Cause Of 1946 Tsunami
Becomes A Mystery
Aleutian Finding Topples Theory That
Sea Floor Landslide Was To Blame

By Doug O'Harra
Anchorage Daily News
Scientists traveled to the Aleutian Chain last summer to check out a colossal submarine landslide blamed for one of the most devastating tsunamis of the 20th century.
They wanted to find out how sea-floor life responded to such a huge disturbance and produce detailed charts.
What they got was a shock of seismic proportions.
Instead of a 12-mile-wide avalanche dropping 30 to 40 miles down the continental slope into the abyss of the Aleutian Trench, sonar surveys and the remotely operated underwater vehicle Jason II found regular ocean bottom, eroded and crusty and largely undisturbed.
There was no slide.
And now no one knows what triggered the legendary tsunami of 1946, which destroyed Scotch Cap lighthouse on Unimak Island and killed 159 people in Hawaii.
"Basically, we found sea floor evidence that will cause tsunami modelers to rethink the cause and characteristics of the 1946 tsunami," wrote research associate Tony Rathburn and professor Lisa Levin, principle investigators with the project at Scripps Institution of Oceanography. "The sea floor landslide ... that seemed to be evident from previous, low-resolution (charts) does not exist."
A Hawaiian geophysicist working on the project now believes that an underwater slide probably hit farther east along the Aleutian slope, near a feature that might be the toe of a previously unknown avalanche. Triggered by an earthquake, this slide would have created a tsunami. When the wave rolled into shallow water, the theory goes, it accelerated into a bore of incredible power, surging along the shore of Unimak Island with hurricane force.
"You think of a 40-foot wave or a 50-foot-high wave traveling at 140 mph," said Gerard Fryer with the University of Hawaii at Manoa in an interview earlier this month. "It's an astounding image. It's almost beyond our comprehension."
The summer expedition also gathered unprecedented biological information about the ocean off Alaska's coast. During seven dives and several weeks of cruising, the team gathered hundreds of samples, images and photos. Rathburn and Levin are spending the winter analyzing what may be dozens of new species.
Along the way, they found a bizarre methane seep in pitch darkness more than two miles down on a steep slope: fuzzy carbonate rocks "weeping" with upside-down life, unknown worms, gigantic bacteria, seas stars, octopuses, clams.
It was like an oasis fueled by gas, Levin said.
"It wasn't altogether clear why the animals were growing down," she said. "There never has actually been a community of animals that live on rocks but get their (energy) from methane. ... But when you go someplace that nobody has ever been before, you see things you don't expect. That's my experience of the deep ocean."
"We were excited, perplexed, fascinated," Rathburn added. "There are some really strange things going on with the seeps there that seem to be different from what's going on elsewhere in the Pacific basin."
The new findings mark the third season of amazing discoveries along the Aleutian Chain by scientists using subs and underwater vehicles. Over the past few years, biologists and marine geologists have documented coral gardens, new species of fish and invertebrates, and Alaska's first submerged volcano rising to a coral-shrouded cone of black rock deep in Amchitka Pass.
Details of this new fish-rich habitat have spurred intense debate at the North Pacific Fishery Management Council about restricting trawling and other bottom-contact commercial fishing gear.
This summer, four scientific teams took turns voyaging on the Scripps research vessel Roger Revelle, using the Jason II to explore seamounts, coral habitat, the Adak underwater canyon, and submarine slides. The work was sponsored by the West Coast National Undersea Research Program of the National Oceanic and Atmospheric Administration.
The group led by Rathburn and Levin focused on what Fryer had named the Ugamak Slide, located south of Unimak Island. The slope of the North American tectonic plate pitches into the Aleutian Trench, where the Pacific plate gets subducted into the Earth.
It was there, about 1:30 a.m. on April 1, 1946, that an earthquake originally measured as magnitude 7.3 struck with surprising effect. A second quake hit 27 minutes later.
Within about 30 minutes of the second jolt, a huge wave smashed into the Scotch Cap lighthouse on the tip of Unimak Island, reducing a new structure of reinforced concrete to rubble and killing the five men inside.
"Terrific roaring from ocean heard, followed almost immediately by terrific sea, top of which rose above cliff and struck station, causing considerable damages," reads the 2:18 a.m. entry in the U.S. Coast Guard log from the radio communications station on the cliff above.
At dawn, personnel found a beach littered with debris. Human body parts were discovered too, some washed 115 feet above sea level to the cliff top. The victims had basically been shredded.
The idea that this five-year-old lighthouse, built on a platform 30 feet above the sea, could be vulnerable to a 115-foot tsunami was unimaginable, Fryer said.
"Nobody living had seen a tsunami like this," Fryer said.
Meanwhile, another tsunami was traveling across the Pacific at nearly 500 mph. Within five hours, a train of waves reached Hawaii and rolled into settlements without warning, ultimately destroying 1,400 structures. One 30-footer inundated the Hilo waterfront, killing 96 people.
The destruction prompted the U.S. Congress to establish the Pacific Tsunami Warning Center in Ewa Beach, Hawaii, and create a Pacificwide warning system. (The West Coast and Alaska Tsunami Warning Center, located in Palmer, was added after the 1964 quake.)
For years, scientists argued about the source of the tsunami. How could such a relatively small earthquake make the entire Pacific Ocean slosh like bath water?
Even more inexplicable, how could it also have created such an enormous wave at Scotch Cap?
Fryer has argued that a huge underwater slide must be the explanation for both tsunamis, not the earthquake. He thought he had found the culprit with a sonar image taken a few years ago. But now he believes that the older sonar data didn't produce an accurate picture of the bottom and that the Ugamak Slide doesn't exist.
But other tsunami modelers disagree with Fryer, saying that the earthquake was large enough to trigger the Pacificwide tsunamis while an as-yet unidentified submarine avalanche probably caused the damage at Scotch Cap.
"The 1946 event is not a textbook earthquake. It's not an earthquake that follows the rules," said professor Emile Okal, a tsunami expert at Northwestern University. "It was a much larger earthquake (than originally measured) because it was a very slow earthquake (that) released much of its energy at very long periods."
Last summer's findings deepened the mystery, said Rathburn, also a professor at Indiana State University.
"This is what happens when all of your highfallutin theories come to grip with reality," he said. "It's like theorizing on the moon or Mars, and then actually going there and looking at the place. Lots of things change, and it actually makes things more exciting."
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