What if there had been a megatsunami and no one noticed? It almost happened last year when a rockfall occurred halfway up the east coast of Greenland. The wave it created was initially 200 meters high, but there was very little coverage of it – we must confess that IFLScience missed it too. In our defense, no one was hurt and scientists have only now reported on the most interesting part of the event: the creation of a second type of wave that vibrated the waters of a fjord for a week.

The tsunamis we are most familiar with are caused by volcanic eruptions or underwater earthquakes. Tsunamis can also be caused by rocks falling into the sea – the most dramatic was the impact that killed most dinosaurs, but they are usually caused by avalanches. This type of tsunami is surprisingly common – a tsunami in 1958 caused the highest waves ever recorded.

We rarely hear about events like this because they tend to occur where there is plenty of ice, such as near the poles. Six years after a 100-meter-high megatsunami devastated a small community in western Greenland, another one occurred on the opposite coast. But when Angela Carrillo-Ponce of the German Research Center for Geosciences GFZ and her colleagues examined the seismic data, they noticed something unusual.

From the initial rapid release of a large amount of energy, a signal could be observed, but it was accompanied by a second sound, called a very long period (VLP) signal. This, the team found, came from a seiche – a standing wave that causes a mass of water to vibrate. You may be familiar with seiches from the amazing discovery of a fossil bed created by a seiche on the day the dinosaurs went extinct, or from the home of the world’s rarest fish. You may also have created a seiche in your bathtub as a child by moving the water back and forth at a frequency that matched the vessel’s resonance, or you may have created a seiche in physics class when your teacher demonstrated standing waves.

The signal these researchers found, however, is on a different time scale. “The fact that the signal from a sloshing wave triggered by a rockfall in a remote area of ​​Greenland can be observed globally and for over a week is exciting, and as seismologists, this signal is what caught our attention the most,” Carrillo-Ponce said in a statement. “Analyzing the seismic signal can give us some answers about the processes involved and could even lead to improved monitoring of similar events in the future. If we had not studied this event seismically, we would not have known about the seiche that was created in the fjord system.”

Social media posts gave scientists the first clues that something big had happened on the east coast of Greenland, followed by reports of large waves from the (at the time unoccupied) Danish naval base on the nearby island of Ella.

The origin of the tsunami was located in a side valley of Dickson Fjord, where satellite images showed a rock formation 300-400 meters above the waterline that was once visible there and is no longer visible today. Researchers concluded that a landslide had swept a mixture of glacial ice and rock into the water and down the valley for about 1 mile until it reached the fjord proper.

No one has seen what happened to the water in the fjord. Seismic data suggest that the first tsunami was still about 60 metres high for most of the length of the bay, but quickly subsided when it reached the open sea.

However, some of the energy bounced back and forth within the 20-kilometer-long and 2-kilometer-wide fjord, creating a seiche. The seiche was initially about 2.6 meters high and bounced back and forth between the side walls with a period of just under two minutes, and decayed exceptionally slowly.

Icefalls have produced VLP signals that we have detected before, but the length of this signal surprised researchers, especially given the distance from the stations that were able to detect it. “It’s pretty impressive to see that we were able to use high-quality data from stations in Germany, Alaska and North America and that these records were strong enough for at least a week,” said Carrillo Ponce.

The cause of the landslide is unknown, but since many rocky but unstable parts of Greenland’s cliffs are held together by ice, it is plausible that climate change contributed. Notably, most events of this type have occurred in late summer.

A tsunami is considered “mega” if it is more than 100 meters high at its source, even if it quickly falls below this height again.

The study is published in open access in The Seismic Record.