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Wednesday, October 22, 2014

Marine Debris Evidence Of Massive Hawaiian Tsunami

Brett Smith for - Your Universe Online

The massive tsunami that struck Japan in 2011 was a wake-up call for Honolulu officials, who began working on updating the island's evacuation plans and procedures immediately after the wall of water struck. Hawaii’s renewed attention on tsunamis also led to the unlocking of a scientific mystery surrounding a massive sediment deposit on the island of Kauai.

According to a new study from Hawaiian researchers – the sinkhole, which is nearly 330 feet from shore and guarded by 23-foot-high walls, was filled by marine debris from another massive tsunami, which struck roughly 500 years ago.

Published in the journal Geophysical Research Letters, the new study concluded that the older tsunami was probably caused by a 9.0-magnitude earthquake near the Aleutian Islands and measured around 30 feet high.

Hawaiian folk lore is full of tales about gigantic waves swamping the islands, but scientific evidence of these events was only first discovered in the 1990s, in the form of the Makauwahi sinkhole, a collapsed cave on the island of Kauai.

Over six feet below the ground, paleoecologist David Burney – who made the discovery – came across a stratum of sediment denoted by coral bits, mollusk shells and coarse beachfront sand that could only have originated from the sea. He suspected that a massive wave could have brought the debris there, but the distance from shore and natural levees of the surrounding area meant this wave would have to be an extremely rare and epic event.

After the 2011 earthquake caused the water to rise roughly 128 feet above normal sea level in Japan, scientists began reevaluate the sizes of waves that could hit the Hawaiian islands. Current evacuation maps are based mostly on a 1946 tsunami, which was brought on by an 8.6-magnitude quake which caused water to rise only about 8 feet.

“(The 2011 earthquake) was bigger than almost any seismologist thought possible,” said Rhett Butler, a geophysicist at the University of Hawaii at Manoa and author of the new study. “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?”

In the new study, researchers used a simulation to calculate how a tsunami would flood the Kauai shoreline. The team modeled earthquakes with magnitudes between 9.0 and 9.6 coming from several places along the Aleutian-Alaska subduction zone - a 2,100-mile ocean trench extending down the southern coast of Alaska and the Aleutian Islands where the Pacific tectonic plate is plummeting beneath the North American plate.

The scientists discovered that the distinctive geometry of the eastern Aleutians would steer the largest post-earthquake tsunami energy straight at the Hawaiian Islands. Models also revealed that an earthquake with a magnitude of more than 9.0 in the right location could generate water levels that would reach up to 30 feet high, easily passing over the Makauwahi sinkhole levee.

Using radiocarbon dating, the researchers also found that the Makauwahi deposit matched several other similar deposits found along the western coasts of Canada and the United States. The analysis indicated that an epic tsunami or multiple large tsunamis probably occurred between 350 and 575 years ago.

“(The study authors) stitched together geological evidence, anthropological information as well as geophysical modeling to put together this story that is tantalizing for a geologist but it’s frightening for people in Hawaii,” said Robert Witter, a geologist at the US Geological Survey was not involved in the study.

Image 2 (below): The researchers simulated earthquakes with magnitudes between 9.0 and 9.6 originating at different locations along the Aleutian-Alaska subduction zone, and found that the unique geometry of the eastern Aleutians would direct the largest post-earthquake tsunami energy directly toward the Hawaiian Islands. The red circles are centered on Kaua‘i and encircle the Big Island. Credit: Rhett Butler

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