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Sinkhole Reveals Hawaii's Risk for Another Monster Tsunami

Oct 21, 2014 12:17 PM EDT

A giant sinkhole discovered in Hawaii, piled high with marine debris, provides evidence of the largest tsunami ever to hit the islands, and reveals the risk of another monster tsunami in the future, new research shows.

About 500 years ago a 9.0-magnitude earthquake in Alaska triggered a mighty wave, sending a wall of water 30 feet (nine meters) high surging toward Hawaiian shores. The disaster left behind up to nine shipping containers worth of ocean sediment in a sinkhole just recently discovered on the island of Kauai.

The tsunami was at least three times the size of the damaging 1946 tsunami, which was driven by an 8.6-magnitude earthquake off the Aleutian Islands. Mammoth tsunamis, like the one described in the study, are rare, and likely happen once every thousand years. This is the same probability as the 2011 Tohoku earthquake that struck Japan, according to Gerald Fryer, a geophysicist at the Pacific Tsunami Warning Center in Ewa Beach, Hawaii.

Despite the slight chance, the new research reveals that another large Alaskan earthquake could cause a mammoth tsunami in Hawaii in the future.

"You're going to have great earthquakes on planet Earth, and you're going to have great tsunamis," lead researcher, Rhett Butler, a geophysicist at the University of Hawaii at Manoa, said in a statement. "People have to at least appreciate that the possibility is there."

Hawaiian officials have already started bracing for impact. The study's results have prompted Honolulu to revise its tsunami evacuation maps, doubling the area of evacuation in some locations, to better protect the county's one million residents.

Stories have circulated around Hawaii for generations about such monster waves, but then researchers used radiocarbon dating and earthquake models to confirm that the debris from the sinkhole - first found in the late 1990s - was indeed from a mammoth tsunami. They found that anything over a reading of 9.0 would be enough to trigger a wave 26 to 30 feet (eight to nine meters) high.

"I've seen the deposit," said geophysicist Gerald Fryer at Hawaii's Tsunami Warning Center, who was not involved in the study. "I'm absolutely convinced it's a tsunami, and it had to be a monster tsunami."

The findings were published in the journal Geophysical Research Letters.

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