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Across an ocean, Alaskans hear an underwater volcano erupt

NOAA via GOES West satellite

Some thought they heard snow sliding off a roof. Others imagined a moose at the window.

But the thunderous bellows Alaskans heard early Saturday morning came from something else entirely: the eruption of an underwater volcano nearly 6,000 miles away, by the South Pacific Kingdom of Tonga. 

The eruption spurred a tsunami warning for the entire West Coast of the U.S. and generated destructive waves as close as Tonga and as far away as Peru.

The largest waves to hit Alaska were just over three feet tall, in King Cove.

But the ordeal was still alarming for Alaskans who were jolted awake by what sounded like explosions early Saturday morning.

“And it wasn’t just one boom. It was several, over and over and over," said Sally Cassano. When she heard the burst from her home in Clam Gulch, her mind started racing.

“My first guess was, there’s gotta be fireworks," she said. "Except I could feel it under my feet on the deck. So then I said, ‘My God, it sounds like bombs in the distance.’ I mean, that can’t be, because we’re military to the gills up here and I can’t imagine there being bombing going on. So then the third guess was, ‘Oh my goodness. There's all these capped gas wells that have been drilled out in this area. I hope they're not exploding.'"

Cassano realized it was an effect from the eruption when she took to Google. And she wasn’t the only one who was curious. Alaskans were flooding local Facebook groups to ask their neighbors if they had heard the same.

Across Kachemak Bay in Seldovia, Bretwood Higman was asleep and didn’t hear anything.

But he has been thinking a lot about the eruption and its effects in the days since. Higman is a geologist who studies tsunamis and other disasters. And he said he’s still trying to figure out why Alaskans were able to hear the eruption, so far away.

What he does know is the eruption sent an atmospheric pressure wave across the U.S.

He speculates that wave may have created the sound many heard hours after the eruption. That could explain why it reached Alaska even though people who were much closer didn’t report hearing anything.

“I guess the questions I have — can that longer period, that atmospheric wave, actually produce audible sound as well, for instance, as it hits the range of mountains that are on the south coast of Alaska?”

Besides the soundwaves, the waves of water generated from the explosion are puzzling, too.

Higman said scientists usually think about tsunamis caused by eruptions like a rock sending small waves through a lake, with the water furthest away seeing tiny ripples.

But this eruption was different.

He said the waves that hit places like Peru were much larger than would be predicted given the size of the wave in Tonga. That could again have something to do with that pressure wave.

Higman said he’s still talking to scientists from all over the world about what happened, and how the waves could be so much larger than they expected.

“Because we got this one pretty wrong, I think this suggest that there is important stuff to learn there," he said.

Higman hopes a silver lining is that researchers will work to better anticipate hazards from other volcanoes, like the Okmok Volcano in the Aleutian Islands. 

Some scientists have labeled this eruption a once in a millennium event. It’s certainly nothing like Cassano has ever experienced. She said it was disconcerting to not know at first what was creating the sound she heard from her deck by the bluff.

“And then once I saw that the U.S. National Weather Service was confirming that that’s what it was — seriously I felt much better, that that’s what it was," Cassano said. "But of course, feeling badly for Tongans.”

Tsunamis pummeled Tonga in the wake of the eruption, though it’s still unclear the breadth of the damage there, since communication has faltered.

That’s made it hard for Alaskans with relatives there to check in with family. Cassano said she has a cousin who has been unable to reach her family in Tonga.

The nation is now asking for immediate help getting food and freshwater to Tonga. Meanwhile, the Hunga-Tonga-Hunga-Ha’apai volcano continues to erupt at a lower intensity.

Sabine Poux is a producer and reporter for the Brave Little State podcast of Vermont Public. She was formerly news director and evening news host at KDLL in Kenai.

Originally from New York, Sabine has lived and reported in Argentina and Vermont and Kenai.
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