Signs of ‘monster tsunami’

LIHUE — Makauwahi sinkhole on Kauai’s Southside has revealed just how large tsunamis in Hawaii can get.

The answer: much bigger than previously thought.

For generations, Hawaiians have told stories about colossal tsunamis in the islands, and in the late 1990s David Burney, a paleoecologist at the National Tropical Botanical Garden in Kalaheo, discovered the first possible evidence.

“When we first dug through this deposit in 1997, I was pretty sure we had found something remarkable,” he wrote in an email from Europe. “For several years, as we excavated more of the debris layer — a pile of huge broken stones that included a lot of beach rock, as well as fish skeletons, marine shells and splintered wood up to four feet thick in some places inside the sinkhole — I had to conclude that only a huge tsunami could make such a deposit.”

The mouth of the Makauwahi sinkhole is separated from the shore by 328 feet of land and 23-foot high walls, according to a release about the recent study.

After looking at other tsunami deposits, including the 1946 layer on the North Shore of Kauai, Burney was convinced it had to be a much bigger tsunami. He published a technical monograph in 2001 about his theory, but said it “didn’t create much of a stir in the scientific community.”

Recently, however, additional research found that the mass of debris is evidence that at least one mammoth tsunami — larger than any in Hawaii’s recorded history — did strike the islands, and that a similar disaster could happen again.

Scientists say a wall of water up to 30 feet high surged onto Hawaiian shores about 500 years ago, triggered by a 9.0-magnitude earthquake off the coast of the Aleutian Islands. The wave is believed to have left behind up to nine shipping containers worth of ocean sediment in Makauwahi, according to the study. The tsunami was at least three times the size of a 1946 tsunami, the most destructive in Hawaii’s recent history.

Burney said it was the Japanese tsunami of 2011, along with modeling efforts of Dr. Rhett Butler and his colleagues, that finally drew attention to this find.

“I had been saying for over a decade that this evidence was significant for Civil Defense, and I’m just relieved that they are listening now,” he wrote. “Regardless of how rare such an event might be, if another one occurred in the islands without a suitable evacuation plan, it would cause major loss of life, as the similar event in Japan did.”

Tsunamis of this magnitude are rare events. An earthquake in the eastern Aleutian Trench big enough to generate a tsunami like the one in the study is expected to occur once every thousand years, said Gerard Fryer, a geophysicist at the Pacific Tsunami Warning Center in Ewa Beach, Hawaii.

Nevertheless, the 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, according to the release.

“You’re going to have great earthquakes on planet Earth, and you’re going to have great tsunamis,” said Butler, a geophysicist at the University of Hawaii at Manoa and lead author of the new study. “People have to at least appreciate that the possibility is there.”

The study was published online Oct. 20 in Geophysical Research Letters, a journal of the American Geophysical Union.

The Tohoku earthquake hit Japan in 2011, causing water to surge inland like a rapidly rising tide, reaching heights up to 128 feet above the normal sea level. That’s when 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 8 feet up the side of the Makauwahi sinkhole.

“(The Japan earthquake) was bigger than almost any seismologist thought possible,” Butler said the release. “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?”

Fryer is confident that more evidence of the massive tsunami will be found, confirming that events of this magnitude have rocked the island chain in the not-so-distant past.

“I’ve seen the deposit,” he said. “I’m absolutely convinced it’s a tsunami, and it had to be a monster tsunami.”


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