A mass of marine debris discovered in a giant sinkhole in the Hawaiian islands provides evidence that at least one mammoth tsunami, larger than any in Hawaii’s recorded history, has struck the islands, and that a similar disaster could happen again, new research finds. Scientists are reporting that a wall of water up to nine meters (30 feet) high surged onto Hawaiian shores about 500 years ago. A 9.0-magnitude earthquake off the coast of the Aleutian Islands triggered the mighty wave, which left behind up to nine shipping containers worth of ocean sediment in a sinkhole on the island of Kauai.
The tsunami was at least three times the size of a 1946 tsunami that was the most destructive in Hawaii’s recent history, according to the new study that examined deposits believed to have come from the extreme event and used models to show how it might have occurred. Tsunamis of this magnitude are rare events. An earthquake in the eastern Aleutian Trench big enough to generate a massive tsunami like the one in the study is expected to occur once every thousand years, meaning that there is a 0.1 percent chance of it happening in any given year — 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.
Nevertheless, the new research has prompted Honolulu officials to revise their tsunami evacuation maps to account for the possibility of an extreme tsunami hitting the county of nearly 1 million people. The new maps would more than double the area of evacuation in some locations, according to Fryer.
“You’re going to have great earthquakes on planet Earth, and you’re going to have great tsunamis,” said Rhett Butler, a geophysicist at the University of Hawaii at Manoa and lead author of the new study published online in Geophysical Research Letters, a journal of the American Geophysical Union. “People have to at least appreciate that the possibility is there.”
Hawaiians have told stories about colossal tsunamis hitting the islands for generations, but possible evidence of these massive waves was only first detected in the late 1990s when David Burney, a paleoecologist at the National Tropical Botanical Garden in Kalaheo, was excavating the Makauwahi sinkhole, a collapsed limestone cave on the south shore of Kauai.
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?”
To find out, the study’s authors used a wave model to predict how a tsunami would flood the Kauai coastline. They simulated earthquakes with magnitudes between 9.0 and 9.6 originating at different locations along the Aleutian-Alaska subduction zone, a 3,400-kilometer (2,113-mile) 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 researchers found that the unique geometry of the eastern Aleutians would direct the largest post-earthquake tsunami energy directly toward the Hawaiian Islands. Inundation models showed that an earthquake with a magnitude greater than 9.0 in just the right spot could produce water levels on the shore that reached eight to nine meters (26 to 30 feet) high, easily overtopping the Makauwahi sinkhole wall where the ocean deposits were found.
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Massive debris pile reveals risk of huge tsunamis in Hawaii