First global inventory of megatsunamis to know their impacts on global warming

This inventory, recently published in the scientific journal GeoHazards, reveals that ‘megatsunamis’ have been caused by massive landslides or rock avalanches of great magnitude, sometimes induced by large earthquakes, and details their location and geographical distribution, with a notable concentration in the glacial fjords of Alaska and Norway. Likewise, it warns about the potential increase in these extreme events in glacial areas, as a direct consequence of global warming. “This catalog represents a crucial tool to understand the connection between geological risks and climatic conditions and can help prevent risks,” says Mercedes Ferrer, researcher at IGME-CSIC and one of the two co-authors of the research.

‘Megatsunamis’ or giant waves are extreme events that impact cliff coasts in any region. They are conditioned by the presence of a body of water and the occurrence of a large landslide that, upon entering the body of water, causes a sudden vertical displacement of the water. Until now there was no universal and accepted definition of the term, as it was associated with prehistoric oceanic events linked to large asteroid impacts, gigantic collapses of volcanic islands or very destructive transoceanic earthquakes, such as those in Indonesia in 2004 and Japan in 2011. Unlike tsunamis, the waves of megatsunamis reach great heights, up to several hundred meters, when they impact the coasts.

The main problem when investigating these phenomena is the scarcity of well-documented geological and historical records. Although ‘megatsunamis’ must have been relatively frequent throughout Earth’s history, the vast majority were not observed or documented and have left no geological record.

Causes of megatsunamis

The earliest recorded event was triggered by the massive explosive eruption on the island of Thera (Santorini) in Greece, around 1600 BC. C. Of those that have been documented, most were generated by large subaerial landslides, in some cases by submarine landslides, and only a few are related to violent volcanic eruptions. The first are those that caused the highest waves, due to the violent entry of large masses of rock fallen from considerable heights into bays, fjords or lakes.

The study by IGME-CSIC and the UCM began with the analysis of the relationship between the maximum heights of the waves and the causes of historical tsunamis documented in the world—mainly caused by earthquakes (75%) and landslides (14%). — showing that those of seismic origin rarely generated waves of more than 20 meters. Above 30 meters, the vast majority were caused by subaerial or submarine landslides.

The research proposes a definition of ‘megatsunami’, based on an objective criterion of the maximum height reached by the waves, of 35 meters or more. This threshold distinguishes an exclusive group of 40 events, representing 1.5% of all documented historical tsunamis. The two existing global databases of historical tsunamis, supplemented by numerous publications, were used as main sources of information. In these bases there are about 2,800 recorded events, 700 of them with waves more than one meter high. For the new catalogue, each historical event was meticulously reviewed and researched, with references to original sources wherever possible, resulting in the correction of inaccurate data and erroneous ancient interpretations. In total, more than 300 publications have been consulted from 1888 to 2024.

There are only a few records from the 19th century, although the number increases in the 20th century. Only in the first 20 years of the 21st century have as many been documented as in the previous century, which is related to the advance of scientific and technological knowledge for the detection of these events. Of the 40 ‘megatsunamis’ included in the catalogue, some had extreme heights of over 100 metres. During the research for the catalogue, a new “megatsunami” has even been documented that took place before 1786 in Lituya Bay (Alaska), which the authors mention, but do not include in the list due to lack of information on the exact date. .

Precisely in Lituya, the largest ever recorded took place in 1958. It was caused by a rock avalanche triggered by a magnitude 7.8 earthquake that impacted the waters at the head of the bay. The fall of between 35 and 40 million cubic meters of rocks and ice into the sea, from a height of 600 meters, generated a wave of 524 meters, devastating 10 km2 of forest. The next recorded took place a few years later, in 1963, in the Vaiont reservoir in the Italian Alps, when a landslide that occurred while the infrastructure was being filled caused a wave of about 250 meters, destroying entire towns. The last one of similar dimensions occurred in 1980 in Spirit Lake (USA), reaching 260 meters and was a landslide associated with the eruption of the Saint Helena volcano.

Impact of global warming

The authors conclude that, currently, in glacial areas the combined effects of steep slopes, fractured rock masses and ice retreat play a crucial role in the occurrence of large landslides and avalanches. Global warming, which leads to increased temperatures, impacts the high and cold latitudes of both hemispheres, especially in the frozen regions, causing glacial retreat that generates instability of the slopes, which is why they point out that “it is probable “that global warming drives an increase in the frequency of large rock slides.”

After this retreat, they explain, the rock walls are exposed to erosion and tension changes, modifying their balance, which can cause large rock falls and avalanches that, when falling violently into the water in fjords and bays, can trigger enormous waves. “It is important to direct attention towards the prevention and mitigation of possible damaging effects in these regions prone to large landslides and associated ‘megatsunamis’,” they conclude.