As a seismologist at UC San Diego’s Scripps Institution of Oceanography in La Jolla, Carl Ebeling admits that seismological issues that leave him with questions are few and far between. So when one presented itself a year ago, he couldn’t resist jumping in to learn more.
In September 2023, scientists around the world detected a mysterious seismic signal that lasted for nine days. It was considered puzzling for two main reasons. First, the signal looked nothing like what earthquakes produce on seismographs. Instead, it oscillated with a 92-second interval between its peaks, too slow for humans to perceive. Second, the signal stayed strong for days, while more common seismic events weaken more rapidly.
As a first step, Ebeling checked to see if it was an equipment malfunction. Finding none, he reached out to other stations to see if the signal was registering on their equipment. It was.
“It had been a couple of days at that point, which is much longer than other types of signals, like an earthquake, which can last for about 10 minutes or an hour,” Ebeling said. “We knew at that point something weird was going on.”
“We like to think we know everything in the field of seismology,” he added. “So when we saw this very unusual signal, I was scratching my head. Even though we like to look for new things, we like to think we have seen everything. When we first saw [the signal], we had nothing to base our hypotheses on. Our models weren’t fitting. We thought of everything from aliens to a volcanic eruption.”
Scientists around the world started online discussions of what could be causing the strange seismic waves. The discussions turned up reports of a mountaintop collapsing into a remote fjord in East Greenland around the time the seismic signal was first detected.
A team of 68 scientists from 41 research institutions used their seismic recordings along with field measurements, satellite images and computer simulations to track the signal’s source. They analyzed satellite and ground images to document the enormous volume of rock and ice (33 million cubic yards) that fell into the fjord — a deep, narrow water inlet between cliffs.
The landslide triggered a mega-tsunami about 650 feet tall and waves that rocked back and forth inside the fjord for nine days. The rhythmic sloshing is a phenomenon known as a seiche. La Jollans may be familiar with the term given that late Scripps Oceanography researcher Walter Munk named his house after it.
The scientists also analyzed the seismic waves to model the dynamics and trajectory of the avalanche as it moved down the glacial gully and into the fjord.
“It was very weird and very unexpected,” Ebeling said.
No one was hurt during the tsunami, but the waves destroyed some $200,000 in infrastructure at an unoccupied research station on Ella Island.
Robert Anthony, a geophysicist with the U.S. Geological Survey’s Earthquake Hazards Program and co-author of the seismic study published Sept. 12 in the journal Science, said “It was exciting to be working on such a puzzling problem with an interdisciplinary and international team of scientists. Ultimately, it took a plethora of geophysical observations and numerical modeling from researchers across many countries to put the puzzle together and get a complete picture of what had occurred.”
Though the magnitude of the landslide and subsequent seismic signal was rare, such events are believed to have happened before — and likely will again. And next time, Ebeling said, it could happen in a more populated area.
“Gravity happens all the time and glacial thinning caused this collapse, so especially with climate change, we could see this more in the future,” he said.
But by having the data and record of the event, seismologists around the world are “able to use this information more actively [to] help us be better informed,” Ebeling said. “One event doesn’t tell us everything, [but] it is possible this could prompt other investigations and we can go back through this data.”
“It’s possible we have seen this before and not known it,” he added. “So to go back and go through these records allows us to think outside the box.” ♦
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Publish date : 2024-09-14 08:00:00
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