A massive tsunami in Greenland unleashed waves for a week

MADRID, (EUROPA PRESS). – On September 16, 2023, a gigantic wave hit a fjord on the east coast of Greenland. In some places, traces of the flood reached 200 meters high.

Researchers led by Angela Carrillo Ponce of the German Research Centre for Geosciences (GFZ) have analysed seismic signals from earthquake measuring stations around the world and discovered another unusual event: a standing wave triggered by the megatsunami was tossing back and forth in the narrow bay of the uninhabited Dickson Fjord for more than a week. The international team published their work in The Seismic Record.

The tsunami was triggered by a large landslide. Earthquake measuring stations up to 5,000 kilometres away recorded the shaking caused by the landslide as a short signal. However, there was also a very long period (VLP) signal that was recorded by seismometers for more than a week.

“The mere fact that the VLP signal of a wave moving back and forth caused by a landslide in a remote area of ​​Greenland can be observed worldwide and for more than a week is exciting. That is why this signal has been of such concern to us in seismology,” explains Angela Carrillo Ponce, a PhD student in the “Earthquake and Volcano Physics” section of the GFZ.

Fortunately, the researcher adds, there were no injuries. Only one military base, which was unmanned at the time of the tsunami, was devastated.

Analysis of seismic signals (shock waves that travel thousands of kilometres in the earth’s crust) showed that after the landslide, a so-called standing wave formed in the fjord. Initially, the parts of the flank that fell into the water triggered a giant wave that spread across the entire fjord to the island of Ella, located off the coast, more than 50 kilometres away. At the point where the rockfall entered the fjord, the maximum height was more than 200 metres, and on the coast, on average, 60 metres.

Parts of the wave apparently overflowed from the steep banks of the narrow fjord, forming a standing wave that moved back and forth for more than a week. However, this wave was only about a metre high.

Such standing waves and the resulting long-period signals are already known from research. These VLP signals are often associated with large calving events at the edges of glaciers.

“In our case, we also recorded a VLP signal,” says Carrillo Ponce, adding that “what was unusual was its long duration.”

What was especially impressive was that the data from seismic stations in Germany, Alaska and elsewhere in North America were of very good quality for analysis. A comparison with satellite images confirmed that the cause of the early seismic signals corresponded well with the strength and direction of the rockfall that triggered the megatsunami. In addition, the authors were able to model the slow decay and dominant oscillation period of the VLP signals.

This gives researchers hope that they will be able to detect and analyse other similar events in the past. It is obvious that the retreat of glaciers, which once filled entire valleys, and the thawing of permafrost are causing an increase in landslides. Climate change is accelerating the melting of glaciers and could therefore increase the risk of megatsunamis.