On September 16, 2023, seismographs around the world detected an anomalous seismic signal. It was not an earthquake, whose tremors last for a few seconds or, at most, a few minutes if the replicas are added. The oscillation was repeated every 90 seconds and remained for nine days. They immediately determined the origin in a fjord of eastern Greenland. The Danish Marina sent a mission to see what was happening and discovered that there had been a caused by the sliding of a large amount of rocks and ice. Now, an investigation based on satellite data and published in confirms that the wave remained wandering all that time until it dissipated.
“The event was anomalous for two reasons,” says the researcher at the University of Oxford (United Kingdom) and the first author of this work, Thomas Monahan. “First, the occurrence of the Megatsunami in September and October 2023 [hubo un segundo evento el 11 de octubre]caused by a landslide that hit the fjord. It reached 200 meters high! ”, Details in an email. The second anomaly was its duration.” What he did to this particularly strange event was that this tsunami wave was subsequently stabilized in a stationary wave (also known as Seiche) that remained in the fjord for more than a week, “he adds. Seiche comes from the late nineteenth century, when a Swiss hydrologist watched Alpines produced large oscillations, usually due to strong winds.
To reach this conclusion, the researchers used a tool that did not exist in 2022. It was that year when Spacex launched the Swot mission (acronym for oceanic topography and surface waters). It is a joint effort of the American NASA, the space agencies of the United Kingdom and Canada and the National D’Etudes Spatial Center in France. The satellite carries on board a radar interferometry system that measures the height in water height throughout the planet.
Using SWOT data, the researchers prepared elevation maps of the Dickson Fjordo, where it all started, at various times before and after the two tsunamis. These showed clear slopes throughout the canal, with differences of height of up to two meters. The marks on these maps were presented in opposite directions, which indicated that the water moved forward and backward through the Fjord, like a Seiche in the Alpine lakes. To reinforce their conclusion, they connected the tsunami in the fjord with the observation of small movements of the earth’s cortex measured thousands of kilometers away. This connection allowed them to rebuild the characteristics of the wave, even in periods not observed by the satellite. They also rebuilt the weather and tide conditions to rule out that their measurements were due to other events such as the wind.
“The earthquake was driven by a wave that was swollen in a narrow channel. This, in turn, was driven by a landslide. Water can splash for a long time, since there is not much to dissipate its energy, hence the long duration of the signal,” summarizes the professor of the University of Oxford and senior author of the study, Thomas Adcock. On the second event, that of October 11, which remained for a week, although with half magnitude, Adcock does not know if they are related, although he thinks that it is also due to a landslide.
This work is confirmed by the one published in the year following the event. That study, in which dozens of scientists intervened, from psychologists to mathematicians, already pointed out the origin of everything: “This type of tsunami is generated by the detachment of material, which can be a mixture of ice, the moraine of the glacier. This impacts a generally narrow basin, such as a fjord, a narrow bay, which suddenly evicts the entire water,” says the entire water, ”says the entire water, Mathematician of the University of Malaga Manuel J. Castro, expert in the study of geophysical fluids and participated in the modeling of the event in the Science study. “The wave is sweeping the fjord, and then, when that comes out in an open sea. It is not a tsunami like that of 2004, where we are talking about a 1,000 kilometers failure, which generates a wave that affects the entire Indian. Here it is something very local, which generates a devastating tsunami, but local,” he adds.
According to estimates, it was not that the collapse glacier and a large mass of ice fell into the sea. The reality was another and there enters the mathematical modeling of the Castro group. “We have evidence that the glacier is retiring, which generated an unstable area, which was what generated that avalanche,” says the Spanish mathematician. It is estimated that about 25 million cubic meters of rock and earth ended up in the water. “There you had ice that somehow held that material and has now retired, generating areas that are very unstable, slope areas that did not have that stability problem and as a consequence of the removal of ice are candidates to generate avalanches,” Castro completes.
On the ultimate cause, the thaw, the authors of the study point to climate change. “It is difficult to predict whether a seismic event similar to this will happen in the future due to the unique circumstances that produced it. However, the Arctic is experiencing rapid changes as a consequence of climate change and we are witnessing the appearance of new extreme events,” says Monahan. “The landslide that caused the megatsunami occurred by the collapse of a glacier in warming. There is no doubt that similar warming is occurring in many other Arctic glaciers,” he ends.
