged up the opposite side of the head of the bay to a height of , and was still many tens of metres high further down the bay when it carried eyewitnesses Howard Ulrich and his son Howard Jr. over the trees in their fishing boat. They were washed back into the bay and both survived.

Analysis of mechanism

The mechanism giving rise to megatsunamis was analysed for the Lituya Bay event in a study presented at the Tsunami Society in 1999;"The Mega-Tsunami of July 9, 1958 in Lituya Bay, Alaska: Analysis of Mechanism"
– George Pararas-Carayannis, Excerpts from Presentation at the Tsunami Symposium of Tsunami Society of 25–27 May 1999, in Honolulu, Hawaii, US this model was considerably developed and modified by a second study in 2010.

Although the earthquake which caused the megatsunami was considered very energetic, it was determined that it could not have been the sole contributor based on the measured height of the wave. Neither water drainage from a lake, nor a landslide, nor the force of the earthquake itself were sufficient to create a megatsunami of the size observed, although all of these may have been contributing factors.

Instead, the megatsunami was caused by a combination of events in quick succession. The primary event occurred in the form of a large and sudden impulsive impact when about 40 million cubic yards of rock several hundred metres above the bay was fractured by the earthquake, and fell "practically as a monolithic unit" down the almost-vertical slope and into the bay. The rockfall also caused air to be "dragged along" due to viscosity effects, which added to the volume of displacement, and further impacted the sediment on the floor of the bay, creating a large crater. The study concluded that:

A 2010 model that examined the amount of infill on the floor of the bay, which was many times larger than that of the rockfall alone, and also the energy and height of the waves, and the accounts given by eyewitnesses, concluded that there had been a "dual slide" involving a rockfall, which also triggered a release of 5 to 10 times its volume of sediment trapped by the adjacent Lituya Glacier, as an almost immediate and many times larger second slide, a ratio comparable with other events where this "dual slide" effect is known to have happened.

Examples

Prehistoric

* An astronomical object between wide traveling at per second struck the Earth 3.26 billion years ago east of what is now Johannesburg, South Africa, near South Africa's border with Eswatini, in what was then an Archean ocean that covered most of the planet, creating a crater about wide. The impact generated a megatsunami that probably extended to a depth of thousands of meters beneath the surface of the ocean and probably rose to the height of a skyscraper when it reached shorelines. The resultant event created the Barberton Greenstone Belt.
* The asteroid linked to the extinction of dinosaurs, which created the Chicxulub crater in the Yucatán Peninsula approximately 66 million years ago, would have caused a megatsunami over tall. The height of the tsunami was limited due to relatively shallow sea in the area of the impact; had the asteroid struck in the deep sea the megatsunami would have likely been tall. Among the mechanisms triggering megatsunamis were the direct impact, shockwaves, returning water in the crater with a new push outward and seismic waves with a magnitude up to ~11. A more recent simulation of the global effects of the Chicxulub megatsunami showed an initial wave height of , with later waves up to in height in the Gulf of Mexico, and up to in the North Atlantic and South Pacific; the discovery of mega-ripples in Louisiana via seismic imaging data, with average wavelengths of and average wave heights of , looks like to confirm it. David Shonting and Cathy Ezrailson propose an "Edgerton effect" mechanism generating the megatsunami, similar to a milk drop falling on water that triggers a crown-shape water column, with a comparable height to the Chicxulub impactor's, that means over for the initial seawater forced outward by the explosion and blast waves; then, its collapse triggers megatsunamis changing their height according to the different water depth, raising up to . Furthermore, the initial shock wave via impact triggered seismic waves producing giant landslides and slumping around the region (the largest known event deposits on Earth) with subsequent megatsunamis of various sizes, and seiches of in Tanis, away, part of a vast inland sea at the time and directly triggered via seismic shaking by the impact within a few minutes.
* During the Messinian (ca. 7.25–ca. 5.3 million years ago) various megatsunamis likely struck the coast of northern Chile.
* Reservoir-induced seismicity at the end of or shortly after the Zanclean Flood (ca. 5.33 million years ago), which rapidly filled the Mediterranean Basin with water from the Atlantic Ocean, created a megatsunami with a height of nearly which struck the coast of Spain near what is now Algeciras.
* A megatsunami affected the coast of south–central Chile in the Pliocene