Megatsuns

First scare ...

On December 26, 2004, as a result of an earthquake with a magnitude above 9, a Sumatra in the Indian Ocean was formed by a tsunami, which led to the deaths of more than 230 thousand people with victims mainly in Indonesia, Sri Lanka and other coastal regions of the Indian Ocean. In the history of modern civilization, this was the most catastrophic natural event, with the largest number of victims. In some regions, the wave height reached 30 m. The apocalyptic picture is a land flooded by water kilometers from the coast. In prehistoric times, such an event could easily serve as the basis for the myth of the biblical flood.

But what if the height of the wave would not be dozens, but hundreds of meters? (Tsunami with a wave height of more than 100 m is called Megatsuns.) As if delusional at first glance this question looked, events of this scale have repeatedly occurred in the past (see Fig.). Another evidence is the study published in the journal Science Advances on October 2, 2015 [1], which claims that 73 thousand years ago, a landslide to the ocean in the Atlantic, which caused megatsuns with a wave height of more than 270 m. The earlier calculations carried out for another, went on the volcanic island of the foil of the islands of the Green Cape in the Atlantic. The hypothetical megatsuns in the Atlantic Ocean with the center in the Canary Islands showed that as they move away from Canar, the wave height decreases, but still, when the populated coast of the Atlantic on both sides of the ocean reaches the wave, from units to several tens of meters [2]. Obviously, if such an event happened today, this would mean colossal human sacrifices, a global economic crisis and, possibly, the end of the modern Western, and, perhaps, eastern civilization. At least in the form that we are used to.

... then explain ...

The usual tsunami (approximately three quarters of cases) is caused by strong earthquakes leading to the shift of the ocean bottom. At the same time, a small wave with an amplitude of tens of centimeters is formed over the epicenter of the earthquake. However, this phenomenon is characterized by a wavelength of over 500 km, which allows it to move across the oceans at the speed of a reactive aircraft. When the wave runs into the shallow part of the shelf, its length is reduced, and the amplitude increases. At the same time, the height of the wave in the coastal zone depending on its initial parameters and the configuration of the coastal zone can reach tens of meters, but extremely rarely more than 40-50 m in the most favorable conditions.

Megatsuns are formed in a different mechanism. They need a large collapse or landslide in the water. At the same time, conditionally, the megatsuns can be divided into two types: when the landslide goes into (1) shallow pool or (2) deep -sea pool. In the first case, the water simply spilles to the other side. This was the only Megatsun, which happened on July 9, 1958, in the Gulf of Litu in Alaska. As a result of a large earthquake with a magnitude of about 8 in this bay, which has the shape of a letter T about 25 km long and an average width of 3 km, a mass of rocks and ice collapsed. This led to the fact that a wave spilled onto the opposite shore, which destroyed all living things up to 524 m. In other parts of the bay, the height of the wave varyed from 200 to 10 m. At the time of the tsunami there were several fishing boats, the fishermen from which the father and son - survived the catastrophe. Similarly, a wave of tsunami on Lake Chekhalis in Canada was formed when a landslide in it went on December 4, 2007. True, the maximum wave height reached “only” 38 m. There were no victims only because there were no people who wanted to relax on the lake in December. However, there was a landslide in the summer, the dead would be almost guaranteed.

In the case of a landslide gathering in a deep -sea basin, for example, from the volcanic island to the ocean, the maximum amplitude at the wave occurs at the place of landslide. Nature does not tolerate emptiness - figuratively speaking, the ocean seeks to fill the emerging cavity after the landslide It was according to this mechanism that the megatsuns were formed on the islands of the Green Cape in the described article [1]. In a similar way, the historical tsunami arose on November 18, 1929, when, after a strong earthquake, underwater landslide from the slope of Newfoundland came down. True, the maximum height of the wave that claimed 28 lives was “total” 27 m.

The Paleotsuns associated with the landslides are recorded in a number of lakes: on Lake Baikal in Russia [3], Tahot in the USA [4] and Geneva in Switzerland [5]. Such tsunami can occur in artificial reservoirs. For example, a landslide in Italy on October 9, 1963 went into the Vyont reservoir, a landslide that had formed through a dam and caused the death of almost two thousand people [6]. The case with the tsunami during the construction of the dam Wyont deserves a special mention, since the landslide gathering was predicted in advance. As a measure of prevention of disaster, the level of filling the reservoir was left 42 m below the upper mark of the dam. However, this was not enough. The case itself is notable for the fact that it occurred after the Megatsuns in the Bay of Lituya and the catastrophic consequences of the landslide gathering could be foreseen. However, the station engineers and the management company did not warn residents, and some of them even gathered on the dam to look at the tsunami. Truly murderous arrogance!

... and finally reassure

Studies of recent decades convincingly show that catastrophic events associated with the gathering of landslides in large and small water basins can be considered an infrequent, but ordinary geological phenomenon. Such events took place in the past and is guaranteed to happen in the future. A positive moment - in their rarity, negative - in the fact that landslide processes are activated when the level of water pools changes, when the slopes become unstable. And this is exactly what can be expected in connection with global climate warming. In addition, we ourselves influence the stability of the slopes, forming reservoirs and engaged in construction. In this sense, the case of the Viont disaster should serve as a vivid warning. It is possible that humanity will still see natural and technogenic disasters associated with the gatherings of landslides in water pools, but, most likely, will avoid the megatsuns destructive for all mankind in the foreseeable future. In any case, the megatsun phenomenon requires additional study both from the point of view of the discovery of such disasters in the past and in the sense of theoretical calculations. It seems that the ice in this direction has already moved.

1. Ramalho RS, et al., 2015. Hazard Potential of Volcanic Flank Collapse Raidsed by New Megatsunami Evidence. Science Advances, v. 1, no. 9, E1500456, DOI: 10.1126/Sciadv.1500456.

2. Ward SN, and Day, SJ, 2001. Cumbre Vieja Volcano - Potential Collapse and Tsunami at La Palma, Canary Islands. Geophyical Research letters, v. 28, p. 3397–3400, DOI: 10.1029/2001GL013110.

3. Ivanov AV, et al., In Press. Catastrophic Outburst and Tsunami Flooding of Lake Baikal: U-Pb Detrital Zircon Provenance Study of the Paleo-Manzurka MegaFlood Sedments. International Geology Review, DOI: 10.1080/00206814.2015.1064329.

4. Moore JG, et al., 2006. Tsunami-Generated Boulder Ridges in Lake Tahoe, California-Nevada. Geology, v. 34, p. 965-968, DOI: 10.1130/G22643A.1.

5. Kremer K., et al., 2012. Giant Lake Geneva Tsunami in Ad 563. Nature Geoscience, v. 5, p. 756-757, DOI: 10.1038/ngeo1618.

6. Barla G., and Paronuzzi, P., 2013. The 1963 Vajont Landslide: 50th Anniversary. Rock Mechanics and Rock Engineering, v. 46, 1267-1270, DOI: 10.1007/S00603-013-0483-7.