18-year study reveals the origin of tsunamis

American scientists from the Georgia Institute of Technology have established the mechanism of formation of giant destructive tsunamis.

These huge waves occur in the open ocean and usually exist for less than a minute before disappearing. They can reach a height of 20 meters or more. They are currently observed all over the world. They can represent danger to navigation and coastal areas.

The researchers analyzed of high-frequency laser measurements at the Ekofisk oil platform in the central North Sea, which have been carried out over 18 years. The scientists concluded that giant tsunamis are not formed by chance.

Scientists analyzed nearly 27,500 half-hourly records of rough seas collected between 2003 and 2020. These recordings, recorded every half hour, show, how much the sea surface rises compared to mean sea level. They include data on major storms, such as the Andrea wave in 2007. 

Under normal conditions, waves are caused by wind, which blows over the surface of the sea. It’s like blowing on a cup of drink, causing small ripples on the surface In the sea, given enough time and space, this ripple can turn into large waves.

Researchers have focused on understanding, how waves suddenly become abnormally large and rise above neighboring smaller waves. One suggestion is that the process is based on modulation instability — a phenomenon described by complex mathematical models. When waves are compressed in a narrow channel, the theory of modulation instability describes their wave-like motion well. However, in open ocean conditions, this theory is no longer as effective. In open environments, such as the North Sea, waves can propagate freely in different directions.

Researchers can generate killer waves in a confined channel in the laboratory, where they obey modulation instability. However, without channel confinement, killer waves typically do not obey these laws of physics and do not form in the same way in the open sea. 

The scientists analyzed sea state data, using statistical methods to identify the patterns underlying tsunami occurrence. Their results demonstrated, that abnormally high waves are not formed as a result of modulation instability, but through a process called constructive interference. 

Constructive interference occurs when two or more waves merge into one. This effect is enhanced by the natural asymmetry of the waves, whose crests are usually sharper and steeper than flatter troughs. Tsunamis form when a large number of smaller waves line up and their steeper crests begin to overlap. These killer waves rise and fall in less than a minute, following a so-called quasi-deterministic pattern in space and time. This pattern is recognizable and repeatable, but with a degree of randomness.

In an ideal ocean, this randomness would be almost non-existent. This would allow tsunamis to reach almost infinite heights. But it would take an eternity to see one, such wave because so many waves would have to line up perfectly.

In the real ocean, as a wave gains height and energy, it cannot stay above a certain point of no return. The top of the wave overflows over the edge and disintegrates into foam, releasing the remaining energy. Constructive interference can occur with many types of waves. A general theory called quasi-determinism of waves, developed by oceanographer Paolo Boccotti, explains, how killer waves are formed both in the ocean and in other wave systems.

In a turbulent flow of water flowing through a confined channel, killer waves occur in the form of short-term rotating eddies. Meanwhile, Paolo Boccotti’s theory indicates, that oceanic tsunamis are not completely random and unpredictable. When a really big wave is formed, the surrounding waves follow a recognizable pattern formed by constructive interference. 

The researchers applied the theory of Boccotti to identify and describe these patterns in recordings of the rough North Sea. The giant waves, observed in these recordings, carry a certain signature and signature of a series of waves, that can help explain, how abnormally large wandering waves are formed. 

For example, on November 24, 2023, a powerful storm hit the North Sea. A camera on the «Ecofisk» platform captured a huge killer wave 17 meters high. Scientists used the theory of quasi-determinism and an artificial intelligence model to study the nature of these waves. 

The results of the analysis showed, that the formation of a giant wave was consistent with the theories of quasi-determinism and constructive interference. The big wave was formed from many others, which repeatedly overlapped Understanding how killer waves are formed can help engineers and designers build safer ships and offshore platforms, as well as better predict risks.

Sources: The Conversation; ScienceAlert