Last year, a landslide sent a wall of water 200 meters (650 feet) high rushing down a Greenland fjord. Thankfully, not only was no one hurt but hardly anyone noticed. Seismologists across the globe were confused by signals they had never seen before. Fortunately, a research station at the mouth of Dickson Fjord, while abandoned for the winter, had automatic recording devices that have revealed an abundance of information. Even a recent paper revealing a week-long standing wave left out the fact it made the entire planet vibrate to a detectable degree for nine days.

One would normally expect the creation of a tsunami twice as high as the Statue of Liberty to attract at least a little attention, but constrained to an uninhabited fjord in Greenland that wasn’t the case. The first time many people outside the area heard of the event was last month, almost a year after it happened, when a paper revealed that waves had bounced back and forth across the fjord for a week. These waves were perpendicular to the direction of the original tsunami, which flowed down the length of the fjord.

Although the standing wave was confined to the fjord, the Earth is one connected system, and the energy sloshing against the walls caused the crust to shake so hard it was picked up on seismometers on other continents.

“When we set out on this scientific adventure, everybody was puzzled and no one had the faintest idea what caused this signal,” said Dr Kristian Svennevig of the Geological Survey of Denmark and Greenland in a statement.  “All we knew was that it was somehow associated with the landslide. We only managed to solve this enigma through a huge interdisciplinary and international effort.”

Earthquake disaster movies usually feature the wild swings of seismometers as a major quake hits, followed by subsidence, and sometimes aftershocks an unpredictable time later. The signals picked up in September 2023 looked entirely different, with a peak every 92 seconds, reoccurring for days with little loss of strength.

At first seismologists worldwide had no idea what was responsible, but they soon realized the timing was too close to reports of a landslide in Greenland to be coincidence. From there they set out to work out the relationship, using satellite images to see what had changed at the location and eventually visiting the site.

The seismic consequences of standing waves like this, also known as seiches, have been seen before, but these have lasted less than an hour and only been detected within 30 kilometers (19 miles) of the event. Clearly this was built on a different scale, thanks to the tightness of the fjord, offering an unprecedented opportunity to understand the consequences and causes of such a large event.

The landslide itself is probably caused by climate change, Svennevig and co-authors believe. The extreme temperature swings between Greenland’s summer and winter mean that avalanches have always been common in spring as ice melts, but global heating is making that worse. 

“Climate change is shifting what is typical on Earth, and it can set unusual events into motion,” said Dr Alice Gabriel of the University of California, San Diego. Melting permafrost, reduced buttressing from ice, and changed precipitation patterns can all contribute to an event like this.

The authors calculated 25 million cubic meters (33 million cubic yards) of rock and ice tumbled down a 600-900 meter (2,000-3,000 foot) 45-degree slope into a side channel of the fjord. The volume is almost twice that of the world’s largest building, the Boeing Everett Factory. Satellite images reveal at least four previous rockslides and one subsequent one, showing how much warmer conditions are messing with the site.

Svennevig, Gabriel and more than 60 co-authors from 15 countries modeled the way the original tsunami would behave in the fjord’s distinctive shape, and saw how it turned into a seiche. This is a phenomenon produced when the speed of the wave and size of the tube in which it is traveling are right to create resonance, like a note blown on a musical instrument.

The research base on Ella island at the fjord’s mouth – which provided some of the data to understand the event – suffered $200,000 damage, and some archeological sites are suspected to have been destroyed. Tiny as that is in the context of other consequences of a warming planet, the authors noted it could have been much worse, as cruise ships often venture into the fjord’s mouth. A tsunami issuing from the fjord at the wrong time, even highly reduced in size by its passage, could have been the new iceberg for sinking giant ships.

The study is published in Science.