Landslide-Driven Megatsunamis Threaten Hawaii

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Sunglint surrouncs half of the Hawaiian Islands. CREDIT: NASA.

Content provided by Becky Oskin, OurAmazingPlanet

It's almost unimaginable: a tsunami more than 1,000 feet (300 meters) high bearing down on the island of Hawaii.

But scientists have new evidence of these monster waves, called

megatsunamis, doing just that. The findings were presented Wednesday  (Dec. 5) at the annual meeting of the American Geophysical Union.

Unlike tsunamis from earthquakes, the Hawaiian tsunamis strike when the

island chain's massive volcanoes collapse in humongous landslides. This

happens about every 100,000 years, and is linked to climate change,

said Gary McMurtry, a professor at the University of Hawaii in Honolulu.

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Sitting about 30 feet (10 m) away from today's Ka Le (South Point)

seashore are boulders the size of cars. Some 250,000 years ago, a

tsunami tossed the enormous rocks 820 feet (250 m) up the island's

slopes, said Fernando Marques, a professor at the University of Lisbon

in Portugal. (The boulders are closer to the shore now because the main

island of Hawaii is one of the world's largest volcanoes, and its

massive weight sends it sinking into the Earth at a rate of about 1

millimeter a year.)

McMurtry's team found two younger and slightly smaller tsunami deposits

at South Point on the main island of Hawaii, one 50,000 years old and

one 13,000 years old. He suggests the tsunami source is the two Ka Le

submarine landslides, from the flanks of the nearby Mauna Loa volcano.

The waves carried corals and 3-foot (1 m) boulders 500 feet (150 m)

inland.

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Deadly, landslide-triggered tsunamis happen at volcanic islands around the world, and are a potential hazard for the Eastern United States.

"We find them everywhere, but we don't know of any historical cases, so

we have to go back in time," said Anthony Hildenbrand, a volcanologist

at the University of Paris-Sud in France, who helped identify the

ancient tsunami deposit.

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The falling rock acts like a paddle, giving the water a sudden push. While landslide tsunamis

may have a devastating local effect, they lose their power in the open

ocean and don't destroy distant coastlines like earthquake tsunamis.

The giant landslides seem to happen during periods of rising sea levels,

when the climate is also warmer and wetter, Hildenbrand told

OurAmazingPlanet. Researchers speculate that the change from lower sea

level to higher may destabilize a volcanic island's flanks, and heavier

rains could soak its steep slopes, helping trigger landslides.

There are at least 15 giant landslides that have slid off the Hawaiian

Islands in the past 4 million years, with the most recent happening only

100,000 years ago, according to the U.S. Geological Survey. One block

of rock that slid off Oahu is the size of Manhattan.

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