Tsunamis: New Mechanisms Revealed by the Hunga Tonga Explosion Last January

A team of scientists has identified the mechanism responsible for the extraordinary tsunami that spread across the world after the Tonga volcano erupted in January this year. The phenomenon is largely explained by the explosion that accompanied the event.

The eruption of Hunga Tonga-Hunga Ha’apai volcano on January 15, 2022 was the largest volcanic eruption of the 21st century, the last comparable event dating back to the 1883 eruption of Krakatoa. It was accompanied by the largest atmospheric explosion ever recorded, with a power 100 times that of the Hiroshima atomic bomb.

The event was the source of numerous atmospheric disturbances – sudden increases of 2 hPa were felt as far as Europe – and it also caused the formation of an exceptionally fast tsunami, the speed of which was slightly less than the speed of sound.

According to an international team including researchers from the Universities of Lisbon and Cardiff, this extraordinary tsunami is explained by the formation of gravity waves as the volcano explodes. Their findings were published in the journal last week Nature.

“The idea that tsunamis could be generated by atmospheric waves triggered by volcanic eruptions is not new, but this event was the first to be recorded using modern, dense instrumentation anywhere in the world, allowing us to finally to unravel the exact mechanism behind these unusual phenomena. ‘ says Usama Kadri of Cardiff University’s School of Mathematics, one of the co-authors of the study.

Cloud formations created by gravity waves downstream from Amsterdam Island, a volcanic island in the Indian Ocean [Nasa – Wikimedia Commons]

Gravitational waves are very long waves generated by volcanic eruptions, earthquakes, and other violent events. They move under the influence of gravity and can cross the oceans or the atmosphere at the speed of sound.

“In a sense, the Tonga eruption occurred in an ideal location, namely in shallow water, resulting in a large release of energy into the atmosphere near the water surface. The interaction of gravity waves with the ocean was inevitable.”

Using sea level, atmospheric and satellite data from around the world at the time of the eruption, the team showed that the tsunami was powered by gravitational waves triggered by the eruption and moving very rapidly through the atmosphere.

(A) Snapshots of atmospheric pressure change.  (B) Snapshots of sea surface elevation change Clear wave signals are marked with arrows.  (C) Snapshots of seafloor pressure change. [Wikimedia Commons - Nature/Science/Université de Lisbonne et de Cardiff](A) Snapshots of atmospheric pressure change. (B) Snapshots of sea surface elevation change Clear wave signals are marked with arrows. (C) Snapshots of seafloor pressure change. [Wikimedia Commons – Nature/Science/Université de Lisbonne et de Cardiff]

A comparison of atmospheric and sea level data showed a direct correlation between the first signs of gravity wave-induced air disturbance and the onset of a tsunami in many locations around the world.

The team explains that the energy transfer in the ocean was caused by a phenomenon known as nonlinear resonance, where gravitational waves interact with and amplify the tsunami they generate.

They estimate that the tsunami traveled 1.5 to 2.5 times faster than a volcano-triggered tsunami and traversed the Pacific, Atlantic and Indian Oceans in less than 20 hours at a speed of about 1,000 km/h crossed.

Location of meters and readings at events [Wikipedia - Nature/Science/Université de Lisbonne et de Cardiff]Location of meters and readings at events [Wikipedia – Nature/Science/Université de Lisbonne et de Cardiff]

“Because the tsunami was also caused by a fast-moving atmospheric source, it propagated directly into the Caribbean and across the Atlantic without having to travel around the South American land mass like a tsunami. This explains why the Tonga tsunami reached the Atlantic coast almost 10 hours before what was expected for a “normal” tsunami,” adds Dr. Ramalho, another co-author of the study, added.

“The Tonga tsunami gave us a unique opportunity to study the physical mechanism of formation and amplification of global tsunamis through resonance with gravity waves. These resulting new insights should lead to improved real-time warning systems.

Philippe Jeanneret with the journals Science and Nature

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