Suzanne's Mom's Blog

Photo: Soren Ryesgaard.
A boat looks tiny cruising by the rockslide gully of a destroyed glacier in Greenland recently. Rockslide plus global warming equals a surprise.

I don’t remember reading about this mysterious rumble at the time, but I’m always interested when scientists are stumped. I like how they approach figuring out a new phenomenon. Here’s hoping that science will continue to guide us the future. For one thing, unlike business or politics, it is usually collaborative, as seen in this story.

Kasha Patel wrote at the Washington Post, “The strange rumble was detected mid-September last year. An odd seismic signal appeared at scientific stations around the globe, but it didn’t look like the busy squiggles of an earthquake. A day passed, and the slow tremor still reverberated. When it continued for a third day, scientists worldwide began assembling to discuss what was causing the grumble in the ground.

“Some initially thought the seismic instruments recording the signal were broken, but that was quickly nixed. Maybe it was a new volcano emerging before their eyes, others said. …

“ ‘No one had ever seen this. We have nothing to compare it with,’ said Kristian Svennevig, a geologist at the Geological Survey of Denmark and Greenland.

“Nine days later, the vibrations greatly dissipated. But the mystery of the USO [Unidentified Seismic Object] lasted much longer. A year later, the puzzle has been solved, according to a study published in the journal Science [in September 2024]. It took about 70 people from 15 different countries and more than 8,000 exchanged messages (long enough for a 900-page detective novel) to crack the case.

“The short answer: A mega-tsunami created waves that sloshed back and forth in a fjord in Greenland, creating vibrations that traveled around the world.

“The long answer begins in the atmosphere. As greenhouse gas concentrations increase due to climate change, those heat-trapping gases accelerate ice melt particularly around Earth’s poles. On Sept. 16 last year, that extra heat thinned a glacier in eastern Greenland over time so much that it could no longer support the mountain rock above it.

“A 500-foot-thick piece of metamorphic rock, about a third of a mile wide and long, fell and triggered a massive landslide. Rock and ice, enough to fill 10,000 Olympic-size swimming pools, let loose as fast as 47 meters per second and ran for more than a mile. The avalanche plunged into the Dickson Fjord, triggering a 650-foot-high tsunami — one of the highest seen in recent history.

“Farther away from the fjord, tsunami waves reaching 13 feet high damaged an unoccupied research station and destroyed cultural and archaeological heritage sites, including an old trapper hut that had never been affected by tsunamis during its century-old history. …

“Meanwhile, in the fjord, the mega-tsunami wave traveled back and forth in the inlet and created a standing wave called a seiche. We often see small-scale seiches — this rhythmic oscillation in water — in a swimming pool or bathtub. This tsunami source was so energetic that the seiche radiated seismic waves globally, shaking the planet for nine days before it petered out.

“Members of the Danish military sailed into the fjord only days after the event to collect drone imagery of the collapsed mountain face and glacier front and scars left by the tsunami.

“Of course, Svennevig and many of his close colleagues didn’t fully know of the connection between the tsunamigenic landslide and the seiche as the events unfolded, which is detailed in the study.

“At the time, they were scratching their heads about the data at the seismograph stations. The seiche appeared as a single slow vibration, like a monotonous-sounding hum, as opposed to the frantic lines of a typical earthquake reading. The wave peaked every 92 seconds, which is slow compared with an earthquake. …

“ ‘When we started doing this research, nobody had any ideas about what was the root cause,’ said Carl Ebeling, a co-author of the study and a seismologist at the Scripps Institution of Oceanography at the University of California at San Diego. ‘Even for a large landslide under normal circumstances, it would be hard to see that on a global scale, so something special is going on here.’

“As some scientists investigated the peculiar seismic data, another group of authorities and researchers had heard of a large tsunami in a remote fjord in eastern Greenland. The two teams, among others, joined forces, quickly growing into a 24/7 international collaboration via a messaging system. The group brought a variety of local field data and remote, global-scale observations.

“ ‘We knew there was a landslide and a tsunami. You could pull the seismic signal of those,’ Svennevig said. ‘But then there was this other seismic signal that continued for nine days, and they were taken from roughly the same area, so they must be associated somehow.’ …

“The breakthrough came when they received new bathymetry data of the fjord, similar to a topographic map, from the Danish military, which allowed them to better map the seabed in the computer models. Once incorporated, the team used an unprecedentedly high resolution model to show how the landslide direction, along with the uniquely narrow and bendy fjord channel, led to the nine-day seiche.”

More at the Post, here.

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