In Lituya Bay (Alaska, USA) there were only three small boats with six people on board on the night of July 10, 1958, when a chaotic scene began: roars, shaking and gigantic waves. An 8.3 magnitude earthquake caused 90 million tons of ice and rocks to fall into the water, creating colossal waves. One of them reached 524 meters and is the largest ever recorded. The walls of water destroyed 10 square kilometers of forest, the few surrounding buildings, the bay lighthouse and two of those boats. It is the largest of the 31 megatsunamis recorded in human history.
One of the big differences with tsunamis is, logically, the height of the waves: from 40 meters they are considered megatsunamis. Another difference is found in the causes: earthquakes are the main origin of conventional tsunamis; In the case of megatsunamis, their main reasons also include large rock falls, land or underwater landslides, volcanic eruptions and the fall of large meteorites into the ocean. All this is detailed by Mercedes Ferrer, CSIC researcher at the Geological and Mining Institute of Spain, in her book Megatsunamisalong with the ways to identify them and which have been the most relevant.
While 58 tsunamis have been recorded in the last hundred years, there is historical and geological evidence of 31 megatsunamis in human history. Ferrer explains in his book that they are so rare because the phenomena that cause them are not common. The most common are large avalanches: “They originate from a large and sudden displacement of material that enters the sea or a lake,” the author clarifies.
Paradoxically, megatsunamis are not necessarily the most destructive nor are they always synonymous with devastation. Conventional tsunamis can travel thousands of kilometers and reach very distant shores. On the other hand, when megatsunamis are associated with landslides, they become very concentrated phenomena: they can be devastating locally, but the energy decreases when they move away from the origin. “They have occurred mainly in isolated areas where there are no human settlements, so in general they have not affected populations or infrastructure,” develops the geologist.
In their wake, these phenomena leave a large amount of material that they drag from the seabed, such as animal remains, mollusks and blocks of sand, which provide evidence that a colossal tsunami occurred there. Thanks to that, there are now records of these exceptional events.
The most devastating recent tsunamis, classified as megatsunamis due to the height of the waves, occurred in Indonesia in 2004 and Japan in 2011. In the first, the walls of water reached 50 meters in the Indian Ocean, after an underwater earthquake of magnitude 9.1 off the coast of Sumatra that caused more than 230,000 fatalities, making it the deadliest. The one in Japan was caused by another large earthquake of the same magnitude, generating waves of 40 meters, and killing more than 20,000 people.
Reservoirs, lakes and rivers
Five years after the largest wave in history, another megatsunami occurred and the only one attributed to humans. On October 9, 1963, in the Italian Alps, a rock mass of 270 million cubic meters fell from Mount Toc towards the Vaiont reservoir. Although the stability problems of the slope were known, no measures had been taken to address them. “The sudden landslide expelled about 50 million cubic meters of water and generated a wave of more than 260 meters on the opposite slope. The large mass of water flowed into the Piave River valley and destroyed the municipality of Longarone and other towns downstream. More than 2,000 people died and many others disappeared or were injured as a result of the direct effect of the wave,” says Ferrer.
Megatsunamis do not only occur in the open sea, as they can occur in freshwater bodies. In the Grijalva River, located in Chiapas (Mexico), a period of intense rains caused a sudden slide of 55 million cubic meters of rocks and earth on the right slope of this river, causing a wave of more than 50 meters that destroyed the population of Juan Grijalva. Although part of the population managed to flee when they heard the roar, more than 30 people lost their lives. “The town literally disappeared under the lash of the waves. The river rose more than 30 meters in a few days and more than a million hectares were flooded,” Ferrer described.
The prehistoric megatsunamis of the Canary Islands have been a case highly reviewed by the scientific community. Ferrer began his studies in this matter by studying these events that occurred in the last hundreds of thousands of years, caused by large landslides of important parts of the islands. Based on the geological remains that have been found, it is estimated that the waves reached heights of 180 and 270 meters in Teno and Isla Baja (Tenerife), respectively, 125 in Piedra Alta (Lanzarote) and 290 in the Agaete valley (Gran Canaria). .
When will the next one happen?
These phenomena are absolutely unpredictable. They can occur at any time or place, so it is not possible to know when the next tsunamis or megatsunamis will occur. However, there are currently effective tools to reduce its effects. Jorge Macías, professor at the University of Malaga, is a member of the EDANYA research group that works with statistics for the early detection of tsunamis. When an earthquake occurs, specialists perform a real-time simulation to estimate how long it will take for a wave to arrive. They also prepare action maps in case of disaster.
In the case of megatsunamis caused by landslides, “there is not much room for action,” Macías acknowledges. However, in the areas most prone to avalanches there are mechanisms to detect them. In December 2015, the UN General Assembly established November 5 as World Tsunami Awareness Day.
But not There is scientific evidence that supports that climatic conditions influence the generation of megatsunamis, it is a factor that cannot be ruled out. Of those 31 recorded, six have taken place in the last 23 years. The researcher warns of an unusual frequency of giant waves caused by avalanches recently. And she recognizes that there are now more mechanisms that allow these phenomena to be identified in remote areas, but climatic conditions can influence the generation of megatsunamis in areas where the ice is melting and causing large rock falls.
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