It started on 16 September 2023. Broadband seismometers started rhythmically rocking, first near Greenland and then worldwide, within hours. Each station tracked the same signal, a low, droning 10.88-millihertz wave recurring every 92 seconds.
The noise lasted an incredible 9 days. Seismic signals are often sharp and abrupt. This signal was more like the sound of a gong radiating across Earth itself. It was officially dubbed a USO, or an unidentified seismic object.
“It’s not something we ever observed before.”
“Like a UFO, it was really very puzzling,” said Alice Gabriel, a computational seismologist at the Scripps Institution of Oceanography. “It’s not something we ever observed before.”
Researchers pinpointed the source of the USO to a massive rockslide in East Greenland’s Dickson Fjord but were unsure why the wave lasted so long. Then, on 11 October, it happened again. Seismographs started swinging to the same tone from the same location.
Further research showed this fjord had a signature song, one it’s been singing for decades.
Cold Case
Gabriel is among the 68 researchers from 15 countries who solved the USO mystery in a recently published Science article.
The seismic event started with a landslide. A thinning glacier unleashed 25 million cubic meters of rock and ice, which crashed down a steep gully into a narrow fjord. “You had basically all the check boxes for a very tsunamigenic landslide,” said Kristian Svennevig, a landslide geologist at the Geological Survey of Denmark and Greenland and the lead author on the paper.
The landslide triggered a colossal 200-meter (650-foot) tsunami inside the basin. Ancient Inuit campsites and century-old trapper huts disappeared as the wave pulsed toward the ocean. There were no casualties, but it was a close call because a cruise ship had passed near the area the day before. Another cruise ship sailed down the coast the next day, reporting significant damage to an empty research station 72 kilometers (45 miles) from the slide.
The landslide seemed like the obvious source of the signal, but researchers struggled to connect the tsunami with the 9-day noise. They guessed the seismic signal was caused by a seiche, a long-lasting wave that sloshes back and forth like a splash in a bathtub. But seiches typically need a sustained energy source like a windstorm. This event appeared to run on its own momentum.
“We had no literature, no examples of anything like that naturally occurring.”
“We couldn’t get our observations to explain the seismic signal,” said Svennevig. “We had no literature, no examples of anything like that naturally occurring.”
They also had a computation issue. Tsunami models usually simulate an event over hours, not weeks. The team needed supercomputers and declassified bathymetry data from the Danish military to build a model detailed enough to re-create the seismic event.
The team’s model eventually confirmed that Dickson Fjord’s unique geography could sustain the seiche. After the landslide slammed 500 trillion newtons of force into the fjord, the basin’s parallel walls ricocheted the water back and forth roughly 10,000 times. A bend in the fjord prevented the energy from fully bouncing out to sea.
F as in Fjord
The research team finally had an answer to where the tone came from, but they weren’t done asking questions.
How many more times had Dickson Fjord struck this tone?
The team dug through decades of seismic data to find out. Their search turned up four more instances in which Dickson Fjord produced the same frequency between 2005 and 2017, just at shorter durations and smaller amplitudes than the September 2023 event. After they were cross-checked with satellite observations, those seismic signals pointed to landslides no one had recorded before.
Fjords “kind of all have their song.”
Each seiche is like a slow-moving guitar string, said Ben Holtzman, a geophysicist at Columbia University and the Massachusetts Institute of Technology who runs the Seismic Sound Lab. The walls of the fjord, he explained, limit the vibration of the wave, like tuning knobs do for guitar strings. That’s why every seiche in Dickson Fjord strikes the same note. And that note? It’s basically an F, Holtzman said, but 14 octaves below F2, the second lowest F key on a piano.
The results demonstrate a new way to identify remote landslides. “If you saw something at 10.88 [millihertz] again, you might ask, ‘Hey, is that that same fjord going off?’” said Jackie Caplan-Auerbach, an acoustic seismologist at Western Washington University who was not involved in the research.
As Earth warms, more fjords may start singing. Thinning glaciers and thawing permafrost have triggered landslides in Greenland for decades. The study shows another way to monitor these events, especially in remote and unpopulated parts of Greenland as well as in other fjord-rich regions like Norway, New Zealand, Canada, and Chile.
“[Fjords] all have their song,” said Caplan-Auerbach. With passive instrumentation, seismologists can “sit back and listen and hear what the planet has to provide for us.”
—J. Besl (@J_Besl), Science Writer
Citation: Besl, J. (2024), Finding the frequency of a fjord, Eos, 105, https://doi.org/10.1029/2024EO240454. Published on 9 October 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0
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