The discovery of a massive debris pile in a giant sinkhole in the Hawaiian islands suggests that the region was hit by a mammoth tsunami about 500 years ago. It was larger than any in Hawaii's recorded history, so scientists are now worrying that a similar disaster could happen again.

Top image: Breach of the Miyako Seawall in Japan on March 11, 2011 (Toru Hanai/Reuters).

By studying the deposits left by the tsunami, and by creating computer models to show how it might have occurred, researchers from the University of Hawaii at Manoa have painted a picture of the cataclysmic event.

Sometime between 1425 and 1665 A.D., a 9.0 earthquake (and possibly as high as 9.2) struck the eastern Aleutians. It triggered a tsunami that, by the time it reached the Hawaiian islands, featured a wall of water up to 30 feet (9 meters) high. It left behind up to nine shipping containers worth of ocean sediment in a sinkhole on the island of Kauai. It was three times bigger than the tsunami that hit the region in 1946.

Tsunamis like this are extremely rare, and the paleogeologists are ascribing a 0.1% chance of it happening in any given year. That's the same degree of risk as the 2011 Tohoku earthquake that struck Japan in in 2011.

Despite the low probability, the new research has prompted Honolulu officials to revise their tsunami evacuation maps to account for the possibility. The new maps more than double the area of evacuation in some locations.

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

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Two meters (six and a half feet) below the surface he encountered a layer of sediment marked by coral fragments, mollusk shells and coarse beach sand that could only have come from the sea. But the mouth of the sinkhole was separated from the shore by 100 meters (328 feet) of land and seven-meter (23-foot) high walls. Burney speculated that the deposit could have been left by a massive tsunami, but he was unable to verify the claim.

The deposits remained a mystery until the Tohoku earthquake hit Japan in 2011. It caused water to surge inland like a rapidly rising tide, reaching heights up to 39 meters (128 feet) above the normal sea level. After that tsunami deluged the island nation, scientists began to question Hawaii's current tsunami evacuation maps. The maps are based largely upon the 1946 tsunami, which followed a magnitude 8.6 earthquake in the Aleutian Islands and caused water to rise only two and a half meters (8 feet) up the side of the Makauwahi sinkhole.

"[The Japan earthquake] was bigger than almost any seismologist thought possible," said Butler. "Seeing [on live TV] the devastation it caused, I began to wonder, did we get it right in Hawaii? Are our evacuation zones the correct size?"

The researchers used a wave model to predict how a tsunami would flood the Kauai coastline. They simulated earthquakes between 9.0 and 9.6 magnitude originating at different locations along the Aleutian-Alaska subduction zone, a 2,113-mile (3,400-kilometer) long ocean trench stretching along the southern coast of Alaska and the Aleutian Islands where the Pacific tectonic plate is slipping under the North American plate.

The red circles are centered on Kaua'i and encircle the Big Island. Image credit: Rhett Butler

Models showed that the unique geometry of the region would direct the largest post-earthquake tsunami energy directly towards the Hawaiian islands, and that a 9.0 quake in just the right spot could produce water levels on the shore reaching 26 to 30 feet high (8-9 meters).

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More evidence is needed to confirm these findings, but in the meantime, the researchers are telling Hawaiian officials not to take any chances. The county hopes to distribute the new maps to residents by end of the year.

Read the entire study at Geophysical Research Letters: "Paleotsunami evidence on Kaua'i and numerical modeling of a great Aleutian tsunami".

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